DEM File structure

Today, we know a lot about the pointers and variables in a DEM subfile (check this WIKI) , but not about the actual hill shading bit located in DEM2.

For additional findings see Exploring DEM. Document is updated regularly.

Incorporating DEM subfiles into your IMG

DEM Files operate independently of other subfiles.

As long as

• the FID and PID in all your subfiles are the same
• the area of your img is larger than that of the DEM
• there are at least 4 zoom levels in your main IMG

you can attach your DEM subfile to the end of your IMG eventhough some of the blocks provide unexpected results.

Create an IMG using say mkgmap with name: 12345678.img. Use TYPWiz to give your map colour and create a TYP file. Load TOPO IMG covering the same area into GMaptool ,use the Split option and select subfiles. Click Split All Rename DEM subfile as 12345678.dem Add both 12345678.IMG and 12345678.DEM file to Gmaptool Use Join option & tick set mapset FID; enter say FID:4664 PID:1 Next to Output File: give the path and name of detination file , ie c:\87654321.img Click Join All Check existence of this new file Import this file and TYP file into Mapsource using MapsetToolkit When exporting to your gps you may have to enable a 'Shaded Relief" option.
DEM Investigator
A GUI that illustrates base heights for each tile ; you can alter data found in DEM1 DEM2 and DEM3