It's not a big deal but you mentioned you didn't understand why GPS's get wigged out when moving along a cut ridge road...
I can answer that!!! GPS works, of course, by measuring distance to multiple satelites. The path between the sat and the GPSr is roughly a straight line... Certain parts of the atmosphere can be modelled, others can't and that's why you have to have 4 sats for a good position.
In the case of the cut ridge road, some of the signals coming from sat will bounce off of the wall and make it to the GPSr's antenna following a path substantiallly longer than the direct path. It's called multipathing and can be seen on a TV with a conventional antenna when you get "ghosting" The tv isn't sure which signal to use so it uses both. The GPS has to make a choice, but it can't always make the right choice. So it starts to calculate where it is based on these longer signal paths and the position gets out of whack.
Hope that helps!