ABCs of GPS configuration, Pt. 2
Understanding datum settings
If you donít know the correct datum but can still locate the wreck or reef you want to fish, you can always make the adjustment once you arrive.
Datum settings are different from the “Position Format” or “Location” settings.
Do you know what the datum selection means in your unit?
Geodetic datum is the basic starting point for mapping and grid formulas. A datum is a set of reference points converted to the Earth’s surface against which position measurements are made. It is based on a model of the shape of the Earth for computing positions.
Horizontal datums are used for describing a point on the Earth’s surface in latitude and longitude, or in other coordinate systems. There are hundreds of locally-developed horizontal datums around the world. Your GPS unit probably has a choice of over 150 different datums; they are usually referenced to some convenient local reference point.
Modern datums, based on increasingly accurate measurements of the shape of the Earth, are intended to cover larger areas. The WGS 84 datum, which is almost identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum.
In layman’s terms, this horizontal datum was at first a mathematical model of the size and distances of Earth. The most-accurate datums in the North America hemisphere were given designations such as North American Datum of 1927 (NAD27) and North American Datum of 1973 (NAD73).
Each country or cartographer could and usually did determined what the shape of the Earth was at their particular location. You will note in your GPS selections for Japan, Sydney Australia, Switzerland, British Admiralty of 1937 as well as many, many other different choices.
In the United States, the standard was NAD27 for many years, but after man was able to utilize space radar mapping, it was found that there was a discrepancy in what was believed to be the shape of the Earth; it was found that there was a bulge in the Earth around the equator. Satellite mapping allowed a more-accurate definition of the actual shape of the Earth.
This led to the reconfiguration of the datum that was used to correctly describe distances on the surface of the Earth. These newer datums are designated as North American Datum of 1983 (NAD83) and Word Geodetic System of 1984 (WGS84). For the consumer, they are basically the same value. These are the most-accurate datums. And it is the default setting in most GPS units and the new standard used in topo and marine mapping.†
Usually the difference between NAD27 and the default World Geodetic Survey of 1984 (WGS84) in our part of the world is about 158 feet to the west and about 112 feet to the north of one another. While not necessarily that important on a 300-foot-by-300-foot-by-seven-story-high oil platform in the Gulf of Mexico, an incorrect datum value might make you completely miss an underwater reef, submerged wreck, oyster reef or the opening end of a rock jetty.†
Most Louisiana Department of Wildlife and Fisheries reefs are marked by NAD 27 datum. All of the offshore oil and gas platforms are also plotted using NAD27 datum.
By entering the same numerical values for the latitude and longitude position of the reefs or platforms that were plotted using a different datum setting in your unit, the positions may be off by 150-plus feet.†
When entering a position, try to determine what datum was used to plot the position. Change the datum setting in your GPS unit to that datum, and then enter the values. After completing the entries, change back to WGS84, which is now usually the standard; the GPS unit will do the conversion for you.
If you don’t know what datum was used for the position, use the WGS84 settings, but use the information with caution. When at the location, correct the settings by reading the actual position value in your GPS unit while using the WGS84 setting.†
By viewing the actual positions, you will note that the numerical value for the location is different. The position location is the same, but when expressed in a different datum the actual latitude-longitude numbers will be different. That means if you entered the values for a NAD27 position when your unit was set to display a WGS84 position, the GPS would guide you to a location other than what you expected.†
Or, if you prefer, there is a free download from the U.S. ARMY Corp of Engineers called CORPSCON Version 6.0 that converts positions.
The download can be found at www.agc.army.mil/Missions/Corpscon.aspx.
A commercial version is available at www.expertgps.com/convert-coordinates.asp, or NADCON NAD27 - NAD83 Coordinate Conversions at http://www.fcc.gov/mb/audio/bickel/DDDMMSS-decimal.html.
Download the program(s), put it on your desktop and access it whenever you need to perform a conversion. They are neat, handy programs that will convert one datum to another. It is worth the time and minimal effort it takes to do the downloads.†
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