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NAD vs. WGS84 When do you change?


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The rule of thumb is to use the Datum that cooresponds with what you are doing. For example, if you are using your GPS to enter data manually from Geocaching.com, use WGS-84 on your GPS to enter the coordinates.


When you pick up a topo map you need to look at the bottom of the map, usually around the centre of the map, for the Datum. It can be different from map to map in the same area and even if you have an older map and then buy a newer one of the same place.


For example, my map of Toronto is NAD27, the next map N/W of Toronto,

"Bolton" is NAD83. The map south of Bolton is also NAD27 but west of Bolton is NAD83! You really have to watch the datum and NEVER make an assumption!


As has been stated before, the difference between NAD83 and WGS84 are very slight and therefore I leave my GPS in WGS84 when looking at a NAD27 map.


That said, you could argue that you should change your GPS to NAD83 when using a NAD83 map and you'd be 100% correct.


The good news is, if your GPS is in NAD27 mode and you mark a waypoint it's stored, AFAIK in WGS84 internally. This means that no matter what datum you use to display your coordinates or store them on your computer they are still valid. A NAD83 waypoint plotted directly on a NAD27 map will be off of course but if you set the GPS datum to match the map datum it will match no matter what.





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what i do is i set the primary datum to wgs84 and the secondary datum to nad27. im luck here in nl because all the map sheets are either nad27 (1:50000) or nad83(1:250000) and since nad83 and wgs84 are nearly identical there is less fiddling.


the user manual with my mag 315 says to user wgs84 unless you are using a chart of a different datum. with my setup i can easily use the two common datums here in nl.


just a question, has anyone been in a situation where wgs84 and nad83 have been off by more than, say 100m?

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