No subject

is that +nadgrids takes precedence over +towgs84, at least=20
in Proj4 (perhaps not in OGRSpatialReference etc.). =20

That would mean that in your definition of CRS 902056,
where you use both +towgs84=3D674.374,... and +nadgrids=3D at null,=20
the +towgs84 would be ignored and +nadgrids=3D at null would be=20
used, which is the same thing (as far as I understand) as=20
saying that the longitude/latitude in CRS 902056 is the same=20
as longitude/latitude in WGS84. =20

But that's not your intention, is it? =20
The datum of EPSG:2056 is CH1903+, and the datum of=20
EPSG:21781 is CH1903 (without the +). =20

I thought CH1903+ was some kind of improved version of CH1903,
with less distortion, although I don't know if your gridshift=20
file is intended for CH1903 or for CH1903+.

Anyway, I don't the see the reason for using +nadgrids=3D at null
in your definition of CRS 902056.  I would have expected either
the +towgs84 parameter or the +nadgrids=3Dchenyx06.gsb.

Best regards,

--
Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 G=F6teborg
Visitors: Sankt Eriksgatan 5
SWEDEN
mikael.rittri at carmenta.com
www.carmenta.com=20

________________________________

From: proj-bounces at lists.maptools.org =
[mailto:proj-bounces at lists.maptools.org] On Behalf Of Ziegler Stefan
Sent: den 19 januari 2009 17:00
To: proj
Subject: [Proj] Problems using grid shift files


Dear List

we have some problems using grid shift files. We are using a grid shift =
file which was developed by the Swiss Federal Office of Topography [1]. =
We just duplicated epsg 21781 (to 921781) and 2056 (to 902056) in the =
epsg file and added "+nadgrids=3Dchenyx06.gsb" and "+nadgrids=3D at null":

21781:=20
+proj=3Dsomerc +lat_0=3D46d57'08.66"N +lon_0=3D7d26'22.50"E =
+ellps=3Dbessel +x_0=3D600000 +y_0=3D200000 =
+towgs84=3D674.374,15.056,405.346 +units=3Dm +k_0=3D1 +no_defs=20

2056:
+proj=3Dsomerc +lat_0=3D46d57'08.66"N +lon_0=3D7d26'22.50"E =
+ellps=3Dbessel +x_0=3D2600000 +y_0=3D1200000 =
+towgs84=3D674.374,15.056,405.346 +units=3Dm +k_0=3D1 +no_defs

921781:=20
+proj=3Dsomerc +lat_0=3D46d57'08.66"N +lon_0=3D7d26'22.50"E =
+ellps=3Dbessel +x_0=3D600000 +y_0=3D200000 =
+towgs84=3D674.374,15.056,405.346 +units=3Dm +nadgrids=3Dchenyx06.gsb =
+k_0=3D1 +no_defs=20

902056:
+proj=3Dsomerc +lat_0=3D46d57'08.66"N +lon_0=3D7d26'22.50"E =
+ellps=3Dbessel +x_0=3D2600000 +y_0=3D1200000 =
+towgs84=3D674.374,15.056,405.346 +units=3Dm +nadgrids=3D at null +k_0=3D1 =
+no_defs

4326:
+proj=3Dlonglat +ellps=3DWGS84 +datum=3DWGS84 +no_defs

904326:
+proj=3Dlonglat +ellps=3DWGS84 +datum=3DWGS84 +nadgrids=3D at null

Here are the results:

21781 -> 2056: correct!
921781 -> 90256: correct (with grid shift file)!

but:
921781 -> 2056: wrong!
921781 -> 4326: wrong!
921781 -> 904326: wrong!

Is the grid shift only applied if the destination definiton has a =
"+nadgrids=3D at null"? But then why is the transformation to 904326 not =
correct? Has this something to do with this bug report [2]? Or I'm just =
mixing up some parameters? Same results with Proj 4.5 and 4.6.1. Any =
help is appreciated.

regards
Stefan

[1]: =
http://www.swisstopo.admin.ch/internet/swisstopo/en/home/topics/survey/lv=
03-lv95/chenyx06/distortion_grids.html
[2]: http://trac.osgeo.org/proj/ticket/22

Mit freundlichem Gruss
Stefan Ziegler
Leiter Aufsicht

Kanton Solothurn
Bau- und Justizdepartement
Amt f=FCr Geoinformation
R=F6tistrasse 4
4501 Solothurn
Telefon 032 627 75 96
Telefax 032 627 75 98
stefan.ziegler at bd.so.ch
http://www.so.ch