Finally! after 2 years of trying I stumbled across the tools needed and finally had enough braincells active to understand the process of making your very own custom GPS map (for use on garmins only). it is as simple as 1,2,3, (4)!!
1) You need to capture your map. the garmin units use a Vector based system so you will need to use a tool to draw/trace the features you want in your map. I started with quakemap and have actually registered it. the vectors get exported to XML format
2) you then translate it. I have been using gpsmapedit a free program to convert to the strange format of .MP. so far I have not found a way to batch convert the vectors so I painstakinly go through, right click on each one and convert to poly line.
3) Compile it! this tool has been around for some time, but untill the recent addition of GUI interfaces to actually draw the vector content it seemed very unapealing. the free version does all that I need so far and can be found at cgpsmapper.com they have some website that will do the conversion online (and I presume keep a copy of what it compiles) and I Think the online tool has more options that the free one. just something to keep in mind. some people have used another program called mapdekode, but I haven't looked into that yet.
4) I almost forgot this step, UPLOAD it! there is an accompanying program called sendmap that can do the job, or if your feeling a littly giddy you can try the hack on mapsource to view and upload it with garmins own tools. I've used sendmap. fast, simple, and it works.
I made a quick map (about 160 routes) around my place of residence. that coverse probably a 10 mile by 10 mile region. I'm hoping it will help me find all those tricky hidden backroads that you can only seem to find when you have a satelite view. this map (to my amazement) compiled to 10k. it was 450k as an XML file. thats great news, because my unit, a rhino 120 has 8meg.
if this isn't a good enough thing to get you guys going, read this other summary HERE I'm sure there are better and faster ways to do this, but right now i'm just happy its posible. i'll let you know if I find better ways to do it.
img2gps, front end for sendmap
spatial data for POI starting point
convert to .MP format
globalmapper.com $250 but may be worth it.
dlg from usgs