Projection Grouping and Characterization

The projected coordinate systems are grouped together and the groups are assigned a code. This topic describes these projection groupings in terms of properties such as the method used to map the earth’s surface onto a flat plane. The topic contains two elements: a description of the properties and the mapping of the projection groups to the properties.

The MgCoordinateSystem class has a method called GetProjectionCode which returns an integer. The meaning of the integer value can be determined from the set of integer constants contained in the MgCoordinateSystemProjectionCode class. These integer constants map to a set of coordinate system categories that are used to group the coordinate systems. The following code shows the retrieval of the projection code and its comparison to one of the projection code constants.

// csDict is defined above
// "CANADA-ALBERS" is in the Alber group of projections
MgCoordinateSystem cs = csDict.GetCoordinateSystem("CANADA-ALBERS");
int projectionCode = cs.GetProjectionCode();
if (cs.GetProjectionCode() == MgCoordinateSystemProjectionCode.Alber)
{
	// do something
}

The mapping of the projection code values to properties is programmatically extracted from the CS Map file CSdataPJ.c. In CSdataPJ.c there is a set of macro definitions of the form “#define cs_PRJFLGS_<GroupName> <OR’d set of bit flags>”. The bit flag definitions are contained in cs_map.h and are of the form “#define cs_PRJ_FLG_<FlagName> <bitDefinition>”.

Each projection code has a flag indicating the projection method. The four possibilities are flat plane (azimuthal), conical, cylindrical, and other. Other is defined as being neither flat plane, conical or cylindrical. The presentation of the projection code properties is divided into four tables according to the projection method used for the coordinate systems belonging to group represented by the projection code.