A primer: using map coordinates on this website

The pencil points out a trail intersection with UTM NAD 27 coordinates E563910 and N5390690. The UTM 1-kilometer grid is shown on this map sheet as fine black lines. Click to enlarge.

Some ‘Getting There‘ descriptions use Universal Transverse Mercator (UTM) coordinates to locate obscure outcrops or landmarks. Northwest Geology Field Trips use the NAD 27 Zone 10 coordinate system, as this corresponds to the grid overlaid on some (but not enough!) USGS 7.5’ topographic maps, pretty handy in the field.  I occasionally use a hand-held GPS receiver on site, and round off UTM coordinates to ± 10-20 meters depending on signal imprecision. You may wish to preview these geology field trips on Google Earth. If you do, note that WGS 84 is the UTM coordinate system used in GE, or optionally latitude and longitude (decimal degrees; degrees, minutes, seconds; or degrees and decimal minutes). In Google Earth, use Tools, Options, 3-D View to set the coordinate system you prefer. There are some complex NAD 27 to WGS 84 conversion methods online at this website. You can skip the whole computer or mathematical conversion by using just your GPS receiver at home, if you have one, to convert directly from WGS 84 to NAD 27, or to latitude/longitude, or vice-versa. Enter waypoints given on this website in your GPS unit using NAD 27 (use your ‘settings or setup’ menu to find that system- you may have the choice of numerous NAD 27 subsystems, look for ‘conus’ for ‘continental US’). After entering the NAD 27 waypoints, go back into settings and change to WGS 84, or latitude/longitude. Then bring up the waypoints you entered as NAD 27 units, and voila, they should appear in the other coordinate system. This may vary with the type of GPS receiver you use. Mine is, admittedly, ancient, dating waaaaaaaaaaay back  to 2001 or so. Write the coordinates down (yes, use paper and pencil!) and go back to Google Earth to find those points.

I made a transparent acetate grid that I lay over my USGS 1:24,000 maps to help me locate places quickly, whether I already have measured UTM from GPS, or if I want to estimate the UTM without a GPS. You can do that, or you can purchase a purpose-made ‘UTM grid reader’ from ‘Map Tools‘. There are other retailers, but I like this model and the price is good, only $3 each.

A transparent plastic UTM grid overlay for maps.

Your comments on the topic are appreciated. I’m not a wizard with this stuff, so go easy on me, please!

Please be sure to read Howard’s comment below for further important points.


One Response

  1. Great tips, Dave–

    A few points:

    1) The US National Parks Service has a really good explanation of the UTM system at http://www.nps.gov/history/nr/publications/bulletins/nrb28/

    2) As you point out, UTM users MUST remember to include the datum they are using (NAD 27, WGS 84, NAD 83 etc.) when they record UTM coordinates of, say, an outcrop or fossil locality, as there is a significant bust between NAD 27 and WGS 84. But they also have to record the UTM *zone*, which I didn’t see mentioned in your primer: because the Earth is divided into 60 zones (6 degrees each, x 60 =360 degrees), your easting and northing coordinates will have 60 possible locations (in the northern hemisphere), unless you specify which zone the coordinates refer to. Your area (Pacific Northwest) is divided across zones 10 and 11, so it’s important to record which zone you’re in (this will be displayed automatically on the GPS and Google Earth).

    3) I’ve noticed that one or two of your field trip localities are in Canada, so it will be useful for your readers to know that Canada has been converting its topo maps to the NAD 83 datum. Newer maps will use NAD 83, while older editions will still be in NAD 27. Therefore, map users need to check which datum is being used–it’s printed on the lower margin of Canadian topo maps. NAD 83 is a good thing, because its coordinates are identical (to within a tiny fraction of a hair’s breadth) to the WGS 84 datum used on Google Earth.


    (Calgary, Canada)

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