Possible error in database links to cache maps?

[Guest arffer]

Question:

When displaying a cache from the database, links are provided to three resources:

mapblast, terraserver, and topozone.

 

If you examine these links, they link to these three resources by providing the cache location in the decimal degree notation that matches the database's dd mm.mmm for the cache.

 

This is fine for the links to both mapblast and terraserver, as both of these sites state that they use NAD83 which for this discussion is identical to geocaching's WGS84.

 

However, I am questioning the use of this data in the link to topozone, which uses NAD27. If I am correct, there can be up to a 200 meter discrepancy in the topozone map that is displayed for a cache.

 

If the answer is that +/- 200 meters is okay because the map is only supposed to help us plan our route, and not actually find the cache, I guess I can live with that, but that size error could potentially put the map marker on the wrong side of a river/gultch/canyon etc.

 

Any thoughts?

[Guest topografix]

Jeremy knows about this bug.

 

If you're using EasyGPS, you can get to the TopoZone map by choosing View Waypoint Online. EasyGPS does the conversion to NAD27 before linking to TopoZone.

 

------------------

Dan Foster

TopoGrafix: GPS Software, Waypoints, and Maps

http://www.topografix.com

[Guest arffer]

Thanks for the reply topografix,

 

For what its worth, I picked a sample cache in Florida that I thought should have some displacement between WGS84 and NAD27, and I ran the geocache WGS84 co-ordinates through 5 different utilities to convert it to NAD27.

 

One of the utilities is a multi-regression (seisoft's) that SHOULD have the best accuracy. I then compared the other 4 results to see how much varience there was.

 

For what its worth (very little?), here are the results:

 

Magellan 315 datum setting: 13 feet

jeeep: 4 feet

easygps: 9 feet

seisoft Molodensky: 16 feet