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Why add a DEM (digial elvevation model) file? It will show you elevation at any spot on your map, and it enables drawing an elevation profile (distance vs. elevation) in your GPS, of an active route, and in Mapsource. Nice for topo maps.
The secret is to get Microdem, at
http://www.usna.edu/Users/oceano ... m/microdemdown.htm. It will open and convert geotif files and DTED DEM (.dt1) files into the .bil dem format required by Garmin. I tried 3DEM, Quantum GIS, GRASS, the ERDAS ER Viewer, and none of those did the trick. Ironically, Microdem doesn't convert a .bil file to a .bil file, even if it's been converted to another format, e.g. .tif. One quirk about Microdem: if you open it as MODIS or QuickView, it will reopen in that program, even if you select Microdem shortcut. You must go to Options, and in the Menu box, select Regular, then OK...it will open next time with Microdem. It's easy then: just select File, Open, Open DEM, browse to find the DEM file you want; once it's loaded, select File, Save DEM, then BIL. It will make the files in the Garmin BIL format.
Where to get the DEM files? The ASTER data is very good, it is available at
https://wist.echo.nasa.gov/~wist/api/imswelcome/ and there is a nice tutorial there how to do it. It provides a DEM geotif file, about 25 MB for a 1 degree tile. It's easiest to select the latitude and longitude in the 4 corners.
You can also get the SRTM data at
http://edcsns17.cr.usgs.gov/NewEarthExplorer/ You can download as .bil or DTED. The bil file is not in Garmin format: it is in decimal seconds (not decimal degrees), and not in double precision digits, and Microdem won't "convert" it. It's better to download as DTED, and then use Microdem to convert to .bil. This data has a lot of missing cell values, and is not as detailed at the ASTER data. It will be about a 2.8 MB file for a 1 degree area. Again, here, easiest to enter location in the box "Area Selected." The results will bring up an Elevation option. Select that and you'll get the files. There is also the National Elevation Dataset (NED) at
http://seamless.usgs.gov/ if you're just looking at basically the United States. I haven't used that yet.
In the Mapsource Creator Product (MPC) program, you'll need to load a shape file along with the .bil file (see the tutorial in the MPC program). You can get shapefiles at
http://geonames.nga.mil/ggmagaz/ Select Output in the upper left, then, in Output Instructions, select Shapefile. Select an area under regions. There is a lot of good data here you could use in GPSMapEdit. Alternately, make an .mp file with GPSMapEdit, only an area polygon of the size you want your finished map to be. In the label field, put in a Garmin area type, found in the help file in MPC, e.g. GOLF_COURSE. You can experiment with different names. I haven't tried DATA_BOUNDS yet, but that may crop the map to an irregular (not square) polygon, which would be good. Save the .mp file, then copy cgpsmapper.exe (only $40 now for routable...I think we owe that to Stan), and the sendg.dll file in the cgpsmapper folder, to the folder where the .mp file is. Run cgpsmapper in cmd, that is, go to Start, Run, type in cmd, then hit enter. Type in CD
This stands for change directory. After CD, type in the path to the .mp file, e.g.
C:\Documents andSettings\Owner\Desktop\Garmin Shape Then, hit Enter. Then, type in cgpsmapper shp nameofyour mpfile.mp and hit enter. It will spit out a ton of shape files in the folder. You can delete all of them except the polygon.shp, polygon.shx and polygon.dbf files. In the .dbf file, I just opened it with Excell, and changed the Label heading to GRMN_TYPE
In that column should be a Garmin Area type, the GOLF_COURSE or whatever. There are .dbf editors if you're doing anything complicated. Put NAME in the first column. Doesn't matter what the name is. It will be some number, e.g., 24, that GPSMapper uses, I think, to define its type. Save the .dbf. Now, it's ready to use with MPC, along with the BIL file you made. Follow tutorial in MPC. Make sure to check allow DEM export.
Once you made an .img, the DEM is embedded in it, cropped to size. If you want to extract the DEM, you must first load it in Mapsource, using MapSetToolKit, or GmapTool. Get Map Converter, from Garmin, at
http://www8.garmin.com/support/download_details.jsp?id=3897
Run MapConverter (select only the mapset with the .img you want to "disect,") and download to a convenient folder. Dig into the .gmapi Folder, and you'll find the Garmin DEM you want. Now you can add this DEM to an .img you have made through GPSMapEdit. You need to make sure you change the name of the DEM file to the INTERNAL name of your .img. Then you will join the two files with GmapTool (from
http://www.anpo.republika.pl/download.html#gmaptool). Load your .img file, and the DEM file that you've renamed, go to JOIN, select an Output file (use an 8 digit number for .img name), create a Mapset name, set a Mapset FID (any arbitrary number, less than 6 digits, I think?), and PID as 1. Check "Don't create a MPS subfile" and "Include other data." Then hit the "join all" button. This will create a .img with the Garmin DEM embedded in it. You can then load the .img in Mapsource with the GmapTool kit. I think this is preferable than MapSetToolKit, because I think that may by default create an empty DEM, whereas with GmapTool, (you create Mapsource files in the Split tab (sounds funny, I know)), you can leave the Add Empty DEM box unchecked. This will create install and uninstall .bat files. If you have trouble with the map in Mapsource (program shuts down), you can run uninstall.bat, or use MapSetToolKit to remove the mapset. If you haven't done this before, it may take some experimenting, and internet searching, to get this right.
If you have a map with Garmin DEM in, for example, a commercial topo map, you can extract the DEM file and use it in your custom map. Only one Garmin DEM per .img, so you may need to split your .img to the available DEM sizes. It's in Spanish (use Google translate), but here's how:
http://gpsando.blogspot.com/2008 ... m-los-mapas-de.html 
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