[Proj] towgs84 values for EPSG:27700

[support.mn at elisanet.fi]

Hello,

Spatial Reference Org

http://spatialreference.org/ref/epsg/27700/
http://spatialreference.org/ref/epsg/27700/proj4/

gives:

+proj=tmerc
+lat_0=49
+lon_0=-2
+k=0.9996012717
+x_0=400000
+y_0=-100000
+ellps=airy
+datum=OSGB36
+units=m
+no_defs 

which is the OSGB 1936 / British National Grid. The
OSGB36 datum is defined differently in both
applications. There are several different definitions
for the OSGB36 and more are added as people need
definitions for different purpose.

So the reason for the difference is that the OSGB36
datum is not any standard value.

Regards: Janne.

------------------------------------------------------------------------

Hermann Peifer [peifer at gmx.eu] kirjoitti: 
> Hi,
> 
> I am working with PROJ Rel. 4.7.1 and GDAL/OGR from trunk.
> 
> testepsg and gdalsrsinfo tell me that the towgs84 values for EPSG:27700 
> are 375,-111,431,0,0,0,0 whereas cs2cs reports for the same EPSG code: 
> 446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894
> 
> Is this expected behaviour?
> 
> Hermann
> 
> 
> $ testepsg epsg:27700 | grep PROJ.4
> PROJ.4 rendering of [epsg:27700] = +proj=tmerc +lat_0=49 +lon_0=-2 
> +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy 
> +towgs84=375,-111,431,0,0,0,0 +units=m +no_defs
> 
> 
> $ gdalsrsinfo epsg:27700 | grep PROJ.4
> PROJ.4 : '+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 
> +y_0=-100000 +ellps=airy +towgs84=375,-111,431,0,0,0,0 +units=m +no_defs '
> 
> 
> $ cs2cs -v +init=epsg:27700 +to +proj=latlong +datum=WGS84
> # ---- From Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +init=epsg:27700 +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 
> +x_0=400000
> # +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs
> # +towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894
> #--- following specified but NOT used
> # +ellps=airy
> # ---- To Coordinate System ----
> #Lat/long (Geodetic alias)
> #
> # +proj=latlong +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
> ^C
> 
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