Woah. UTM looks pretty nice

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One of the things I've always found difficult about navigating with standard topo maps (and a major motivation for being able to download maps into my GPS) is the pain in the ass that's finding your position based on lat/long. Let's take your typical 1:24000 scale map of the Bay Area:

  • The scale is about 1" for each minute of longitude and 2.5" for each minute of latitude.
  • On the USGS quad I'm looking at, tick marks are every 2.5 minutes of longitude and every 3 minutes of latitude.

Using a map like this and starting with GPS readings, finding your position involves converting minutes and seconds to inches (with different scaling factors for lat and long) and then measuring to find the right point on the map.

It turns out—and I feel pretty stupid for not knowing about this already— that there's a much easier way: Universe Transverse Mercator. The basic idea is that you divide the earth into sectors which are small enough to be treated as rectangular and then you can describe any position within the sector by measuring the distance (in meters) from the corner. Generally your GPS can emit UTM coordinates and good topo maps come labeled with UTM grid lines, so finding your position is a simple matter of locating the nearest grid ref and doing a little interpolation. You can even get nice little map tools that let you measure UTM distances on maps of common scales (especially the 1:24000 scale used on the most useful topos). This is dramatically easier; I've known about it for less than 8 hours and I'm already quite a bit better at finding my position with it than I ever was with lat/long.

1 Comments

I took a map class (thanks US DoD) years ago and learned all about UTM...you are right. Easy.

Have you tried a GPS receiver with SiRF receiver tech? Wow compared to "regular".

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