Why don the UTM Grid References used in Gillean Dafferns Kananaskis Country Trail guidebooks make sense with my Gem Trek map?
Daffern bases her grid references on the NTS government topographic maps (we don’t know why she still does this, either), and many of these NTS maps (the ones published before 1983) are based on NAD 27 (North American Datum from 1927). All of the Gem Trek maps are based on NAD 83 (datum from 1983), which is a more up-to-date representation of the shape of the earth. On Gem Trek’s Bragg Creek-Sheep Valley map, for example, to convert from NAD 27 to NAD 83, on the Easting, subtract 80 metres and on the Northing, add 209 metres. Depending on which Gem Trek map you are using, Daffern’s grid references will differ by about 200 metres. Or, a particular grid reference may not differ at all, if she is basing it on a NAD 83 government topo map. Unfortunately, when she gives a grid reference, she doesn’t say which datum she is using. If you have a GPS, you can figure out the conversion factor yourself for any Gem Trek map. With the map datum on your GPS set to NAD 83, choose a spot approximately
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