Draw the map's missing squares as gaps, and its rings in your own units

Two faults reported from in front of the window, both of them the map saying
something confidently and wrongly.

Tiles that did not arrive.  Resizing the window does not refetch the map: the
window deliberately fetches more world than it shows, so a resize still fits
inside what is in hand and gets stretched to the new size.  It refetches when
the view leaves that box, which is what a new station does -- and that fetch
asks a volunteer-funded server for a hundred tiles that have never been on
this disk, all at once, at the sharper zoom the bigger window chose.  Some of
them are refused.

What the window did with a refusal was draw it.  A tile that never arrived
leaves its square of the canvas black, and black is not a neutral colour
here: the brightness is inverted on the way in, because a printed map is ink
on paper and this picture is the other way round.  So the darkest possible
square came out as the brightest thing on the picture, a glowing rectangle
where the map should be.  Measured on a reproduction, one missing tile in
eighteen put thirteen thousand pixels at full brightness -- and dragged the
floor of the map's own contrast down to black with it, so thirteen thousand
four hundred and ninety pixels changed in all: the whole map was redrawn
dimmer to make room for a square that was not there.  Then it was kept, cached
under the view it was fetched for, until the view moved again.

So the missing squares are asked for again at once, and only those, the rest
being on the disk by then; what is still missing is drawn as bare ground and
left out of the reckoning when the darkest and brightest of the map are worked
out, which puts the same reproduction at two pixels changed rather than
thirteen thousand four hundred and ninety, a hairline where a cell is averaged
over part of a tile and part of nothing; and the map is kept as provisional
rather than as the last word, asked for again half a minute later, four
attempts in all, each retrying its own misses once.

Found while measuring that: the politeness pause between requests was being
paid on every tile, including the ones read straight back off the disk.  Two
hundred and twenty tiles at an eighth of a second is twenty-six seconds of
sleeping to redraw a view that was entirely cached, and it would have made
asking again for three missing squares cost the wait for the two hundred that
were not.  The constant's own comment already said it should only be paid on a
tile that was not already there.  Now it is.

The APRS map's units.  Setting imperial changed nothing at all about the
window: the unit it measures in was hardcoded to kilometres, and that one
value drives the ring labels and the scale along the bottom; and --radius was
always read as kilometres, so the rings were not merely mislabelled, they were
at the wrong distance from the flag.  A ring is what a distance gets judged
against by eye, and one labelled in a unit it was not drawn in is a wrong
answer given confidently.  The aircraft side has done this properly all along
-- a radius read in whatever unit the speeds are in, and no unit suffix on the
setting because the suffix belongs to the other setting -- so this now mirrors
it exactly.  At --radius 100 in imperial the outermost ring stands seventy-
five statute miles from the flag and says so, where it used to stand seventy-
five kilometres and say kilometres whatever you had asked for.

Twenty-one new tests against sixteen deliberately broken builds.  One
survived, and removing what it broke was the right answer rather than
strengthening a test: a check that the remembered request still matched the
map in hand could not be made to fail, a request for a different view being
taken up only after the slot it guards is already full.  The tests do not
trust the drawing to mark its own homework -- the one that matters walks north
from the flag by each ring's radius and measures the great-circle distance
with a haversine written in the test, then checks that against the printed
label.  Full suite 2685 passed.  Built as 2026-09-21_03.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016PsWPTweCT6pwxKngvVxcg

README.md

@@ -1646,6 +1646,37 @@ map is enlarged after all. Somebody else's tile server is not a thing to fetch
 a thousand tiles from for one picture. A smaller `--radius` buys the detail
 back, since the same budget then covers less ground.
 
+**Tiles that do not arrive.** Resizing the window is the demanding case: a
+wider picture picks a sharper zoom, and a hundred tiles that have never been
+on this disk are asked for at once. A busy server refuses some of them, and
+what the window used to do with a refusal was draw it.
+
+A tile that never arrived leaves its square of the canvas black, and black is
+not a neutral colour here — the brightness is inverted on the way in, so the
+darkest possible square comes out as **the brightest thing on the picture**: a
+glowing rectangle where the map should be. It also dragged the floor of the
+map's own contrast down to black, so every other pixel was drawn dimmer to
+make room for a square that was not there. And it was kept: the map was
+cached under the view it was fetched for, and the hole stayed until the view
+changed.
+
+So three things happen instead. The missing squares are **asked for again**,
+straight away, because the usual reason for a refusal is the ninety-nine tiles
+asked for just before it — and only the missing ones, since the rest are on
+the disk by then. What is still missing is **drawn as bare ground** rather
+than as a light, and left out of the reckoning when the darkest and brightest
+of the map are worked out, so a hole costs nothing but itself. And the map is
+**remembered as provisional**: the window keeps drawing it and asks for the
+rest of it half a minute later — four attempts in all, each of which retries
+its own misses once, so a square gets eight chances before one that will not
+come is accepted as one that is not there.
+
+The politeness pause between requests is now paid only on a tile that had to
+be fetched. It had been paid on every tile including the ones read back off
+the disk, which put twenty-six seconds of sleeping into redrawing a view that
+was entirely cached — and would have made asking again for three missing
+squares cost the wait for the two hundred that were not.
+
 `--map-brightness PERCENT` (70 by default) is how far up its range the map is
 drawn, and **the vector themes bend the middle of that range down hard** —
 because a tinted photograph of a county behind the vectors is the one thing
@@ -2498,6 +2529,17 @@ trails, `g` the map underneath, `[` and `]` its brightness, `+`/`-` the range,
 `q` quits. `--radius` sets how far it reaches to begin with and `--theme` picks
 from the same five.
 
+**The window measures in whatever unit you are being shown.** `--units
+imperial` puts statute miles round the rings, along the scale at the bottom
+and on `--radius` itself, and `--units metric` puts kilometres on all three.
+The rings are then *drawn* at the distance they are labelled: the outermost
+one at `--radius 100 --units imperial` stands seventy-five statute miles from
+the red flag, measured on the ground, not seventy-five kilometres with miles
+written beside it. A ring is the thing a distance gets judged against by eye,
+so a ring labelled in one unit and drawn in another is a wrong answer given
+confidently — and the tables printed afterwards have always followed this
+setting, so the window disagreeing with them was the window being wrong.
+
 **Where the aerial is** puts the red flag on the map, centres the range rings
 and gives every station a distance and a bearing. Set it with `--at LAT,LON`,
 or in the menu under **Receiver at** — and if you have already told the

bandsaunter/__init__.py

@@ -9,7 +9,7 @@ and transcribing speech.
 # 2026-08-21_02 is the second build made on the 21st.  The revision is padded
 # to two digits so versions sort as text.
 VERSION_DATE = "2026-09-21"
-VERSION_REVISION = 2
+VERSION_REVISION = 3
 
 __version__ = f"{VERSION_DATE}_{VERSION_REVISION:02d}"
 

bandsaunter/aprs.py

@@ -36,6 +36,7 @@ from .settings import Setting, format_value
 __all__ = ["AprsOptions", "OPTIONS", "OPTION_GROUPS", "defaults", "in_group",
            "channel_text", "find_channel", "report_channels", "Found",
            "use_region", "receiver_at", "coordinates",
+           "map_unit", "radius_in_nm",
            "by_key", "format_option", "describe", "summarise", "Station",
            "Net", "Heard", "listen", "open_device", "open_log", "pump",
            "finish", "report", "watch", "windowed", "blip_for",
@@ -73,7 +74,7 @@ class AprsOptions:
     digipeated: bool = True        # count frames that reached here relayed
 
     # -- the map --------------------------------------------------------
-    radius: float = 50.0           # km the window reaches around the aerial
+    radius: float = 50.0           # how far the window reaches, in map_unit
     theme: str = "night"
     map_brightness: int = 70
     basemap: bool = True
@@ -239,12 +240,17 @@ OPTIONS: tuple[Setting, ...] = (
 
     # -- the map --------------------------------------------------------
     O("radius", "Map reaches", "The map", "float",
-      "how far around the aerial the window reaches, in kilometres",
+      "how far around the aerial the window reaches, in the unit being shown",
       "Which is also the zoom. Fifty kilometres is a region; five is a town "
       "and a digipeater at the edge of it falls off the picture. Stations "
       "outside it are still heard, still logged and still on the map drawn "
-      "afterwards \u2014 they are simply off the edge of the window.",
+      "afterwards \u2014 they are simply off the edge of the window. "
-      unit="km", minimum=1.0, maximum=2000.0, flags=("--radius",),
+      "In whatever unit Show readings in is set to: kilometres in metric, "
+      "statute miles in imperial. No suffix is printed against it here "
+      "because the suffix is the other setting's to give, and a number "
+      "labelled km while the rings beside it are drawn in miles would be "
+      "worse than one labelled nothing.",
+      minimum=1.0, maximum=2000.0, flags=("--radius",),
       example="50"),
     O("theme", "Theme", "The map", "choice",
       "how the window looks",
@@ -423,6 +429,30 @@ def coordinates(text: str) -> tuple[float, float] | None:
     return (lat, lon) if abs(lat) <= 90 and abs(lon) <= 180 else None
 
 
+def map_unit(options: AprsOptions) -> str:
+    """Which unit the window measures in: statute miles or kilometres.
+
+    The same setting that picks the unit for every reading in the tables
+    picks it here.  A window that said kilometres round its rings while the
+    table beside it said miles would be two answers to one question.
+    """
+    return "mph" if options.imperial else "kph"
+
+
+def radius_in_nm(options: AprsOptions) -> float:
+    """How far the window reaches, as the drawing wants it.
+
+    The setting is in whatever unit is being shown -- kilometres in metric,
+    statute miles in imperial -- so that the number typed in and the number
+    written beside the outermost ring are the same number.  Everything
+    inside the drawing is in nautical miles, that being a minute of
+    latitude and so the unit the geometry is already in.
+    """
+    from .flightlog import speed_unit
+
+    return max(1.0, float(options.radius)) / speed_unit(map_unit(options))[3]
+
+
 def receiver_at(options: AprsOptions) -> tuple[float, float] | None:
     """Where the aerial is standing, from here or from the aircraft settings.
 
@@ -1490,8 +1520,8 @@ def watch(console, options: AprsOptions, output_dir: str,
     # out of the palette this writes.
     set_theme(options.theme)
     sky = livemap.Sky(
-        unit="kph", hold=options.hold, home=home,
+        unit=map_unit(options), hold=options.hold, home=home,
-        radius_nm=max(1.0, options.radius) / 1.852,
+        radius_nm=radius_in_nm(options),
         brightness=max(10, options.map_brightness) / 100.0,
         fade=0.0, airports=False, rings=options.window_rings,
         box_opacity=max(0, options.box_opacity) / 100.0,

bandsaunter/basemap.py

@@ -71,6 +71,15 @@ USER_AGENT = (f"bandsaunter/{__version__} "
 # tile that was not already on the disk.
 FETCH_PAUSE = 0.12
 
+# What to do about tiles that did not arrive.  A server that has just been
+# asked for a hundred tiles in a row refuses some of them, and the answer to
+# being told to slow down is to slow down and ask again rather than to draw
+# the gap.  Longer than FETCH_PAUSE because the first pass is what provoked
+# it; once, because a tile that fails twice is usually a tile that is not
+# there, and the caller asks again later anyway.
+RETRY_PAUSE = 0.8
+RETRIES = 1
+
 
 # Where to ask what aerodromes are in a piece of the world.  The same OSM
 # data the tiles are drawn from, asked as a question rather than a picture.
@@ -303,12 +312,55 @@ def fetch_tile(zoom: int, x: int, y: int, url: str = TILE_URL,
     return body
 
 
+def on_disk(zoom: int, x: int, y: int, cache: Path | None = None,
+            **kw) -> bool:
+    """Whether this tile has been fetched before.
+
+    A read off the disk owes a volunteer-funded server no politeness, and
+    paying it anyway is how a view whose tiles are all in hand takes half a
+    minute to redraw: two hundred and twenty tiles at an eighth of a second
+    each, spent sleeping between files that were already there.
+    """
+    where = (cache if cache is not None else cache_dir()) / str(zoom) / str(x)
+    try:
+        return (where / f"{y}.png").exists()
+    except OSError:
+        return False
+
+
+def _one_tile(fetch, zoom: int, tx: int, ty: int, **kw):
+    """One decoded tile, or None if it could not be had or made sense of."""
+    body = fetch(zoom, tx, ty, **kw)
+    if body is None:
+        return None
+    try:
+        tile = decode_png(body)
+    except (PNGError, zlib.error, ValueError):
+        return None
+    if tile.shape[0] != TILE_PIXELS or tile.shape[1] != TILE_PIXELS:
+        return None
+    return tile
+
+
 def mosaic(south: float, west: float, north: float, east: float, zoom: int,
-           fetch=fetch_tile, pause: float = FETCH_PAUSE, **kw):
+           fetch=fetch_tile, pause: float = FETCH_PAUSE,
+           retries: int = RETRIES, retry_pause: float = RETRY_PAUSE,
+           cached=on_disk, **kw):
     """Every tile the box touches, stitched into one image.
 
-    Returns the pixels and where their top-left corner sits in the world, in
+    Returns the pixels, where their top-left corner sits in the world in
-    tile-grid pixels at this zoom, so the resampling below can place them.
+    tile-grid pixels at this zoom so the resampling below can place them,
+    and which tiles actually arrived.
+
+    That last one matters because the canvas starts black, and black is not
+    a neutral colour here: the brightness is inverted further down, so a
+    tile that never arrived is drawn as the brightest thing on the map
+    rather than as a gap.  Saying which squares are real lets the map be
+    drawn without them, and lets the caller know the answer is not final.
+
+    Tiles that did not arrive are asked for again before the map is given up
+    on, because the usual reason for a hole is a hundred tiles having been
+    asked for in the preceding second.
     """
     x0, y0 = tile_of(north, west, zoom)
     x1, y1 = tile_of(south, east, zoom)
@@ -317,32 +369,40 @@ def mosaic(south: float, west: float, north: float, east: float, zoom: int,
     span = 2 ** zoom
     wide, tall = right - left + 1, bottom - top + 1
     if wide <= 0 or tall <= 0 or wide * tall > MAX_TILES * 4:
-        return None, 0, 0
+        return None, 0, 0, np.zeros((0, 0), dtype=bool)
     canvas = np.zeros((tall * TILE_PIXELS, wide * TILE_PIXELS, 3),
                       dtype=np.uint8)
-    got = 0
+    covered = np.zeros((tall, wide), dtype=bool)
-    for row in range(tall):
+    # Rows off the top or bottom of the world are left out rather than
-        for column in range(wide):
+    # asked for: they are not missing tiles, they are places there is no
-            tx, ty = (left + column) % span, top + row
+    # map of, and they stay uncovered so they are drawn as nothing.
-            if not 0 <= ty < span:
+    todo = [(row, column, (left + column) % span, top + row)
-                continue
+            for row in range(tall) for column in range(wide)
-            body = fetch(zoom, tx, ty, **kw)
+            if 0 <= top + row < span]
-            if body is None:
+    for attempt in range(max(0, retries) + 1):
-                continue
+        if not todo:
-            try:
+            break
-                tile = decode_png(body)
+        if attempt and retry_pause:
-            except (PNGError, zlib.error, ValueError):
+            time.sleep(retry_pause)
-                continue
+        missed = []
-            if tile.shape[0] != TILE_PIXELS or tile.shape[1] != TILE_PIXELS:
+        was_here = {(row, column): bool(pause) and cached(zoom, tx, ty, **kw)
+                    for row, column, tx, ty in todo}
+        for row, column, tx, ty in todo:
+            tile = _one_tile(fetch, zoom, tx, ty, **kw)
+            if tile is None:
+                missed.append((row, column, tx, ty))
                 continue
             canvas[row * TILE_PIXELS:(row + 1) * TILE_PIXELS,
                    column * TILE_PIXELS:(column + 1) * TILE_PIXELS] = tile
-            got += 1
+            covered[row, column] = True
-            if pause:
+            # Asked before the fetch rather than after it, because the
+            # fetch is what puts the tile on the disk.
+            if pause and not was_here[(row, column)]:
                 time.sleep(pause)
-    if not got:
+        todo = missed
-        return None, 0, 0
+    if not covered.any():
-    return canvas, left * TILE_PIXELS, top * TILE_PIXELS
+        return None, 0, 0, covered
+    return canvas, left * TILE_PIXELS, top * TILE_PIXELS, covered
 
 
 # ---------------------------------------------------------------------------
@@ -494,17 +554,23 @@ def ground_under(south: float, west: float, north: float, east: float,
     point of putting a coastline under it is that the coastline is where the
     aircraft was.
 
-    Returns None when nothing could be fetched, which the caller draws as the
+    Returns the levels and whether that is the whole answer.  Nothing at all
-    plain grid it drew before.
+    is None, which the caller draws as the plain grid it drew before; a map
+    with squares missing from it is returned all the same, because most of a
+    map is better than none, but it is flagged as not settled so the caller
+    knows to ask again rather than keeping it for the life of the view.
     """
     if width < 1 or height < 1 or north <= south or east <= west:
-        return None
+        return None, True
     if zoom is None:
         zoom = choose_zoom(south, west, north, east, width=width)
-    tiles, origin_x, origin_y = mosaic(south, west, north, east, zoom,
+    tiles, origin_x, origin_y, covered = mosaic(south, west, north, east,
-                                       fetch=fetch, **kw)
+                                                zoom, fetch=fetch, **kw)
     if tiles is None:
-        return None
+        # Nothing arrived.  Settled on purpose: a machine with no network
+        # must not be made to ask for the same tiles five times a second
+        # for the rest of the night.
+        return None, True
 
     # The edges of each output pixel rather than its middle, so that what
     # lands in it can be averaged.  Taking the nearest source pixel instead
@@ -528,11 +594,32 @@ def ground_under(south: float, west: float, north: float, east: float,
     # brightest thing on the picture.
     whole = (0.299 * tiles[:, :, 0] + 0.587 * tiles[:, :, 1]
              + 0.114 * tiles[:, :, 2]).astype(np.float32)
-    luma = _resample(_resample(whole, ys, axis=0), xs, axis=1)
+
-    low, high = float(luma.min()), float(luma.max())
+    # Averaged over the tiles that are really there rather than over the
+    # canvas.  Both sums come off the same box filter, so dividing one by
+    # the other is the weighted mean over the real pixels -- which gets the
+    # cells along the edge of a hole right as well, those being part tile
+    # and part nothing.
+    real = np.repeat(np.repeat(covered.astype(np.float32), TILE_PIXELS,
+                               axis=0), TILE_PIXELS, axis=1)
+    share = _resample(_resample(real, ys, axis=0), xs, axis=1)
+    luma = _resample(_resample(whole * real, ys, axis=0), xs, axis=1)
+    here = share > 1e-6
+    luma = np.where(here, luma / np.where(here, share, 1.0), 0.0)
+
+    if not here.any():
+        return None, True
+    # The darkest and brightest of what was actually fetched.  A hole left
+    # in the reckoning would put the floor at black, which is darker than
+    # any real tile: the whole map would be drawn dimmer to make room for
+    # a square that is not there.
+    low, high = float(luma[here].min()), float(luma[here].max())
     if high - low < 1.0:
         levels = np.zeros_like(luma)
     else:
         levels = 1.0 - (luma - low) / (high - low)
-    return np.clip((levels * (shades - 1)).round(), 0,
+    # And the hole itself is drawn as bare ground rather than as the
-                   shades - 1).astype(np.uint8)
+    # brightest thing on the picture, which is what inverting black gives.
+    levels = np.where(here, levels, 0.0)
+    return (np.clip((levels * (shades - 1)).round(), 0,
+                    shades - 1).astype(np.uint8), bool(covered.all()))

bandsaunter/flightmap.py

@@ -1854,7 +1854,10 @@ def ground_for(view: Projection, fetch=None, url: str = "") -> tuple:
         extra = {"fetch": fetch} if fetch is not None else {}
         if url:
             extra["url"] = url
-        levels = basemap.ground_under(
+        # A still picture gets one attempt at the tiles -- there is no
+        # window to ask again from -- so whether the answer was complete
+        # changes nothing here.  It already retried the misses itself.
+        levels, _settled = basemap.ground_under(
             view.south, view.west, view.north, view.east,
             view.width, view.height, shades=GROUND_SHADES, **extra)
     except Exception:

bandsaunter/livemap.py

@@ -72,6 +72,17 @@ RESETTLE_NM = 12.0
 # the map settling rather than wandering, a little is enough.
 GROUND_MARGIN = 0.12
 
+# What to do about a map that came back with squares missing from it.  The
+# usual cause is a resize: the window grows, a sharper zoom is chosen, and a
+# hundred tiles that have never been on this disk are asked for at once --
+# whereupon a volunteer-funded server refuses some of them.  The fetch
+# retries its own misses, so anything left is a server asking to be left
+# alone for a bit, and the wait is long enough to be that.  Counted as well
+# as timed because a square can be missing for good, and asking all night
+# for a tile that does not exist is the same discourtesy more slowly.
+GROUND_RETRY_S = 25.0
+GROUND_TRIES = 4
+
 # The widest a line in a box is allowed to get before it is folded.  An
 # airport's full name and the town it is in run to forty characters on their
 # own and a route is two of them, so one flight from Los Angeles to Dallas
@@ -446,6 +457,12 @@ class Sky:
         self._ground_for = None
         self._ground_box = None
         self._ground_serial = 0
+        # Whether the map in hand is the whole of what was asked for, when
+        # it arrived, and how many times it has been asked for since.
+        self._ground_settled = True
+        self._ground_at = 0.0
+        self._ground_tries = 0
+        self._ground_ask = None
         # The aerodromes under the view, fetched with the map and kept the
         # same way: they come from the same place, cover the same box, and
         # go stale at the same moment.
@@ -469,15 +486,26 @@ class Sky:
                     if not trail or trail[-1][:2] != here[:2]:
                         trail.append(here)
 
-    def set_ground(self, levels, key, box=None) -> None:
+    def set_ground(self, levels, key, box=None, settled: bool = True) -> None:
         """Keep the map that was fetched, and the piece of world it covers.
 
         A failed fetch is kept too, as nothing: otherwise a machine with no
         network asks for the same tiles five times a second all night.
+
+        ``settled`` is whether this is the whole map or only most of it.
+        Most of it is still worth drawing -- the alternative is a bare grid
+        -- but it is not worth keeping for the life of the view, so an
+        unsettled map may be asked for again.
         """
         with self._lock:
+            if key != self._ground_for:
+                self._ground_tries = 0
             self._ground, self._ground_for = levels, key
             self._ground_box = box
+            self._ground_settled = bool(settled)
+            self._ground_at = time.time()
+            if not settled:
+                self._ground_tries += 1
             # Counted rather than compared: the drawing side keeps the
             # dimmed pixels it made last time, and needs to know whether
             # what it made them from is still the same map.
@@ -532,11 +560,41 @@ class Sky:
         with self._lock:
             return self._ground_serial
 
+    def ground_settled(self) -> bool:
+        """Whether the map in hand is the whole of what was asked for."""
+        with self._lock:
+            return self._ground_settled
+
     def want_ground(self, key, box, size) -> None:
         """Say which map is needed.  Painting must never wait on a network."""
         with self._lock:
             if self._ground_for != key:
                 self._wanted = (key, box, size)
+                self._ground_ask = (key, box, size)
+
+    def reask_ground(self) -> None:
+        """Ask again for a map that came back with squares missing from it.
+
+        For the map already in hand, under the key it was fetched with,
+        rather than for whatever the view happens to be this frame.  The
+        view drifts a pixel at a time inside the box that was fetched and is
+        answered from it without asking for anything; asking on each of
+        those drifts would be a refetch every frame rather than a retry.
+
+        There was a check here that the remembered request was still the one
+        the map in hand came from.  It could not be made to fail: a request
+        for a different view is only ever taken up after ``_wanted`` has
+        been filled, and a filled ``_wanted`` has already returned above.
+        Code that cannot be made to matter is code that is not doing
+        anything, so it went.
+        """
+        with self._lock:
+            if self._ground_settled or self._wanted is not None:
+                return
+            if self._ground_tries >= GROUND_TRIES:
+                return
+            if time.time() - self._ground_at >= GROUND_RETRY_S:
+                self._wanted = self._ground_ask
 
     def wanted_ground(self):
         with self._lock:
@@ -1012,6 +1070,11 @@ def _build():
                                      (int(view.width * scale),
                                       int(view.height * scale)))
                 return
+            # Most of a map, with squares missing where tiles did not
+            # arrive.  Drawn -- it is most of a map -- and asked for again,
+            # which is rate-limited inside and does nothing at all once the
+            # map is whole.
+            self.sky.reask_ground()
             # Cutting the view out of the fetched map, dimming it and
             # looking every level up in the palette is about seventy
             # milliseconds over two megapixels, and none of it changes
@@ -1701,14 +1764,18 @@ def fetch_ground(sky: Sky, url: str = "", fetch=None) -> None:
             # the screen.  Rendering the wider box into the window's pixels
             # and stretching it back was a whole-map upscale of a fifth,
             # which is what a sharp map looks like when it looks blurred.
-            levels = basemap.ground_under(south, west, north, east,
+            levels, settled = basemap.ground_under(
-                                          width, height,
+                south, west, north, east, width, height,
                 shades=_ground_shades(), **extra)
         except Exception:
-            levels = None
+            levels, settled = None, True
         # Remembered either way: a map that could not be fetched must not be
         # asked for again every fifth of a second for the rest of the night.
-        sky.set_ground(levels, key, box if levels is not None else None)
+        # A map that came back with squares missing is remembered too -- it
+        # is most of a map and it gets drawn -- but not as the last word,
+        # so the window can ask for the rest of it in a moment.
+        sky.set_ground(levels, key, box if levels is not None else None,
+                       settled=settled)
 
 
 def _airports(sky: Sky) -> None:

packaging/bandsaunter.1

@@ -1,5 +1,5 @@
 .\" Generated by packaging/make-man.py -- do not edit by hand.
-.TH BANDSAUNTER 1 "2026-09-21" "bandsaunter 2026-09-21_02" "User Commands"
+.TH BANDSAUNTER 1 "2026-09-21" "bandsaunter 2026-09-21_03" "User Commands"
 .SH NAME
 bandsaunter \- scan, record and identify radio signals with an RTL-SDR
 .SH SYNOPSIS
@@ -1579,6 +1579,25 @@ the window wants. A drawing is capped at a couple of hundred tiles, which at
 3840 by 2160 is reached: there the zoom has stopped climbing and the map is
 enlarged after all, and a smaller radius buys the detail back.
 .PP
+Resizing the window is the demanding case: a wider picture picks a sharper
+zoom and a hundred tiles that have never been on this disk are asked for at
+once, whereupon a busy server refuses some of them. A tile that does not
+arrive leaves its square of the canvas black, and black is not neutral here
+\[em] the brightness is inverted on the way in, so the darkest possible square
+came out as the brightest thing on the picture, dragged the floor of the map's
+own contrast down with it, and stayed there for the life of the view. Instead
+the missing squares are asked for again at once, and only those, the rest
+being on the disk by then; whatever is still missing is drawn as bare ground
+and left out of the reckoning when the darkest and brightest of the map are
+worked out; and the map is kept as provisional rather than as the last word,
+asked for again half a minute later, four attempts in all, each retrying its
+own misses once, so a square gets eight chances before one that will not come
+is accepted as one that is not there.
+.PP
+The politeness pause between requests is paid only on a tile that had to be
+fetched. Paid on every tile, as it had been, it put twenty-six seconds of
+sleeping into redrawing a view whose tiles were all in hand.
+.PP
 .B \-\-no\-basemap
 draws the tracks on their own,
 .BI \-\-tiles " URL"
@@ -2715,6 +2734,20 @@ the range, and
 .B q
 to quit.
 .PP
+The window measures in whatever unit is being shown.
+.B \-\-units " imperial"
+puts statute miles round the rings, along the scale at the bottom and on
+.B \-\-radius
+itself;
+.B \-\-units " metric"
+puts kilometres on all three. The rings are drawn at the distance they are
+labelled \[em] the outermost at
+.B \-\-radius " 100"
+in imperial stands seventy-five statute miles from the flag, measured on the
+ground, rather than seventy-five kilometres with miles written beside it. A
+ring is what a distance gets judged against by eye, so one labelled in a unit
+it was not drawn in is a wrong answer given confidently.
+.PP
 .BI \-\-at " LAT,LON"
 puts the red flag on the map, centres the range rings and gives every station a
 distance and a bearing. Left unset, whatever the aircraft side was told is used
@@ -2861,9 +2894,9 @@ Leave it on for a picture of the network; turn it off to find out what you can a
 .SS The map
 .TP
 .B --radius
-Map reaches \[em] how far around the aerial the window reaches, in kilometres (km).
+Map reaches \[em] how far around the aerial the window reaches, in the unit being shown.
 .br
-Setting name \fBradius\fR, default \fB50 km\fR.
+Setting name \fBradius\fR, default \fB50\fR.
 .br
 Accepts: at least 1, at most 2000.
 .TP

packaging/make-man.py

@@ -1030,6 +1030,25 @@ the window wants. A drawing is capped at a couple of hundred tiles, which at
 3840 by 2160 is reached: there the zoom has stopped climbing and the map is
 enlarged after all, and a smaller radius buys the detail back.
 .PP
+Resizing the window is the demanding case: a wider picture picks a sharper
+zoom and a hundred tiles that have never been on this disk are asked for at
+once, whereupon a busy server refuses some of them. A tile that does not
+arrive leaves its square of the canvas black, and black is not neutral here
+\[em] the brightness is inverted on the way in, so the darkest possible square
+came out as the brightest thing on the picture, dragged the floor of the map's
+own contrast down with it, and stayed there for the life of the view. Instead
+the missing squares are asked for again at once, and only those, the rest
+being on the disk by then; whatever is still missing is drawn as bare ground
+and left out of the reckoning when the darkest and brightest of the map are
+worked out; and the map is kept as provisional rather than as the last word,
+asked for again half a minute later, four attempts in all, each retrying its
+own misses once, so a square gets eight chances before one that will not come
+is accepted as one that is not there.
+.PP
+The politeness pause between requests is paid only on a tile that had to be
+fetched. Paid on every tile, as it had been, it put twenty-six seconds of
+sleeping into redrawing a view whose tiles were all in hand.
+.PP
 .B \-\-no\-basemap
 draws the tracks on their own,
 .BI \-\-tiles " URL"
@@ -1713,6 +1732,20 @@ the range, and
 .B q
 to quit.
 .PP
+The window measures in whatever unit is being shown.
+.B \-\-units " imperial"
+puts statute miles round the rings, along the scale at the bottom and on
+.B \-\-radius
+itself;
+.B \-\-units " metric"
+puts kilometres on all three. The rings are drawn at the distance they are
+labelled \[em] the outermost at
+.B \-\-radius " 100"
+in imperial stands seventy-five statute miles from the flag, measured on the
+ground, rather than seventy-five kilometres with miles written beside it. A
+ring is what a distance gets judged against by eye, so one labelled in a unit
+it was not drawn in is a wrong answer given confidently.
+.PP
 .BI \-\-at " LAT,LON"
 puts the red flag on the map, centres the range rings and gives every station a
 distance and a bearing. Left unset, whatever the aircraft side was told is used

tests/test_aprs.py

@@ -1075,6 +1075,32 @@ def test_the_window_gets_the_position_so_the_flag_is_drawn(tmp_path,
     assert "aircraft" not in built["waiting"]
 
 
+@pytest.mark.parametrize("units, unit, nm", [
+    ("metric", "kph", 100 / 1.852),
+    ("imperial", "mph", 100 / 1.150779),
+])
+def test_the_window_is_built_in_the_unit_being_shown(units, unit, nm,
+                                                     tmp_path, monkeypatch):
+    """Not just the helpers: the window that actually gets opened.  The
+    rings, their labels and the scale along the bottom all read off what is
+    handed over here."""
+    from bandsaunter import livemap
+
+    built = {}
+    monkeypatch.setattr(livemap, "available", lambda: True)
+    monkeypatch.setattr(livemap, "show",
+                        lambda sky, title: built.update(
+                            unit=sky.unit, radius_nm=sky.radius_nm))
+    monkeypatch.setattr(ap, "open_device", lambda console, opts: Silence(1))
+    console = Console(width=120, force_terminal=False)
+    with console.capture():
+        ap.watch(console, ap.AprsOptions(rate=RATE, log=False, report=False,
+                                         basemap=False, units=units,
+                                         radius=100.0), str(tmp_path))
+    assert built["unit"] == unit
+    assert built["radius_nm"] == pytest.approx(nm, rel=1e-6)
+
+
 def test_it_says_where_the_position_came_from_rather_than_assuming(
         tmp_path, monkeypatch):
     """A position inherited from another section is a convenience right up
@@ -1109,6 +1135,74 @@ def test_knowing_nowhere_says_what_that_costs(tmp_path, monkeypatch):
     assert "no flag" in out and "--at" in out
 
 
+def _haversine_km(a, b):
+    """Great-circle distance, written here rather than imported, so that a
+    test of how far the rings are drawn is not asking the drawing."""
+    import math
+
+    lat1, lon1 = math.radians(a[0]), math.radians(a[1])
+    lat2, lon2 = math.radians(b[0]), math.radians(b[1])
+    h = (math.sin((lat2 - lat1) / 2) ** 2
+         + math.cos(lat1) * math.cos(lat2) * math.sin((lon2 - lon1) / 2) ** 2)
+    return 2 * 6371.0088 * math.asin(math.sqrt(h))
+
+
+def _rings_as_drawn(options):
+    """Every ring the window would draw: how far it really is from the flag,
+    in kilometres, and what is written beside it."""
+    from bandsaunter.flightlog import move
+    from bandsaunter.flightmap import ring_labels
+
+    home = (47.6062, -122.3322)
+    out = []
+    for nm, text in ring_labels(ap.radius_in_nm(options),
+                                ap.map_unit(options)):
+        edge = move(home[0], home[1], 0.0, nm)
+        out.append((_haversine_km(home, edge), text))
+    return out
+
+
+@pytest.mark.parametrize("units, label, per_km", [
+    ("metric", "km", 1.0),
+    ("imperial", "mi", 1.609344),
+])
+def test_the_rings_are_drawn_at_the_distance_they_are_labelled(
+        units, label, per_km):
+    """Both halves of it.  Writing miles beside a ring drawn at kilometres
+    is worse than writing nothing: it is a wrong answer given confidently,
+    and the ring is the thing a distance gets judged against by eye."""
+    options = ap.defaults()
+    options.units, options.radius = units, 100.0
+    drawn = _rings_as_drawn(options)
+    assert [text for _km, text in drawn] == [f"25 {label}", f"50 {label}",
+                                             f"75 {label}"]
+    for km, text in drawn:
+        said = float(text.split()[0])
+        assert abs(km - said * per_km) < 0.5, (km, text)
+
+
+def test_the_window_reaches_as_far_as_it_was_told_in_the_unit_being_shown():
+    """A hundred set in imperial is a hundred miles, not a hundred
+    kilometres with miles written on it."""
+    metric, imperial = ap.defaults(), ap.defaults()
+    metric.units, metric.radius = "metric", 100.0
+    imperial.units, imperial.radius = "imperial", 100.0
+    assert ap.radius_in_nm(metric) == pytest.approx(100 / 1.852, rel=1e-6)
+    assert ap.radius_in_nm(imperial) == pytest.approx(100 / 1.150779,
+                                                        rel=1e-6)
+    assert ap.radius_in_nm(imperial) > ap.radius_in_nm(metric)
+
+
+def test_the_unit_the_window_measures_in_follows_the_one_being_shown():
+    """The scale along the bottom and the rings read off the same setting
+    as the tables, so the window cannot disagree with the report."""
+    options = ap.defaults()
+    options.units = "metric"
+    assert ap.map_unit(options) == "kph"
+    options.units = "imperial"
+    assert ap.map_unit(options) == "mph"
+
+
 def test_the_box_gives_the_position_in_figures():
     """A mark shows where a station is; a number is what gets written down."""
     rows = dict((label, value) for label, value, _f

tests/test_basemap.py

@@ -215,8 +215,9 @@ def counting_fetcher(answers=None, fail_on=()):
 
 def test_the_tiles_are_stitched_in_the_right_places():
     fetch = counting_fetcher()
-    raster, ox, oy = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9, fetch=fetch,
+    raster, ox, oy, covered = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9,
-                               pause=0)
+                                        fetch=fetch, pause=0)
+    assert covered.all()
     assert raster is not None
     assert raster.shape[2] == 3
     assert raster.shape[0] % bm.TILE_PIXELS == 0
@@ -233,28 +234,33 @@ def test_a_tile_that_does_not_arrive_leaves_a_hole_rather_than_an_error():
     seen = counting_fetcher()
     bm.mosaic(47.0, -122.8, 48.5, -121.0, 9, fetch=seen, pause=0)
     missing = seen.asked[:1]
-    raster, _, _ = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9,
+    raster, _, _, covered = bm.mosaic(
-                             fetch=counting_fetcher(fail_on=missing), pause=0)
+        47.0, -122.8, 48.5, -121.0, 9,
+        fetch=counting_fetcher(fail_on=missing), pause=0, retry_pause=0)
     assert raster is not None
     assert not raster[:bm.TILE_PIXELS, :bm.TILE_PIXELS].any()   # the hole
+    # And the hole is reported rather than left to be mistaken for map.
+    assert not covered[0, 0] and covered.sum() == covered.size - 1
 
 
 def test_nothing_at_all_gives_no_map():
     def nothing(z, x, y, **kw):
         return None
 
-    raster, _, _ = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9, fetch=nothing,
+    raster, _, _, covered = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9,
-                             pause=0)
+                                      fetch=nothing, pause=0, retry_pause=0)
     assert raster is None
+    assert not covered.any()
 
 
 def test_a_corrupt_tile_is_skipped():
     def rubbish(z, x, y, **kw):
         return b"not a png"
 
-    raster, _, _ = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9, fetch=rubbish,
+    raster, _, _, covered = bm.mosaic(47.0, -122.8, 48.5, -121.0, 9,
-                             pause=0)
+                                      fetch=rubbish, pause=0, retry_pause=0)
     assert raster is None
+    assert not covered.any()
 
 
 # ---------------------------------------------------------------------------
@@ -269,14 +275,197 @@ def gradient_tile(z, x, y, **kw):
 
 
 def test_the_ground_comes_back_as_levels_the_map_can_paint():
-    levels = bm.ground_under(47.0, -122.8, 48.5, -121.0, 200, 150,
+    levels, _settled = bm.ground_under(47.0, -122.8, 48.5, -121.0, 200, 150,
-                             shades=32, fetch=gradient_tile, pause=0)
+                                       shades=32, fetch=gradient_tile,
+                                       pause=0)
     assert levels is not None
     assert levels.shape == (150, 200)
     assert levels.dtype == np.uint8
     assert levels.max() <= 31
 
 
+def _patchy(fail_on, note=None):
+    """A tile server that refuses some squares.  What a busy one does."""
+    def fetch(z, x, y, **kw):
+        if note is not None:
+            note.append((z, x, y))
+        if (x, y) in fail_on:
+            return None
+        # Something to see, and different in each tile, so that a hole is
+        # distinguishable from map rather than from a flat field.
+        return solid(40 if (x + y) % 3 == 0 else 230)
+
+    return fetch
+
+
+BOX = (47.0, -123.0, 48.0, -122.0)
+
+
+def _hole_at():
+    """The grid position of one tile inside BOX, worked out here rather
+    than asked of the code whose arithmetic is under test elsewhere."""
+    zoom = bm.choose_zoom(*BOX, width=400)
+    x0, y0 = bm.tile_of(BOX[2], BOX[1], zoom)
+    return zoom, (int(x0) + 1, int(y0) + 1)
+
+
+def _hole_mask(hole, zoom, width=400, height=400, grow=0):
+    """Which output pixels the missing tile covers.
+
+    Worked out from the tile's own corners rather than from the picture, so
+    that a test of what is drawn in the gap is not asking the code that drew
+    it where the gap is.  ``grow`` widens it, for the band along the edge
+    where a cell is averaged over part of a tile and part of nothing.
+    """
+    south, west, north, east = tile_bounds(zoom, hole[0], hole[1])
+    top = (BOX[2] - min(north, BOX[2])) / (BOX[2] - BOX[0]) * height
+    bottom = (BOX[2] - max(south, BOX[0])) / (BOX[2] - BOX[0]) * height
+    left = (max(west, BOX[1]) - BOX[1]) / (BOX[3] - BOX[1]) * width
+    right = (min(east, BOX[3]) - BOX[1]) / (BOX[3] - BOX[1]) * width
+    mask = np.zeros((height, width), dtype=bool)
+    mask[max(0, int(top) - grow):int(bottom) + 1 + grow,
+         max(0, int(left) - grow):int(right) + 1 + grow] = True
+    return mask
+
+
+def test_a_missing_tile_is_a_gap_rather_than_the_brightest_thing_on_the_map():
+    """The canvas starts black and the brightness is inverted further down,
+    so a square that never arrived used to come out as the brightest thing
+    on the picture: a glowing rectangle where the map should be."""
+    zoom, hole = _hole_at()
+    whole, _ = bm.ground_under(*BOX, 400, 400, fetch=_patchy(()), pause=0,
+                               zoom=zoom)
+    holed, _ = bm.ground_under(*BOX, 400, 400, fetch=_patchy({hole}),
+                               pause=0, retry_pause=0, zoom=zoom)
+    assert whole is not None and holed is not None
+    gap = _hole_mask(hole, zoom)
+    assert gap.sum() > 1000, "the hole is not where this test thinks it is"
+    # There was map there, and now there is nothing -- rather than the
+    # brightest shade the picture has.
+    assert whole[gap].max() > 0
+    assert holed[_hole_mask(hole, zoom, grow=-1)].max() == 0
+    # And what leaks in is a hairline round the edge, where a cell is
+    # averaged over part of a tile and part of nothing, rather than a
+    # fraction of the hole: the averaging weighs the real pixels only.
+    assert int((holed[gap] > 0).sum()) < gap.sum() // 100
+    assert holed.max() == whole.max(), "the map lost its brightest shade"
+
+
+def test_a_missing_tile_does_not_dim_the_rest_of_the_map():
+    """Black is darker than any real tile, so counting a hole when working
+    out the darkest and brightest of what was fetched drags the floor down,
+    and every other pixel is drawn dimmer to make room for a square that is
+    not there."""
+    zoom, hole = _hole_at()
+    whole, _ = bm.ground_under(*BOX, 400, 400, fetch=_patchy(()), pause=0,
+                               zoom=zoom)
+    holed, _ = bm.ground_under(*BOX, 400, 400, fetch=_patchy({hole}),
+                               pause=0, retry_pause=0, zoom=zoom)
+    # Outside the gap and the averaged band along its edge, the map is the
+    # map: the hole took nothing else with it.
+    elsewhere = ~_hole_mask(hole, zoom, grow=2)
+    assert elsewhere.sum() > holed.size // 2
+    assert bool((holed[elsewhere] == whole[elsewhere]).all())
+
+
+def test_a_map_with_squares_missing_says_it_is_not_the_whole_answer():
+    """Most of a map is worth drawing and is not worth keeping: the caller
+    has to be able to tell the two cases apart to know whether to ask
+    again."""
+    _zoom, hole = _hole_at()
+    whole, settled = bm.ground_under(*BOX, 400, 400, fetch=_patchy(()),
+                                     pause=0)
+    assert whole is not None and settled
+    holed, settled = bm.ground_under(*BOX, 400, 400, fetch=_patchy({hole}),
+                                     pause=0, retry_pause=0)
+    assert holed is not None and not settled
+
+
+def test_a_tile_that_fails_once_is_asked_for_again():
+    """The usual reason for a hole is that a hundred tiles were asked for
+    in the preceding second, which is a server asking to be slowed down
+    rather than a tile that is not there."""
+    _zoom, hole = _hole_at()
+    refused = {"left": 1}
+
+    def flaky(z, x, y, **kw):
+        if (x, y) == hole and refused["left"]:
+            refused["left"] -= 1
+            return None
+        return solid(40 if (x + y) % 3 == 0 else 230)
+
+    whole, _ = bm.ground_under(*BOX, 400, 400, fetch=_patchy(()), pause=0)
+    healed, settled = bm.ground_under(*BOX, 400, 400, fetch=flaky, pause=0,
+                                      retry_pause=0)
+    assert refused["left"] == 0, "the tile was never asked for a second time"
+    assert settled, "a recovered map is the whole answer"
+    assert bool((healed == whole).all())
+
+
+def test_only_the_tiles_that_failed_are_asked_for_again():
+    """A retry that fetched the lot again would treat a busy server by
+    asking it for everything twice."""
+    _zoom, hole = _hole_at()
+    asked = []
+    bm.ground_under(*BOX, 400, 400, fetch=_patchy({hole}, note=asked),
+                    pause=0, retry_pause=0)
+    twice = [where for where in set(asked) if asked.count(where) > 1]
+    assert len(twice) == 1 and twice[0][1:] == hole
+
+
+def test_retrying_can_be_turned_off():
+    _zoom, hole = _hole_at()
+    asked = []
+    bm.ground_under(*BOX, 400, 400, fetch=_patchy({hole}, note=asked),
+                    pause=0, retries=0)
+    assert len(asked) == len(set(asked))
+
+
+def test_only_a_tile_that_had_to_be_fetched_costs_politeness(monkeypatch):
+    """The pause is an apology to a volunteer-funded server, and a file
+    that was already on the disk was never asked of it.  Paying it anyway
+    is how a view whose tiles are all in hand takes half a minute to
+    redraw -- and how asking again for three missing squares costs the
+    wait for two hundred that are not missing."""
+    slept = []
+    monkeypatch.setattr(bm.time, "sleep", lambda s: slept.append(s))
+    zoom, _hole = _hole_at()
+
+    _r, _x, _y, covered = bm.mosaic(*BOX, zoom, fetch=_patchy(()),
+                                    pause=0.05,
+                                    cached=lambda z, x, y, **kw: False)
+    assert len(slept) == int(covered.sum()) > 1
+
+    slept.clear()
+    bm.mosaic(*BOX, zoom, fetch=_patchy(()), pause=0.05,
+              cached=lambda z, x, y, **kw: True)
+    assert slept == []
+
+
+def test_asking_again_for_what_is_missing_does_not_wait_for_what_is_not(
+        monkeypatch):
+    """Which is what makes a second ask affordable at all: the tiles that
+    arrived the first time are on the disk, so the only thing paid for is
+    the handful that did not."""
+    slept = []
+    monkeypatch.setattr(bm.time, "sleep", lambda s: slept.append(s))
+    zoom, hole = _hole_at()
+    # Everything arrived but the one square, so everything but that square
+    # is now on the disk.
+    bm.mosaic(*BOX, zoom, fetch=_patchy({hole}), pause=0.05, retries=0,
+              cached=lambda z, x, y, **kw: (x, y) != hole)
+    assert len(slept) <= 1, slept
+
+
+def test_a_tile_is_on_the_disk_once_it_has_been_fetched(tmp_path):
+    zoom, hole = _hole_at()
+    assert not bm.on_disk(zoom, hole[0], hole[1], cache=tmp_path)
+    where = tmp_path / str(zoom) / str(hole[0])
+    where.mkdir(parents=True)
+    (where / f"{hole[1]}.png").write_bytes(solid(200))
+    assert bm.on_disk(zoom, hole[0], hole[1], cache=tmp_path)
+
+
 def tile_bounds(zoom: int, x: int, y: int):
     """The corners of one tile, from the inverse of the standard formula.
 
@@ -298,7 +487,8 @@ def test_the_map_is_inverted_so_that_ink_shows_on_a_dark_picture():
     """A printed map is dark ink on white paper; this picture is the other
     way round, so the dark parts of a tile are the bright parts here."""
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 64, 64, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 64, 64,
+                                       shades=32,
                              fetch=gradient_tile, zoom=9, pause=0)
     # The tile is black at the top and white at the bottom, so the picture
     # has to be bright at the top and dark at the bottom.
@@ -313,7 +503,8 @@ def test_north_is_at_the_top():
         return make_png(px)
 
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 40, 40, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 40, 40,
+                                       shades=32,
                              fetch=half_and_half, zoom=9, pause=0)
     assert levels[0].mean() < levels[-1].mean()   # white inverts to dark
 
@@ -325,7 +516,8 @@ def test_east_is_to_the_right():
         return make_png(px)
 
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 40, 40, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 40, 40,
+                                       shades=32,
                              fetch=half_and_half, zoom=9, pause=0)
     assert levels[:, 0].mean() < levels[:, -1].mean()
 
@@ -338,7 +530,8 @@ def test_one_tile_covers_its_own_box_exactly():
         return make_png(px)
 
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 128, 128, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 128, 128,
+                                       shades=32,
                              fetch=corner_marks, zoom=9, pause=0)
     dark = levels < levels.max() / 2              # the white corner, inverted
     assert dark[:4, :4].all()
@@ -349,13 +542,18 @@ def test_no_tiles_means_no_ground_rather_than_an_exception():
     def nothing(z, x, y, **kw):
         return None
 
-    assert bm.ground_under(47.0, -122.8, 48.5, -121.0, 50, 50,
+    levels, settled = bm.ground_under(47.0, -122.8, 48.5, -121.0, 50, 50,
-                           fetch=nothing, pause=0) is None
+                                      fetch=nothing, pause=0, retry_pause=0)
+    assert levels is None
+    # Settled on purpose: a machine with no network must not spend the
+    # night asking for tiles it is never going to be given.
+    assert settled
 
 
 def test_a_box_that_makes_no_sense_is_refused_quietly():
-    assert bm.ground_under(48.0, -122.0, 47.0, -123.0, 50, 50,
+    levels, settled = bm.ground_under(48.0, -122.0, 47.0, -123.0, 50, 50,
-                           fetch=gradient_tile, pause=0) is None
+                                      fetch=gradient_tile, pause=0)
+    assert levels is None and settled
 
 
 # ---------------------------------------------------------------------------
@@ -655,7 +853,8 @@ def test_a_cell_smaller_than_a_source_pixel_takes_that_pixel():
 
 def test_a_gradient_still_comes_out_as_a_gradient():
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 64, 64, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 64, 64,
+                                       shades=32,
                              fetch=gradient_tile, zoom=9, pause=0)
     rows = levels.mean(axis=1)
     assert (np.diff(rows) <= 0.51).all(), "the gradient came out lumpy"
@@ -664,8 +863,9 @@ def test_a_gradient_still_comes_out_as_a_gradient():
 def test_averaging_is_the_same_shape_as_the_picture_asked_for():
     south, west, north, east = tile_bounds(9, 81, 178)
     for width, height in ((40, 40), (137, 91), (300, 200), (17, 5)):
-        levels = bm.ground_under(south, west, north, east, width, height,
+        levels, _settled = bm.ground_under(south, west, north, east,
-                                 shades=32, fetch=gradient_tile, zoom=9,
+                                           width, height, shades=32,
+                                           fetch=gradient_tile, zoom=9,
                                            pause=0)
         assert levels.shape == (height, width)
 
@@ -673,7 +873,8 @@ def test_averaging_is_the_same_shape_as_the_picture_asked_for():
 def test_a_map_asked_for_at_more_detail_than_the_tiles_hold_still_works():
     """Zoomed in past the tiles, a cell covers less than one source pixel."""
     south, west, north, east = tile_bounds(9, 81, 178)
-    levels = bm.ground_under(south, west, north, east, 2000, 2000, shades=32,
+    levels, _settled = bm.ground_under(south, west, north, east, 2000, 2000,
+                                       shades=32,
                              fetch=gradient_tile, zoom=9, pause=0)
     assert levels.shape == (2000, 2000)
     assert levels[0].mean() != levels[-1].mean()

tests/test_livemap.py

@@ -363,6 +363,123 @@ def test_a_map_that_could_not_be_fetched_is_not_asked_for_again():
     assert sky.wanted_ground() is None
 
 
+def test_a_map_with_squares_missing_is_asked_for_again():
+    """A resize picks a sharper zoom and asks for a hundred tiles that have
+    never been on this disk at once, and a busy server refuses some of
+    them.  Most of a map gets drawn; it does not get kept."""
+    from bandsaunter.livemap import GROUND_RETRY_S
+
+    sky = a_sky()
+    sky.want_ground("a", (0, 0, 1, 1), (10, 10))
+    sky.set_ground(np.zeros((40, 40), dtype=np.uint8), "a", (0, 0, 1, 1),
+                   settled=False)
+    assert sky.wanted_ground() is None, "not before the server has a rest"
+    assert not sky.ground_settled()
+    sky.reask_ground()
+    assert sky.wanted_ground() is None, "still too soon"
+    sky._ground_at -= GROUND_RETRY_S + 1
+    sky.reask_ground()
+    # Asked for again, and for the map it already asked for rather than for
+    # whatever the view has drifted to since.
+    assert sky.wanted_ground() == ("a", (0, 0, 1, 1), (10, 10))
+
+
+def test_a_whole_map_is_never_asked_for_again():
+    sky = a_sky()
+    sky.want_ground("a", (0, 0, 1, 1), (10, 10))
+    sky.set_ground(np.zeros((40, 40), dtype=np.uint8), "a", (0, 0, 1, 1))
+    assert sky.ground_settled()
+    sky._ground_at -= 10_000.0
+    sky.reask_ground()
+    assert sky.wanted_ground() is None
+
+
+def test_a_square_that_is_missing_for_good_is_not_asked_for_all_night():
+    """A tile can be absent because there is no such tile.  Asking for it
+    every half minute until morning is the same discourtesy more slowly."""
+    from bandsaunter.livemap import GROUND_RETRY_S, GROUND_TRIES
+
+    sky = a_sky()
+    sky.want_ground("a", (0, 0, 1, 1), (10, 10))
+    asks = 0
+    for _ in range(GROUND_TRIES + 5):
+        sky.set_ground(np.zeros((40, 40), dtype=np.uint8), "a", (0, 0, 1, 1),
+                       settled=False)
+        sky._ground_at -= GROUND_RETRY_S + 1
+        sky.reask_ground()
+        if sky.wanted_ground() is not None:
+            asks += 1
+            sky._wanted = None
+    assert asks == GROUND_TRIES - 1, asks
+
+
+def test_moving_the_view_starts_the_asking_over():
+    """The count is against one view's tiles.  A window that was resized
+    twice has not used up its patience on the second view."""
+    from bandsaunter.livemap import GROUND_RETRY_S, GROUND_TRIES
+
+    sky = a_sky()
+    for _ in range(GROUND_TRIES + 2):
+        sky.set_ground(np.zeros((40, 40), dtype=np.uint8), "a", (0, 0, 1, 1),
+                       settled=False)
+    sky.want_ground("b", (0, 0, 2, 2), (20, 20))
+    sky.set_ground(np.zeros((40, 40), dtype=np.uint8), "b", (0, 0, 2, 2),
+                   settled=False)
+    sky._wanted = None
+    sky._ground_at -= GROUND_RETRY_S + 1
+    sky.reask_ground()
+    assert sky.wanted_ground() is not None
+
+
+@qt
+def test_drawing_most_of_a_map_draws_it_and_asks_for_the_rest(app):
+    """Both halves: the window does not go bare while it waits, and it does
+    not settle for the map it was given."""
+    from bandsaunter.livemap import GROUND_RETRY_S, SkyView
+
+    sky = a_sky(a_blip())
+    view = SkyView(sky)
+    view.resize(900, 650)
+    view._draw_ground(_NoPainter(), view.projection())      # asks
+    wanted = sky.wanted_ground()
+    assert wanted is not None
+    key, box, _size = wanted
+    sky.set_ground(np.full((760, 1050), 12, dtype=np.uint8), key, box,
+                   settled=False)
+
+    drawn = []
+
+    class _Painter(_NoPainter):
+        def drawImage(self, *a):
+            drawn.append(a)
+
+    sky._ground_at -= GROUND_RETRY_S + 1
+    view._draw_ground(_Painter(), view.projection())
+    assert drawn, "most of a map is still worth drawing"
+    assert sky.wanted_ground() is not None, "and worth finishing"
+
+
+@qt
+def test_drawing_a_whole_map_asks_for_nothing(app):
+    from bandsaunter.livemap import SkyView
+
+    sky = a_sky(a_blip())
+    view = SkyView(sky)
+    view.resize(900, 650)
+    view._draw_ground(_NoPainter(), view.projection())
+    key, box, _size = sky.wanted_ground()
+    sky.set_ground(np.full((760, 1050), 12, dtype=np.uint8), key, box)
+    sky._wanted = None
+
+    class _Painter(_NoPainter):
+        def drawImage(self, *a):
+            pass
+
+    sky._ground_at -= 10_000.0
+    view._draw_ground(_Painter(), view.projection())
+    assert sky.wanted_ground() is None
+
+
 # ---------------------------------------------------------------------------
 # The map staying still
 # ---------------------------------------------------------------------------