>In the .prj file that comes with version 5.5, support is given for the
>MapGrid of Australia (geographic coordinates which use the GDA94 datum),
but
>the projection file does not include support for latitude/logitudes with
>GDA94. The question is why is GDA94 - Lat/Lons not included? Should we
be
>using WGS84.
>
>I have inserted the following line in my projection file which (I hope)
>gives the correct GDA94 projection.
>
> "Longitude / Latitude (GDA 94)", 1, 33
>
>Datum 33 is the GRS80 datum that GDA94 is based on. Could someone please
>confirm whether this is corrrect or not.
The "--- Map Grid of Australia 1994 (MGA94) ---" entries in the
MAPINFOW.PRJ
file all use datum 33, GRS80, which is based on the GRS80 ellipsoid. We
believe this to be consistent with the GDA Technical Manual, which you can
peruse at http://www.anzlic.org.au/icsm/gdatm/gdatm.htm.
Your proposal to use:
"Longitude / Latitude (GDA 94)", 1, 33
as a compatible lon/lat system seems reasonable. If there's sufficient
clamor
(send me email), I can include it in the next release of the projection
file.
Using WGS84 also appears to be an acceptable alternative since the
differences
are minuscule. So, if you don't want to construct a custom projection
file,
you could use the existing system for lon/lat WGS84 in the current .PRJ
file:
"Longitude / Latitude (WGS 84)\p4326", 1, 104
Go to http://www.auslig.gov.au/ausgda/gdastrat.htm to start delving into
Australia's overall GDA strategy.
Andrew Dressel
[EMAIL PROTECTED]
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