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It is essential when using a grid co-ordinate system that the receiver
is set on the projection and datum of the map. Otherwise, the
positions given by the GPS receiver will not fit the map. Every
map states the projection and datum on which the map is constructed,
usually found along the bottom of the map. Use this information
in the set-up menu on the GPS receiver. Most of NSA’s maps carry
a Universal Transverse Mercator (UTM) grid, and are constructed on the
Transverse Mercator Projection and WGS 84 (World Geodetic System 1984)
spheroid and datum. For most purposes, WGS can be used with little
loss of accuracy if WGS 84 is not available. It should be noted
that some maps of Series K6611 1:100,000 scale in the Dhofar region
have been constructed on the International Spheroid with European Datum
1950 (ED 50). The GPS receiver should be set-up with these parameters
for use with these maps.
Handheld GPS receivers typically display grid co-ordinates to a resolution
of 1 metre. Users should note, though, that the quoted average
accuracy of these co-ordinates is only of the order of ±15 metres.
There will be times when the accuracy may be better than this but users
should be aware that there are times when the accuracy will be less.
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