[GRASS5] datum transforms...

Paul Kelly paul-grass at stjohnspoint.co.uk
Mon Jan 13 09:46:28 EST 2003


On Mon, 13 Jan 2003, Alessandro Frigeri wrote:

> Hello,
> Nice to hear datum transforms checks in GRASS.
> Will it be possible to create a 'user defined' ellipse/datum set,
> i.e. during startup of a new LOCATION, so that the 'user defined'
> parameters (a,b||f and dx,dy,dz) could then be used in proj calculations?

At present you can do this by manually creating or editing the PROJ_INFO
file in the GRASS database. It should be possible to change g.setproj to
allow a custom ellipsoid option, in which case it would prompt you for the
further parameters:

To extend it to do that you would need to know which combinations /
permutations of ellipsoid parameters were necessary and sufficient for
defining an ellipse. (I mean I'm sure it isn't very complicated; just that
I don't know :) ). E.g. 'a' and 'es' uniquely define an ellipse; perhaps
also 'a' and 'b' do. Do you know what 'f' is? proj doesn't seem to need it
for transformations.

As far as I understand it so far dx, dy and dz are only necessary for
transforming from one ellipsoid to another (e.g. from a local country's
ellipsoid to wgs84- I think this is their specific meaning in GRASS); they
aren't really part of the ellipsoid definition and are only needed for
datum transformations.

Paul Kelly

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