[Proj] Re: Gridshifts give slightly wrong results

[Uwe Schmitz]

> 

<comment>
   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...
</comment>

Paul,

> 
> > 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? 
> http://bugzilla.remotesensing.org/show_bug.cgi?id=368
> Particularly Frank's comments right at the end?
> 
Yes it's interesting, but unfortunately doesn't really help :-(


Thank you for responding
Uwe