You mean the TILE, ISOM, and MTXM sections of the chk, or the tileset data in broodat.mpq and stardat.mpq?
The format of the TILE and MTXM sections are identical, as the CHK specs will tell you. It is simply MapWidth * MapHeight unsigned integers (2 bytes) that reference a certain tile from a tilegroup.
I do not know very much about the ISOM section, except that all 0 will make staredit create rectangular terrain, and filling it with a certain value like 0x4000, 0x6000, etc. will make staredit think there is a certain solid terrain type.
I'll write up info for the tileset files soon...I'll have to go back and look at my code again for a refresher, it's been so long!