User:MPC/GMP Subfile Format
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A reverse-engineering of Garmin's GMP Subfile format
GMP Header
Common Header
The structure of the common header is the same in all subfiles.
| Byte Offset | Length (bytes) | Description |
|---|---|---|
| 0x000 | 2 | Header length. So far only '0x31' and '0x35' has been seen. |
| 0x002 | 10 | Type GARMIN GMP |
| 0x00C | 1 | 0x01 ? |
| 0x00D | 1 | 0x00 |
| 0x00E | 2 | Creation year |
| 0x010 | 1 | Creation month |
| 0x011 | 1 | Creation day |
| 0x012 | 1 | Creation hour |
| 0x013 | 1 | Creation minute |
| 0x014 | 1 | Creation second |
Specific GMP Header
| Byte Offset | Length (bytes) | Description |
|---|---|---|
| 0x015 | 4 | Offset?? Only seen 0x0000 in here so far |
| 0x019 | 4 | Absolute offset of start of .TRE header |
| 0x01D | 4 | Absolute offset of start of .RGN header |
| 0x021 | 4 | Absolute offset of start of .LBL header |
| 0x025 | 4 | Absolute offset of start of .NET header |
| 0x029 | 4 | Absolute offset of start of .NOD header |
| 0x02D | 4 | Absolute offset of start of .DEM header |
| 0x031 | 4 | Absolute offset of start of .MAR header (Bluechart G2) |
Copyright Message Section
| Byte Offset | Length (bytes) | Description |
|---|---|---|
| after GMP header | ?? | Copyright messages. This is a section of ASCII text, null-terminated. There may be several null-terminated sentences present here. The copyright messages fill the .GMP file up to the start of the first subfile present. |
The offsets
Each offset present in the GMP Header is the absolute position in the .GMP file where you'd expect to find the first byte of the Common Garmin Header for that subfile. The Common Header of each subfile tells you exactly what type of file it is part of, though the offsets in the GMP Header seem always to be in the same order.
No idea what the offset at 0x0015 might indicate.
Presumably, any of the offsets could be zero if the .GMP file doesn't have that sort of subfile amongst its contents?