[Proj] Cape & Hartebeeshoek94 datums

[Oscar van Vlijmen]

> From: Zoltan Szecsei
> Date: Thu, 28 Dec 2006 18:54:55 +0200
> Subject: [Proj] Cape & Hartebeeshoek94 datums
> 
> Hi,
> I'm using:
> cs2cs +proj=latlong +ellps=clrk80  +to +proj=latlong +ellps=WGS84
> 19.5 -34.5
> 19d30'E 34d29'49.424"S 0.288
> to convert some coords but the resulting values seem incorrect.
> Is it because I need (somehow?) to specify the datums?
> I've checked out proj -ld and it doesn't show up.
> Can I get the result in decimal degrees to about 9 or 10 places?
> I am new to proj, so please could someone assist by showing me what I am
> doing wrong, or how to get the Cape & Haartebeeshoek94 datums into the
> equation.

If there is no datum definition available for cs2cs, you can provide one
yourself. One part is the ellipsoid designation of source and target, the
other part, missing in your equation, is the datum transformation. In the
case of Cape -> Hartebeesthoek94 there is an official, simple geocentric
shift known for the whole area:
dx = -134 m, dy = -110 m, dz = -292 m
<http://w3sli.wcape.gov.za/Surveys/Mapping/wgs84.htm>
with an accuracy of 15 m (so 9 or 10 decimals are a bit silly).
For higher accuracy: ask the Chief Directorate Surveys & Mapping for local
parameters.
The 3 shift parameters mentioned above are the first 3 parameters in the
cs2cs +towgs84 set. How to construct a cs2cs command line? Read the manual
or wait for a decent tutorial.
Should you want to project the Hartebeesthoek coordinates, note that the
coordinate system in the transversal mercator projection is south-west
oriented, so you may need to juggle with some signs (+/-).