Idea: human brains are good at pattern matching and figuring out where
things are on a very simple map. Use GPS to provide basic data, and
allow quick panning and zooming on that date, to "feel" the geometry
of a place.

GUI principle:

Operation is tap or touch-drag:
- touch Zout, drag down = zoom out (using logarithmic scale)
- touch Zin, drag up = zoom in
- touch Pan, drag anywhere = pan
- touch Rotate, drag = rotate by relative angle
- touch Longer, drag right = increase tail length
- touch Shorter, drag left = decrease tail length
- tap Time/Distance = switch tail unit of measure between time and distance
  (switch preserves on-screen tail length, so we can use this to measure
  speed)
- tap Menu = bring up menu
- dragging into a touch/tap zone has no effect, only touch/tap has

Indicators:
- Scale = indicates scale of trace drawn. NNN for meters, NNNk for
  kilometers
- Time/Distance = total length of tail shown
- Compass = rotation of canvas

Menu:
- Reset = center on current position, north is up, scale = 100m
- Exit

Trail shown:
- current position if fat dot, maybe in different color than rest
- last GPS fixes of specified length as drawn as line
- last segment is interpolated (i.e., the tail length shown is exact)

Layering:
- touch zones (central circles, compass, zoom and tail length areas
  and start ellipses) are at bottom
- tail (uses entire screen)
- current position (can be on entire screen)
- indicators

Interaction:
- updates must be instantaneous (we we can't do this, simplify
  further, e.g., by only showing trail head and end)
- all indicators and the trail are updated on all actions
- trail is updated in real time, from GPS
- canvas does not move when current position changes (maybe make this
  an option)
- therefore, we keep the interface very very simple
- when dragging, allow some departure from area (e.g, Zin-drag does
  zoom, even if drifting out of the zoom area)
- dragging works at pixel accuracy, e.g., zoom is smooth
- full-screen application, to maximize information

Future extension (this is where it gets sexy):
- add ability to show trail of other person, received via GPRS or
  (with manual trigger, because it's expensive) SMS
- likewise, send own position to other person