Hi Ticker, To much to do with to little time to spend. I can't find anything that gives a hint of the extra byte in some TRE S7 rows. TRE-S7 are offset columns UInt32 | UInt32 |UInt32 | Byte | UInt32 columns ------------------------------------------------------- Polygon | Line | Point | Flag byte | Col1 | Col2 | Coln The smallest map you can produce is one area, like a lake, or other land. That gives Polygon- and Col1- column mandatory. Line, Points, Flag byte are all optional with respect to small maps. In well known sources like GPXSee, RGNDisplay and similar, all of them skip the bytes (of my interest). We know how to skip the extra bytes. The amount of extra columns follow map-size and complexity. Smallest map you can produce have 9 bytes per row. (What I have seen.) Polygon | Flag Byte | Col1 If we focus on Col1 .. Coln They are incremental by column. Example with 3 rows and 3 columns: Last row contain "Section sizes", 0x0048 is the size of RGN Section 6. 0x0000 | 0x0002 | 0x0015 0x0001 | 0x0008 | 0x0018 0x0001 | 0x0013 | 0x0032 0x0001 | 0x0013 | 0x0048 Sorry for any mis-postings done. BR. /Thomas