Predefined Datums

Programming with JViews Maps > Ellipsoid and Geodetic Datums > Geodetic Datums > Predefined Datums

Predefined Datums

The IlvHorizontalDatumCollection class manages lists of predefined horizontal datums. The list of predefined datums in the JViews Maps package, or kernel collection, can be retrieved using the GetKernelCollection() method.

Like Ellipsoid collections, horizontal datum collections are read from XML files containing the definitions. The DTD for these definition files is:

In a horizontal datum definition file, you can find:

A horizontal datum

Defined by its name and its parameters. The required parameters are the ellipsoid to be used with this datum and the three-axis shifts dx, dy and dz. Note that you can define the parameters for seven-parameter datums, but the ex, ey, ez and ppm fields are not used in the current version of JViews Maps.

An alias for a horizontal datum

Defined in the kernel collection of Maps. In case of alias, the required information is the id of the datum and the reference of the datum. When retrieving a datum alias from a collection, the datum is searched for using its id as a key, while the name of the returned datum is the one defined in the kernel collection.

The following XML file defines the Afgooye datum, while setting an alias for the NAD 27 datum

<datum-list>
  <datum name="Afgooye"
         region="Somalia"
         ellipsoid="krass"
         dx="-43"
         dy="-163"
         dz="-45"
  />

   <datum-ref id="NAD27"
              ref="NAD27 (CONUS)"
   />
</datum-list>

Note

JViews Maps comes with a default list of datums. Over time, many datums were tuned for given regions of use, while keeping the same name. This causes many datums to have the same name but different parameters. In applications where strict conformance to some datums is needed, you should use your own datum definition instead of relying on the list provided in JViews Maps.

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