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Posted by Bill S on April 2, 2008 at 12:32:06 pm EDT
In reply to National Geographic Topo Question posted by BigSky on April 2, 2008 at 9:39:13 am EDT

Topo does not provide the grid north deviation. You don't really need it anyway for using your baseplate compass with the printed maps. The neat line (the edge of the printed map) on the east and west sides is true north-south. So when using your compass with the map to translate your magnetic bearings to and from the map, you simply use the compass in its protractor mode (one of the important reasons to use a baseplate compass rather than any other type), checking the angle with the neat line. If you are getting the bearings from the computer, Topo automatically gives you the magnetic bearing, using the current value of the magnetic declination, so no conversion is needed. Remember, you will be using the compass in the field, so you only need the magnetic bearing, not the grid and not the true bearing. If you do need to translate to or from the map, just use the T to M translation using the map N-S neatline, as I already said.

I realize that you are trying to make the determination using the UTM grid you print on the map. But it is just as easy to use the neat line, or to use a lat-lon grid to provide the true N-S reference line.

People for some reason believe that UTM is easier to use than lat-lon, because it is a "square" grid. They have been told that lat-lon lines are "curved". However, on the scale the backpacker is going to use, lat-lon is a rectangular grid. The "curvature" is a result of forcing the Earth's curved surface onto a "flat" map. UTM grid lines are in fact much more curved, and only appear "straight" because they are printed on a flat piece of paper and the Earth's features are distorted to match the flat paper. Remember, UTM stands for Universal Transverse Mercator. Remember those maps in your school classroom that show Greenland as bigger than the rest of North America? That's a Mercator map.

And no, you do not need to know any spherical trigonometry to use lat-lon for hiking on a map.

For those of us who realized long ago that UTM is useful only for small areas and has some significant flaws, the coordinate system makes no difference. Whatever the coordinate system (lat-lon, UTM, State Plane, Range-Township), a coordinate system is only an address system. If you can find your way to an address in a city, you can use any one of the systems. Remember that UTM was devised for military usage, primarily for draftees and recruits many of whom had not graduated from high school, with much of the usage being for artillery, mortar, and machine guns ("your target is 350 meters east and 220 meters south of your position"). Other problems include the significant discontinuities at zone boundaries and the variation in the grid angle across even a 7.5 min quad. Here in California, the boundary between zones 10 and 11 (the 120 deg west longitude line) happens to run right through some of the more popular backpacking parts of the Sierra (middle of Lake Tahoe, Mokulmne Wilderness, just west of Yosemite NP), crossing the PCT in the Carson Pass and Luther Pass area. The Zone 11-12 boundary runs through Missoula, MT, and along the Utah western border, as well as some of the more interesting hiking parts of Glacier NP and not far from Great Basin NP. In the East, the 18-19 boundary runs through western NH and almost through Sturbridge Village, MA.

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