Since coordinates on the Earth's surface can be recorded under widely varying assumptions about the shape and size of the Earth and the locations of the poles and prime meridian, cartographers have developed a standard for identifying the frame of reference for a coordinate system. This standard is called the datum. Because the frames of reference differ, a coordinate recorded in one datum usually has slightly different latitude and longitude values from the same point recorded in any other datum.


Some datums are designed to provide a marginally accurate representation of coordinates spanning the entire Earth, while other datums are designed to provide more accurate results in a particular region at the expense of lesser accuracy in other parts of the world. For example, the South American Datum of 1969 (SA69) is tailored to provide good results for maps of the South American continent and surrounding areas, but, as a consequence, provides poor results for the rest of the world.


When combining data from multiple sources into a single map, it is important that all of the coordinate systems being combined specify the projection and datum accurately. Since each datum has slightly different latitude and longitude values for the same coordinates, mixing coordinates from multiple datums together without fully defining the datum introduces inaccuracies into the map. MapViewer will automatically convert different source coordinate systems from different datums to the target coordinate system.


A datum conversion can be used to convert coordinates from one datum to another using the Data | New Projected Coordinates command. Click the ... next to the Target Coordinate System to open the Assign Coordinate System dialog. Click New to define a new projection and datum. The Define Coordinate System dialog has the Conversion Method and Ellipsoid parameters necessary to allow you to define a coordinate system with a custom datum.


Several different Conversion Methods may be used for converting coordinates from one datum to another:


The Molodensky method is the most widely used method of datum conversion. It adjusts latitude and longitude coordinates by taking into account the displacement between two datum's ellipsoids on all three axes. It does not take into account any rotational differences between the two ellipsoids.


The Bursa-Wolfe method is similar to the Molodensky method, but in some instances it produces more accurate results because it takes into account both displacement and rotational differences between two ellipsoids. MapViewer supports the Bursa-Wolfe method for conversions from the WGS84 datum to the following datums: World Geodetic System 1972, DHDN-1, DHDN, Australian Geodetic 1984, ANS84, MRT - Everest Modified, Switzerland - CH1903, NTF France - Paris Meridian, and Pulkovo 1942 - Hungary.



MapViewer supports conversions for over 200 different predefined datums.



See Also

Introduction to Map Projections

Custom Datum Definitions