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Posted by Bill S on April 2, 2008 at 11:28:19 pm EDT
In reply to National Geographic Topo Question posted by BigSky on April 2, 2008 at 9:39:13 am EDT

rdavis said

Quote:

I've used MGRS (and terrain association, of course) to find a person sleeping on the ground from over 15k away, as well as to navigate over entire countries.

15k is a small distance. I'm not sure what the "entire countries" are, though there are lots of countries in the world that are under the 15k size (but, as I have found, don't insult the citizens by implying that their country is unimportant because it is small - some of them take huge umbrage at such an implication).

Anyway, as long as you stay within a zone, or are aware of what happens at the boundaries, UTM is easy (for those who might be confused by rdavis' reference to MGRS in a discussion of UTM - MGRS = Military Grid Reference System. This is a further simplification of UTM, in which the leading digits of Easting and Northing are replaced by letters). A zone is 6 deg wide in longitude, which is about 331 miles at my latitude, 414 miles at the equator, and 207 miles wide at the latitude of Anchorage. When you get far enough north or south, UTM doesn't work at all, and you have to use the UPS (Universal Polar Stereographic) system, in which the square grid system doesn't match N-S or E-W except for 2 lines corresponding to the 0-180 deg longitude great circle and the +90 to -90 deg longitude great circle. UPS is used basically north of 84 deg N and south of 80 deg S latitudes, so doesn't affect most people. UPS does avoid the singularities of the N and S Poles that the lat-lon system has.

As for BigSky's being taught UTM/MGRS in "the military", better specify which "military", son. The Navy teaches lat/lon (the Real Man's coordinate system, if you ask an old Navy man - but of course, if you ask a Navy person, man or woman, the Navy is the only Real Military). Ummm, well, the Marines teach UTM. Of, and the Air Force teaches lat-lon. Basically, the reason is that lat-lon more accurately represents the Earth, and has no discontinuities.

But, when you come right down to it, if you are using a GPS receiver, or are on the computer using a mapping program like Topo, it doesn't really matter which coordinate system you use, as long as you specify it, along with the datum. The GPSR and the computer do all the figuring for you (and for some of us, our PDA or Smart Phone will display the maps and figure out all the coordinate business).

I agree that people tend to stick with what they first learned, at least until they get into a situation where it makes a real difference. If you have to travel over the distances that a blue-water sailor or an airman travels, UTM is awkward at best. If you are lobbing artillery shells, UTM is easier to work between spotter and gunner. If you are on foot or in a tank, it's often easier to know you are to travel so many km east and so many north (or west and south, which is just the reverse, and you call them "clicks" instead of kilometers) than going from a lat-lon to another lat-lon (except that your GPSR will tell you to follow a bearing for a distance, regardless of the coordinate system you used to feed in the destination). Hey, guess what! Bearing and distance is a polar coordinate system, just like lat-lon! And you could say that over short distances you follow the straight line on the map, which becomes a great circle or a rhumb line if you go far enough.

Oh wait, you cheated! You used terrain association, not just MGRS! Which is, of course, the proper way for a backpacker to navigate. You can't follow a bearing very far in real terrain. You have to adjust for terrain.

When you come down to it, the coordinate system doesn't matter, as long as you are consistent. When you go to different countries, you have to be prepared for different coordinate systems on their maps (unless you are using NIMA maps).

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