[Proj] Re: Gridshifts give slightly wrong results
uwe.schmitz at lverma.nrw.de
Tue Mar 13 07:19:18 EST 2007
I've send the following response to Paul and the list
but it doen't show up here, because I'm not a valid
member of the list, I think.
Now I post it through gmane, so it doesn't get lost...
> > Hello,
> > I've upgraded from proj-4.4.7 to 4.5.0 and
> > notice a slight difference of my transformation
> > results when grid shifts are involved.
> > Executing the command:
> > cs2cs -f "%.12f" \
> > +proj=latlong +ellps=bessel +nadgrids=./BETA2007.gsb \
> > +to \
> > +proj=latlong +ellps=GRS80
> > 7.483333333333E 53.500000000000N
> > results in
> > 7.482506019176 53.498461143331 0.000000000000 # 4.4.7 correct!
> > 7.482506019176 53.498461144236 0.000067942776 # 4.5.0
> > Through debugging I found out, that now (V4.5.0) there
> > is an additional step converting the geodetic coordinates
> > to geocentric an back again. This is caused by the different
> > ellipsoids.
> What happens if you add +towgs84=0,0,0 to the second projection
> definition? That might stop it trying to do an ellipsoid
> conversion. Have
> to admit though that I am quite hazy on how this works in PROJ.
thank you for the suggestion, but there is no effect on the
result. If I understand the code correctly, the geodetic to
geocentric conversion takes place, if two distinct non-wgs84
ellipsoids are involved?!
> > IMO, this is a bug, because the datum shift is completely
> > done by adding the shift values.
> Is this relevant?
> Particularly Frank's comments right at the end?
Yes it's interesting, but unfortunately doesn't really help :-(
Thank you for responding
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