The wordwide move to GPS compatible Geocentric Datums in the latter years
of the 20th century created a need to transform pre-geocentric spatial
coordinates to their geocentric equivalents.
Three approaches are widely used:
Canada's Geodetic Survey Division developed NTv2 as a national
high precision transformation between NAD27 and NAD83.
Australia adopted the same format for transformation between AGD66 and GDA94,
and AGD84 and GDA94.
- Low Precision Transformation using Molodensky's Formula,
offering accuracy of about 5 metres.
- Medium Precision Transformation using a Seven Parameter Similarity
Transformation. This offers accuracy of about 1 metre for some Datums
(eg AGD84), but much less for others (eg national AGD66).
- High Precision Transformation using files of coordinate shifts
that deal with distortions in the original coordinate data as well as
the more mechanical datum transformation. Accuracy for this method
is generally better than 50mm, and can be determined for any point.
Australian states originally produced their own grid shift files, and chose
to implement them differently to the Canadian files. The NTv2 spec does not
specify big-endian or little-endian format for binary files (the most commonly
available). Canada opted for big-endian, Australia for little-endian. To make
matters worse the first version of 'Australian' format files used a different
record length for some headers. Subsequently Australian binaries were produced
as strict little-endian versions of the Canadian file format.
Canada's Geodetic Survey Division created Unix based software implementing
the NTv2 transformation as part of its development effort.
Three Australian States produced software to read the Australian format binaries,
all MS Windows based.
GDAit in Victoria
Geod in New South Wales
GDay in Queensland