Does anyone have one of these? If so, do you know how to overlay a plotted trail onto a background map, on your computer?

I own one myself and I have yet to figure that one out. I have been in contact with Lowrance a couple of times with out any luck. If you find out shoot me an email, also if I am able to figure out a way I will let you know.

exmxer;
I use a software program called "OziExplorer" to overlay my GM100 track files on USGS maps. I've got the USGS maps on compact discs, but if you have paper maps you can scan them in and OziExplorer will work with either. The GM100 has many trails built in, and more can be added with the OziExplorer add-on called "OziMC". It requires the compact disc accessory to the GM100. The OziExplorer website address is;
http://www.powerup.com.au/~lornew/oziexp.html
Hope this helps.

Hello!
About maps and such... OZI is a good program, I use it, mostly because the maps are free. You can download any USGS map at www.gisdatadepot.com
The 1:24000 scale (24K) is the popular 7-1/2 minute "topo quad", but you might find the 1:100000 (100K) maps do a better job at covering an trail. The maps are orginized by Country, State, and County, then you must select the map scale and type. Select the desired scale DRG map(DRG=digially rasterized graphic, which comes in a .TIFF fille format after you unZip it). Finally you must know the name of the "quad", because a county may cover several maps.
If you don't know the county or map name, but you do know of a named place on the map you can go to http://mapping.usgs.gov/www/gnis/gnisform.html and do a search.
The OZI program itself is free, that is it has a shareware version. The free version only allows 2 point map calibration which is not very accurate. For serious navigation, spend the $75 USD for the full blown version. OZI now as their own domain at www.oziexplorer.com
Alternatively, you could buy one of the mapping products from Delorme www.delorme.com or MapTech www.maptech.com or National Geographic's Topo Trails www.topo.com All of which interface with common GPS units, but as I said the maps can get expensive.
Have Fun!
Doug