Datums

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, consequently, 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 coordinates being combined use the same datum. Since each datum has slightly different latitude and longitude values for the same coordinates, mixing coordinates from multiple datums together introduces inaccuracies into the map. To address this problem, a datum conversion procedure is available to convert coordinates from one datum to another.

Different methods may be used for converting coordinates from one datum to another:

Method	Description
Molodensky	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.
Bursa-Wolfe	The Bursa-Wolfe method is similar to the Molodensky method, but in some instances it produces results that are more accurate because it takes into account both displacement and rotational differences between two ellipsoids. Didger 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.

If your map includes a NADCON or MRE datum, it is converted to the existing datum in the plot window. If you open a new plot window and import a NADCON or MRE map, the map is converted to WGS 84.

Didger supports the NTv2 datum conversion method. NTv2 is the Canadian government's officially sanctioned method of converting Canadian map data from the old NAD27 datum to the NAD83 datum. NTv2 is based on a hierarchical database of interpolation grids of different resolutions for different regions of the country. NTv2 datum conversions cannot be performed unless an NTv2 grid shift file is installed in the same folder/directory as the Didger program. Visit Natural Resources Canada's Geodetic Reference Systems page on the web for more information about this datum.

Didger supports conversions for over 200 different predefined datums, as well as user-defined datum conversions based on the Molodensky, NADCON, or Inverse NADCON methods.