WORKING WITH COORDINATES AND UNITS

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As mentioned before many datums are only defined for the horizontal plane and thus your gps will continue to use the WGS-84 datum for vertical, altitude, information. This datum defines zero height using a mathematical model of an ellipsoid, which is basically a ellipse spun around its minor axis to form a globe shape. The surface of this ellipsoid is considered to represent zero altitude with points above the ellipsoid representing positive altitude and points within the model negative altitude. This model does not take into consideration the many real life changes caused density differences in the earth and earth motion. What we would prefer is a measure of altitude that represents zero with sea level, or more precisely mean sea level. To get this number your gps uses a table that has been defined for just this purpose. It is called a geoid and can permit the translation of gps computed altitude to its mean sea level equivalent. It should be pointed out that while we seem to have defined altitude very accurately, a handheld gps is not a particularly accurate device in measuring altitude. Because of the geometry of the satellites in the sky and the fact that the earths surface blocks our view of most of the constellation the accuracy of a vertical fix is about 50% worse than the accuracy of a horizontal fix. Unfortunately this is often disconcerting to the user since they generally know their altitude to better accuracy than they know their horizontal location and even may measure it with more precision (feet vs. miles). Do not let this difference cause you to believe the gps itself is not reporting an accurate fix.

Translations among the various datums can be a cause of some errors. Garmin used the the Molodensky transform parameters for those datums and performs transformations as needed. This is a simplified model and can result in errors on the order of 10 meters in some cases. With this transform the WGS-84 and NAD-83 numbers are always the same while a better translation will show a slight difference in these two datums. For most of the US this difference is less than a meter rising to about 2 meters on the west coast. Still less difference than the accuracy of the unit in most cases. Modeling something as complicated as the earth does not lend itself to mathematical transformations so to achieve accurate results some information is contained in tables with model information used to access the entry in the table which is then interpolated to get the answer. It is not known where Garmin depends on models alone and where they supplement with tables.

WGS-84

WGS-84 is a world wide datum and is the master datum for use with a gps. Garmin always stores internal information in this datum. The origin is the center of the earth and then a ellipse is defined using the major and minor axis. Information about ellipse flattening and the gravitational constant is also part of the definition. The latter is used to calculate the geoid height. The model is augmented with stations at precisely defined locations on the earth's surface to pinpoint the accuracy of this system.

As more and more mapping systems have become digital in nature and databases have begun to be the norm for surveying there is a tendency to use the WGS-84 datum more and more. This is exacerbated by a need to share data from around the world that was originally generated on a different datum. Hopefully this trend will continue as it simplifies data sharing and reuse of information.

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