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Being able to check that the datum of your map and GPS match is a very basic, essential GPS skill that you should know before heading into the brush. Same with declination.

That being said, I have to disagree that user errors on declination or datum would have been a likely cause of more confusion. The difference between the relevant datums in that area (nad27? nad24? versus the wgs-84) would have been how much exactly? My guesstimate is less than 1 km, perhaps someone with knowledge about the most likely maps over that area (BruceZed?) would like to chime in. Say the difference is half a kilometer. This is plenty for the GPS coordinates to put you, say on the east river bank when you're absolutely to the west of it. Which will cause confusion and perhaps make you distrust the GPS. However, the map display on your GPS (if you have one - most units sold today do!) will show your positon without any datum shifts, and with your TRUE heading (prior to any declination errors).


GPS will tell you with very good accuracy where you are on the WGS-84 Geode, but lets assume some worse case errors which can occur for a common GPS unit with an inbuilt electronic compass and a paper map (NAD27)

Lets assume that the NAD27 datum hasn't been selected on the GPS reciever and instead is set to the default WGS-84 datum is at its maximum - this will be around 200m.

Lets assume that that WAAS is turned off (default) and the GPS etc has a somewhat limited sky view poor HDOP etc and the GPS error is 40m

Lets assume that the Magnetic declination is quite high at 16 degrees and the Magnetic declination has not been accounted for in the GPS, and it remains zero, i.e. the GPS has been set to True North.

Lets assume that you want to get to a way point on the NAD 27 map and you are using the electronic GPS compass, which has a +-2 degree error and the waypoint you need to navigate to is on a grid bearing of 0 degrees 1000m i.e. 1Km directly north on the NAD27 grid.

If all these errors combine in the worst case we have 200 + 40 + (sin (2 + 16) x 1000) = approx 550 metres away from the waypoint. The situation can get even worse if the Mag declination is entered incorrectly on the GPS i.e. 16 degrees East is entered on the GPS as 16 degrees West.

Now if you happen to now come across a another trail (which may well be the incorrect one) and you let it do the navigating for you then you could quite easily end up being turned around.

But GPS should allow you though to fix this problem quite readily by telling you where you are again once more with another fix, the difficulty once again is if the GPS has not been setup properly (as the GPS user doesn't know how to), is getting the GPS to tell you which direction to go to get back on track.

But once the same mistake is made again and you head off in the wrong direction once more then the navigational panic will probably start.

Even with GPS systems with electronic maps (most GPS electronic maps are inadequate i.e lacking contour height detail information compared to the paper formats), skill is also required to think in 3 dimensions when working with nothing really anything more than contour map lines you find at the higher altitudes.