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Wasmer Consulting NMPlot User's Guide Introduction to Datums
A datum is a detailed survey of a country, continent, or some other portion of the Earth's surface. As part of the survey, the longitudes and latitudes of a large number of points are measured using the best available methods. Once the survey is complete, maps of the region can be constructed.
Over time, the accuracy of surveying methods has steadily improved. Therefore, most regions of the Earth of been surveyed many times, with increasingly accuracy. Each of these surveys is referred to as a datum. With each new datum, the longitude and latitude of a given point on the Earth will change. This change can be as large as several hundred meters.
Adopting a new datum is expensive. Maps must be reprinted. Data files must be converted. Computer programs must be modified. Therefore, at any given time, more than one datum will likely be in use for a given region.
For example, in the United States, both the North American Datum of 1927 (NAD-27) and the World Geodetic System of 1984 (WGS-84) are in widespread use. In addition, maps and data in numerous lesser-used datums are also available.
For this reason, longitude and latitude alone are insufficient to describe the location of a point with an accuracy better than a few hundred meters. If greater accuracy is required, another piece of information is needed: the datum.
Even if you are certain that all of your data and maps are in a single, consistent datum, you still may need to know what that datum is. For example, the datum is required in order to convert from Universal Transverse Mercator (UTM) coordinates to longitude and latitude. This is because the conversion depends on the ellipsoid (a mathematical model of the Earth's shape), and the ellipsoid is part of the datum.
Most datums are local, intended to be used for only a portion of the Earth's surface. However, in recent decades, improved surveying techniques have enabled global datums to be established. These datums can be used to accurately map the entire Earth.
The most recent, and most accurate, datum is the World Geodetic System of 1984 (WGS-84). While future improvements in surveying techniques may result in modifications to this datum, these are expected to be minor: the coordinates of locations should shift by only a few centimeters. Therefore, for most practical applications, WGS-84 represents the ultimate datum.
Once all maps and data are converted to WGS-84, the problems associated with multiple datums will disappear. Therefore, mapping organizations throughout the world are converting to WGS-84.
Unfortunately, the goal of a single global datum has not yet been attained. Converting to a new datum is expensive and time-consuming. Therefore, the conversion to WGS-84 is proceeding slowly, and may take decades. Certainly for the foreseeable future, the existence of multiple datums will be a reality. Anyone who works with geographic data must be prepared to deal with this fact.
NMPlot can convert between most common datums. The 3-parameter Molodensky transformation is used for this purpose. Transformation parameters were taken from Department of Defense World Geodetic System 1984-Its Definition and Relationships with Local Geodetic Systems, NIMA Technical Report TR8350.2, Third Edition, Amendment 1, 3 January 2000.
If errors of several hundred meters are acceptable, you can ignore the entire issue of datums. However, if you require greater accuracy, then you must know the datum of any geographic data you use.
Whenever you obtain geographic data (maps, computer files, tables of locations), make sure that you determine the datum of that data. For maps, the datum can often be found printed in the legend area of the map. For computer files, the datum is often listed in the accompanying documentation. If you cannot determine the datum, ask your source. Any reputable source of geographic data should be able to supply this information.
Similarly, whenever you supply geographic data to others, make sure you tell them the datum. Include the datum in any documentation you provide.
If you are working with computer programs that process geographic data, insure that you know how those programs handles datums. Some programs (such as NMPlot) allow you to specify the datum of your data. Others require that all data be in a fixed datum. Consult the program's documentation.
If you are working with a global positioning system (GPS), make sure you know the datum that your GPS displays locations in. Many allow you to select the display datum.
If you absolutely cannot determine the datum of your data, then WGS-84 should be assumed. However, be aware of the potential errors if this assumption is incorrect.
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