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Map Datums #108
    Date first written: 2000.01.01   Review Date:2004.09.27

The "map datum" of a map affects the coordinates of any given point on the map. Nowdays, all new maps use the WGS84 datum, and so if you get a given set of coordinates on one map, they will always match another map. However, older maps have slightly different datums, which means the co-ordinates can differ by 100 or 200 meters. The datum is like the "origin" of the coordinates.

Don't confuse "datum" with the type of coordinates such as Lat-longs versus UTM. The datum affects both of these coordinate types. There are two main datums in Canada: NAD27 Canada and WGS84. Most of the older 1:50:000 paper maps use the NAD27 datum, but any new maps are WGS84. NAD27 is based on land surveys in 1927, whereas WGS84 is based on satelite measurements in 1984. Depending on the datum used, the co-ordinates you read can differ by almost 200 m. Below is a comparison showing the same waypoint (called my house) in three different datums. Note that the co-ordinates of "MyHouse" are different for each datum. If someone has a NAD27 map, they'd tell you the house was at 49.26681 N, whereas a person with a WGS84 map would say it was at 49.26665. Get it?

----------Datum-------------Lat-Long-------------UTM-----------
MyHouse  NAD27Canada  49:26681 123.16671  10U 0487871 5456911
MyHouse  NAD27CONUS   49.26685 123.16672  10U 0487870 5456916
MyHouse  WGS84        49.26665 123.16801  10U 0487777 5457113    
"CONUS" means Continental USA". You can reproduce this table with your GPS. Just edit a waypoint in your waypoint list to match the NAD27 Canada value, then successively change the datum and position format to get the other values. The last decimal place of a UTM is exactly 1 meter. Note that NAD27Canada is very close to NAD27CONUS, but WGS84 is 197 meters different on the northing (7113-6916=197).

Internally, GPS units record waypoints as WGS84. However they can display them in any map datum, and any position format. So typically when you go on a trip, you'd go into your setup menu and set your GPS to whatever is most convenient for the map. In Canada, that is NAD27Canada, UTM co-ordinates and feet. When you get home, reset your GPS back to WGS84, Lat-Long, and meters, and then it is easy to transfer any waypoints you have recorded into the Bivouac website.

Where is Datum on Canadian Maps? On a Canadian NTS 1:50,000 map, the datum is at the bottom, along with the contour interval, just to the right of the mapsheet title. The map datum affects both the Lat-Long and the UTM.

What Datum does Bivouac Use?
The underlying data in Bivouac.com is all supposed to be entered in WGS84 lat-longs. However waypoints can be displayed in the old datums by using the links on the trip report and road pages. Click the NAD27 link before printing the waypoints for a trip report or route description if you intend to mark those points onto a paper map that is NAD27. Bivouac also has a general purpose calculator for Coordinate Conversion. It will convert both datum and coordinate format at the same time. Eg: Convert from NAD27 UTM to WGS84 Lat-Long.