topomap accuracy?

[Guest turken]

After I placed my first cache, I was surprised that the topomap link placed a marker on the map that was a good 500ft off from where I actually stashed it. (the marker was right on top of a building, and I placed the cache to the north). Granted, I was using an etrex legend with WAAS off (and where can I get info on WAAS anyway?) to mark the spot, but I was wondering if anyone else has had this sort of problem. If so, which is more correct, the GPS reading or the reading given by mousing over the topomap?

[Guest Moun10Bike]

Your GPS is giving the correct position. The problem is that Geocaching.com uses the WGS84 datum (the current standard), while Topozone.com uses USGS maps that use the older NAD27 datum. This can lead to a positional difference of up to around 200 meters in some parts of the US. In order to plot the correct point on Topozone, you will have to convert the cache coordinates to the NAD27 datum and then tell Topozone (manually) to plot them. For more info on datums, see http://joe.mehaffey.com/maps.txt and http://www.alltopo.com/gps_accuracy.htm .