I've thought a bit about this (as you may have guessed) but I guess I prefer the simplicity of a single line to follow, perhaps with supplemental waypoints.
One thing is not everyone uses a Garmin, and some people's Garmins don't support basemaps. A track is much more universal.
But to be honest I haven't delved much into the garmin .img format, and it could be a really nice way to present certain routes/races in a more comprehensive way... just make sure to offer the track too and it should all be good.
If you looked at the Cottonwood map I did, you saw a map with a whole trail network on it. For the Apache map link I had, that only showed the route for the course, along with a few major road and the topo lines and water bodies.
For any courses done, they would be like the Apache map. I also would like to make a simple track overlay file available too. On that, it would just be the route and waypoints, which would draw overtop of the base topo map installed on the unit. And it can be toggled on or off by going into the map setup menu. This would all be in addition to providing the .gpx files for those that would rather use them. Just thinking of different options to give folks, since I have been able to make sense of the custom mapping thing.
Also, if you look at the size of the custom Apache topo map, it is less than 2 meg in size. Any Garmin unit should allow that to load up (minimum internal memory on any Garmin product I know of is 8 meg, even back to a simple Legend device - maybe I'm wrong, but I don't think anyone doing these type of ventures is using a device that won't allow additional maps to be loaded?). Checking the size of the overlay map I created for last years course, that file is only about 200k in size. Plus, as long as you do them all together, even the 'older' models like your Vista HCx or my 60CSx will allow multiple maps to be loaded and merged into a single image file.
Just a thought on some alternatives. I'm planning on doing up some riding area trail maps in full topo and overly version anyway - and I've already converted the NED elevation data and downloaded the TIGER road files, plus some water data (as well as cleaning up my trail network .gpx files and creating .kml files for newer generation units and (i)phone users) and figure it's not hard to do for courses, so, why not?
Dave Harris: If you're checking this thread; You have any POIs to add to the Dixie 200 course? I've got a topo made of the Brian Head/Bryce Canyon/Markagunt/Paunsagunt/Sevier Plateau areas done and was going to make files for the 200, along with my general area ride maps. Have anything extra I can add to it?
Here is a link to the entry on the Cottonwood map thread I put on the NV MTRB board, which shows the areas I am working on creating area Garmin trail maps:
http://forums.mtbr.com/showthread.php?p=7803341#post7803341Let's just get this BS weather to be gone and bring on some wonderful Spring riding temps!