.MP — map files

One ships with the game: AMER2.MP, 12,534 bytes. colwin reads it — colwin map-preview draws it as a PNG, see map preview — and does not write it. This page records the layout, which is solved.

WORD  width          58
WORD  height         72
WORD  UNKNOWN        4
BYTE  terrain[w*h]   plane 0
BYTE  plane1[w*h]    plane 1
BYTE  plane2[w*h]    plane 2

Every byte is accounted for: 6 + 3 × 58 × 72 = 12,534, exactly the file size.

Planar, not three bytes per tile

The evidence is plane 1: it is uniformly zero across all 4,176 tiles. Reading the data as interleaved gives three lanes that all look alike (71, 65 and 76 distinct values, similar histograms); reading it as planes gives one plane with 87 values, one with exactly one value, and one with 15. No interleaving of three per-tile fields produces a lane that is entirely zero.

The planes

Plane 0 — terrain, and the byte is a bitfield rather than an id: the low five bits are a terrain id when bit 0x20 is clear, bit 0x20 means hills and with bit 0x80 mountains, bit 0x40 a river and with 0x80 a major one. That is terrain_from_map_byte (1040:42b5), and it is what turns the 87 distinct values into the 29 terrains of the game’s own TERRAIN0..TERRAIN28 table. The two commonest values are 25 (2,109 tiles) and 26 (810) — ocean and sea lane — together just under half the map. Drawn through the game’s own tile art the plane is the Americas: North, Central and South America, the Caribbean chain, Florida, the Gulf of Mexico, Baja California, with the Rockies and the Andes where they belong.

Plane 1 — all zero. Purpose UNKNOWN. It is not padding: it sits between two live planes. A second shipped file would settle whether it is always zero, and there is only one.

Plane 2 — correlated with land and water, not established as either.

1 tracks water and 2 tracks land, but roughly 250 tiles disagree and the values 3, 5, 6 and 7 are unexplained.

Cross-check

AUTO01.SAV carries 3a 00 48 00 — 58 × 72 — at offset 0x0c: the same dimensions, in a different file, written by a different routine.