[R-sig-Geo] spTransform: wrong results??

[Agustin Lobo]

Regarding the towgs84 parameters, I've found
http://www.mail-archive.com/gdal-dev@lists.osgeo.org/msg02921.html
and from there
http://earth-info.nga.mil/GandG/coordsys/onlinedatum/CountryEuropeTable.html

I'll test these values for my ED50, compare to GlobalMapper results
and let the list know.

Where you say:

2009/7/10 Roger Bivand <Roger.Bivand at nhh.no>:
> On Fri, 10 Jul 2009, Agustin Lobo wrote:
>
>> I'll try and let you know. For the future, no way to compile the
>> towgs84 parameter so that could be automatically used
>> by spTransform? This must be what programs like GlobalMapper do, as
>> you just define input and output datum.
>
> No way. Choosing the 3 or 7 parameter datum transforms has to be the
> responsibility of the user when there is no unequivocal authority. Having a
> known ground control point (a GPS reading from a known point that has a
> known CRS) can be used to see which CRS settings let you reproduce it. But
> with legacy maps, this isn't so easy. GlobalMapper must I suppose be making
> choices and using authorities that it acknowledges, but reading G&D does not
> make me optimistic.

I disagree, you could just use a default table of values letting the
user enter his/her own
in case he/she considers that those of the table are not appropriate
(or do not exist). That's
why default values exist in R.  Let me know if can help.

Agus

Dr. Agustin Lobo
Institut de Ciencies de la Terra "Jaume Almera" (CSIC)
Lluis Sole Sabaris s/n
08028 Barcelona
Spain
Tel. 34 934095410
Fax. 34 934110012
e-mail Agustin.Lobo at ija.csic.es
http://www.ija.csic.es/gt/obster