Why is Topo horizontally/vertically inaccurate?

[Sanaghan]

Post topic moved and expanded on the Topo 6 forum:

http://forum.delorme.com/viewtopic.php?t=11777

[Sanaghan]

I have to correct myself; there is a version for bike training that does do bridges correctly. For an extra $50 you get several improvements....plus that bike trainer thing.

I've put it in as a suggestion to include those functions (except for the bike trainer part) in the next Topo release. Post includes a picture showing how bridges are handled.
http://forum.delorme.com/viewtopic.php?t=4862

[RA-JB]

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!

[LilBudyWizer]

That explaination certainly helped me. I've been trying to figure out how to process/scub the logs. With the big errors it was obvious that it had something to do with hills. The size of some of the errors has been baffling me. The distance to the nearest hillside brings a little reason to the chaos.