The ability to connect projects and data throught a common coordinate system is increasingly necessary with the automation, exchange, and merging of geographic information. However, converting data has become more complicated than in the past due to an increasing number of datums, adjustments, and coordinate systems.
A coordinate conversion or transformation* is the process of bringing a coordinate from one defined coordinate system (or zone) into another through a series of algorithms based on the latitude/longitude position of the point (Figure 4). Coordinate systems based on the same datum retain a perfect mathematical relationship, allowing coordinate values to be precisely transformed between them.
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between datums and coordiate systems.
A datum transformation is the process of bringing coordinate values referenced to one defined datum into another datum systems (Figure 4). The recent geodetic datum adjustments in the United States are not perfectly related to the older datums because they removed errors and distortions in the geodetic network while at the same time redefining the mathematical model. The only way to perform an exact datum transformation is to recalculate the point's position using the original measurement observations that produced the latitude longitude position.
Transformation Considerations
Selection of the most appropriate transformation methods depends upon the data and its potential uses or applications. Considerations include:
Two other important considerations in data conversions are units of measurement and significant figures. The GRS 80 ellipsoid and several recent coordinate systems definitions use the meter as their base unit. Yet most surveying and mapping information collected and produced at the local level is based upon the U.S. Survey Foot. When a conversion from one of these units to the other is performed, it is important to ascertain which standard foot (U.S. Survey or international) is involved.
With the appropriate unit conversion factor and other coordinate system parameters, the number of significant figures carried through the computation is important. Using too few digits will result in an incorrect conversion of data.
Transformation Methods
The only exact method of datum transformation requires the original survey measurement information. This information is used to geodetically recompute positions in the new datum system. (Coordinate data can be exactly converted through a series of mathematical formulas, providing the coordinate systems are referenced to the same datum.) Another method of data transformation uses a least squares approach to "best fit" the data for a region. The quality of the fit is dependent upon the number, distribution, and quality of geodetic control points in the area. These two methods require geodetic expertise to effectively use and analyze results.
A third transformation method uses a set of gridded data models to interpolate approximate corrections, which are then applied to produce the transformed coordinate values. Federally produced software programs such as NADCON, CORPSCON, and VERTCON are based on this commonly used method of addressing datum-to datum transformations.
Software & Tools
WISCON, a product recently developed specifically for Wisconsin's data transformation needs, can be an effective replacement for the federal programs. WISCON is a comprehensive software program that incorporates the NADCON, CORPSCON, and VERTCON packages as well as the algorithms necessary to perform Wisconsin County Coordinate Systems conversions. Use of WISCON is limited to Wisconsin and its four bordering states.
The IGLD85 (International Great Lakes Datum of 1985) is a datum transformation tool that converts NAVD 88 Helmert heights to IGLD85 dynamic heights and vice-versa. This computation can be done interactively, or in batch mode by submitting a file of data points. Many private software venders in the GIS, CAD, and mapping arenas have incorporated federal or other transformation packages into their products. It is important to review the software documentation to ascertain which transformation packages and which versions are being used.
* In the strictest sense of the word, a transformation refers to an approximate method of data conversion. However, common usage today also embraces "exact" conversions.
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When transforming between datums or coordinate systems, it is worthwhile to consider the quality of a point's original value. A transformation cannot improve the quality or better the accuracy of the data. It is up to the user to be aware of and to work within the limitations of the original data.
• The reference datum of the original coordinate values.
• The reference datum of the new coordinate values.
• The accuracy of the original coordinate values.
• The accuracy of the new coordinate values.
Coordinate and datum transformations are performed by various methods. Some of these methods support highly accurate geodetic and surveying work, and others are approximately adequate for mapping and other purposes.
NADCON and VERTCON are now standards within the federal government for datum transformations. The U.S. Army Corps of Engineers has developed a coordinate transformation program called CORPSCON which incorporates NADCON and VERTCON and handles a variety of coordinate systems.
