[OSRS-PROJ] PROJ4 and datums

Andreas Lange Andreas.Lange at Rhein-Main.de
Fri Apr 7 05:37:16 EDT 2000

Markus Neteler wrote:
> Yes, I understand. But my question was more related to
> following, more general, point:
> Do I need datum conversion if reprojecting a map
> from UTM/WGS84 to Transverse Mercator/Bessel?
> The German system is based on Potsdam-Datum, the
> UTM datum I don't know.
> As nad2nad is existing, the datum conversion seems
> to be important.
> Kind regards
>  Markus Neteler

Hi Markus,

i am no geodesist and no specialist in datum conversion. But it seems to
me that grass already includes the bits and pieces for datum shift:

m.datum.shift -> returns geog. coordinates based on a different spheroid
and datum.
m.gc2ll -> converts geocentric to geographic coordinates
m.ll2gc -> converts geographic coordinates to geocentric coordinates
m.ll2u, m.u2ll -> convertes geographic coords to UTM and vice versa
m.proj, m.geo etc.

All those modules require the coordinates in command-line, so they are
useful only for calculating reference points etc. But the code is
included in grass. 

It would be much work to change all modules for projecting/reprojecting
(v.proj, r.proj) to include datum shift because the user must specify
the datum for input/output and the module must use a changed API. But in
theory the code is already included.

Beste Gruesse,


Andreas Lange, 65187 Wiesbaden, Germany, Tel. +49 611 807850
Andreas.Lange at Rhein-Main.de, A.C.Lange at GMX.net
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