WORKING WITH COORDINATES AND UNITS

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US datums

There are several datums that have been or are still being used in the US. One of the NAD-83 is almost identical to WGS-84 and can be considered for navigation purposes to be the same. The differences are well under a meter most places in the US and are caused by a slight difference in the constants used in the calculations as developed by the different agencies administering the datums.

Many existing maps use the NAD27 datum. The difference in coordinates between NAD27 and NAD83 can vary up to 10's of meters. The 1927 datum was determined from less accurate and fewer observations. Factors such as seismic motion have changed station positions over time and computational capabilities did not exist to average readings. The newer datums are geocentric while this datum was based on a point in Kansas, and gravity data was not used in the network adjustment of the 1927 datum. There are other NAD27 datums with other locations chosen as there reference points for areas not too close to Kansas. Most USGS maps were generated using the NAD27 CONUS (continental US) datum.

World Wide Datums

There are datums in use all over the world and you need to be careful to use a datum that matches the map you are using. Study the map legend for this information or check with the map manufacturer. Some map grid systems assume a specific datum is used such as the British system used in Great Britain. Very few paper maps use the WGS-84 datum so if you need to relate your position to a paper map you will have to deal with the datum question.

User Defined Datums

The WGS-84 datum relates to other datum in common use around the world in in terms of differential X,Y,Z coordinates (DX,DY,DZ) and DA and DF which are the differential equatorial radius and differential flattening. This information can be entered into Garmin units that support user defined datums to provide the datum of your choice.

Grids

In addition to defining the origin and model to use for measurement purposes the gps receiver with also have a choice of grids to use to display the horizontal location.

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