Measured on the cartridge, not derived: a map tile is 2x2 screen tiles and
CheckTilePassable matches one id, and the screen agrees with the lower-left tile
of the quadrant. Viridian Forest s (4, 32) reads 3, the second row of its
block, where the first row holds bash4. On open ground most quadrants hold one id
four times over, so the upper-left guess reads correctly on a town and falls
apart in a forest -- which is why the decode is cross-checked against the screen
before it is trusted.
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