Datum Definitions

The datums.ssi file contains definitions for various transverse mercator projections, geodetic spheroid definitions and geodetic datum definitions. These definitions simplify the use of the lat/long to transverse mercator function and the transverse mercator to lat/long function. Additional definitions may be added to support extra transverse mercator projections.

Different transverse mercator projections have varying scope of use. As an example the NSW Integrated Survey Grid is only valid within the Australian Geodetic Datum whereas the UTM projection is globally adopted standard for map projections. Hence a transverse mercator projection is added as follows:

[TM]
name of tm projection
datum applicable (note <ALL> means all datums)
false easting
false northing
central scale factor
zone width
longitude of central meridian

If the datum is not defined in the file it must be added:

[datum]
name of datum
scope of datum (currently unused)
spheroid datum uses

Similarly, the spheroid that the datum uses may need to be added:

[spheroid]
name of spheroid
radius
inverse flattening
eccentricity

The definitions for the Australian Map Grid are shown below to illustrate the necessary definitions for a typical transverse mercator projection:

[TM]
Australian Map Grid
Australian Geodetic 1984 (AGD 84)
500000
10000000
0.9996
6
-177

[datum]
Australian Geodetic 1984 (AGD 84)
Australia and Tasmania Island
Australian National Spheroid

[spheroid]
Australian National Spheroid
6378160.00
298.250000000
0.081820179996