The Hanover Collaboration

Geocaching is the “sport” where you go “online:http:/geocaching.com” and look up some GPS coordinates, and then go to them looking for the hidden treasure. Yeah it’s dorky, but it’s my type of dorky. I’m into it.

I like to explore and people generally hide the stuff in interesting places, many of which I wouldn’t have found on my own. Most are relatively tame, but some are pretty adventurous. I think I might start hiding a few of my own.

But I don’t have a GPS.

This isn’t as much of a problem since I found Mapfinder. Mapfinder will overlay the relevant USGS topo map on top of Google Earth. Since you can also map the geocache on GE, all you’ve got to do is print it. If there are decent terrain cues, with a USGS quad (7.5’) I can orienteer with nearly identical precision to a cheap GPS—better if there is heavy tree cover.

Here are my complaints about Geocaching so far:

1. They use DMS and,
2. They hide stuff too well since…
3. They don’t appreciate the limits on GPS’s precision

Universal Transverse Mercator, is a locally linear coordinate system¹, where you can simply read off the meters to target to the east and north. As in I’m at
0270400E, 4939200N and I want to get to
0270450E, 4929150N, then I should just walk
50m E and 50m S.
On a map, UTM makes a square grid that equally easy to use. The trouble with UTM is that it is local and approximate. Around here (from about Hanover to the tip of Maine and from the coast to past Quebec city), our reference surface is 19T. It is difficult to calculate distances between places with reference surfaces of different longitude (but not the same longitude¹). There is an average length distortion of like .05%, but near the edges it is more like .5%.

¹ The linearization is made by projecting the ground or “datum” (e.g. the WGS84 ellipsoid, the NAD27) to a reference surface. This surface is a segment of a longitudinal “cylinder”, parallel to the ground at the center of the region. It’s radius is chosen to minimize the average longitudinal distortion. Cylinder is in quotes because it’s cross section is the same as the cross section of the datum which is slightly elliptical. This fact means that it is surprisingly complicated to recover latitude from a UTM position.

Degrees, Minutes, Seconds, on the other hand is globaly exact. You have the angles made by you, the center of the earth, the axis of rotation and Greenwich England. You are standing exactly above the datum at point they are telling you. But to figure out how far away anything is, that is going to take trigonometry… Trigonometry and a book about datums… And Matlab would help.

The fact that the geocaching community uses DMS is indicative of that most of them just set a waypoint and follow the arrow. It is more fun if you also have some idea of where you are in addition to the GPS.

People think that because of all those significant figures in their GPS that they can bury a film canister and expect someone else to find it. You’re out in the middle of the woods with a $100 device receiving signals at like -160 to -180dBW! From space! You’re going to have to wait a long, long time before you can have centimeter accuracy.

What they could do is give a reference so that you can make an ad-hoc differential signal correction. For instance if there is something obvious nearby, like a stake, and they gave their measurement of both the stake and the cache, you could measure the stake, compute the difference and then add that to their measurement of the cache to get a much better estimate of where you will find the cache. And if they gave the measurement for the stake and cache in UTM, someone like me could probably just do it with a compass.