Hi,
it can get confusing. WGS 84 is a satellite datum, which incorporates it's
own reference ellipsoid for the purposes of providing positions in
convenient Lat/Lon co-ordinates, rather than it's native 3 dimensional
cartesian co-ordinates. So in effect you are right in concluding that WGS84
is both datum and ellipsoid. This is fine and dandy if you work entirely in
the WGS84 satellite system, but if you wish to relate satellite derived
co-ordinates to existing terrestrial control networks, then you need to
establish a relationship between the two. The radio telescope has been
adopted as the initial point used to tie the reference ellipsoid to a fixed
terrestrial point and thus define the new datum.
>From my reading of the document referenced below, it appears that the
Hartebeesthoek94 Datum is entirely consistent with WGS 84, and there is
therefore no requirement to carry out a datum transformation, provided you
are working with data based upon purely WGS84 satellite-derived co-ordin
ates. If you wish to integrate any data based upon the earlier Cape Datum,
then a datum transformation of the earlier data will be necessary. Not
however that heights in the new Hartebeesthoek94 Datum are still related to
mean sea level, so satellite derived elevations will require correction.
More detailed information on the Hartebeesthoek94 Datum can be found at:
http://www.geomatics.uct.ac.za/research/wgs84/hb94-01.htm
Hartebeesthoek94 Datum
-----Original Message-----
From: Michele Cote [SMTP:[EMAIL PROTECTED]]
Sent: 30 July 1998 15:42
To: [EMAIL PROTECTED]
Subject: MI HARTEBEESTHOEK 94 DATUM
I would like to use the new South African Hartebeesthoek94 Datum in MI.
This datum is based on the WGS84 ellipsoid, with the ITRF91 (epoch
1994.0) co-ordinates of the Hartebeesthoek Radio Astronomy Telescope
used as the origin of the system. (quote from Department of Land
Affairs, South Africa)
However I am little confused. In Appendix G: Creating Custom Datums,
it says I need to specify the shift and rotation of the 'new ellipsoid'
as compared to the WGS84 DATUM. I thought WGS84 was the ellipsoid. Are
there a datum and ellipsoid called WGS84?.
Anyway, since the Hartebeesthoek94 Datum is based on the WGS84
ellipsoid, do I need to create a new Datum or could I just use the WGS84
Datum provided with MI?
If I need to create a new Datum, am I correct in thinking that I must
ask the South African government for the shift and rotation parameters
of the Hartebeesthoek94 Datum as compared to the WGS84 Datum?
Thanks,
Michele Cote
[EMAIL PROTECTED]
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