Join Date: Feb 2003
Location: Colorado Springs, USA!
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It depends upon satellite coverage. If you're riding in deep canyons and/or with a heavy tree canopy overhead you might not get very accurate readings. GPS units are pretty much "line of sight" devices and need as clear a view and as much view of the sky as possible.
Satellite coverage is why I wear mine high up near my shoulder - I found that putting it in my pack or on my hip tended to cause problems with accurate readings. I suppose the IDEAL spot for coverage would be on top of your helmet - but might be a little pricey each time you endo. In two years of riding I've never even bumped it carrying it over my collar bone on my hydra pack strap.
Otherwise they're incredibly accurate.
Here is the absolute best and least expensive software out there for working with GPS units.
TopoFusion is GPS Mapping software for Windows. It downloads maps (Topo and Aerial Photo) automatically from Microsoft's TerraServer and stores them on the hard drive.
And it costs a whopping $40.00