On Tue, 2 Mar 2010, Casey Vandenberg wrote:
I am trying to create a new project using a custom proj4 string. The
parameters I specify are '+proj=utm +zone=6 +ellps=clark66
+towgs84=-5,135,172'. After creating the project however, and verifying the
projection settings using *g.proj -j -f -p* I get the following info:
-PROJ_INFO-------------------------------------------------
name : Universal Transverse Mercator
proj : utm
ellps : wgs84
zone : 6
towgs84 : -5,135,172,0,0,0,0
no_defs : defined
-PROJ_UNITS------------------------------------------------
unit : Meter
units : Meters
meters : 1
+proj=utm +no_defs +zone=6 +a=6378137 +rf=298.257223563
+towgs84=-5,135,172,0,0,0,0 +to_meter=1
Why is it not maintaining the Clarke 1866 Elliposid?
clark66 is the GRASS abbreviation for that ellipsoid. When you define a
co-ordinate system using a PROJ.4 string you need to use the PROJ.4
abbreviation, which is clrk66. So using +ellps=clrk66 should do what you
want.
Alternatively, if I specify a proj 4 string as follows: '+proj=utm +zone=6
+datum=nad27 +towgs84=-5,135,172' and verify the created projection using
g.proj, I get the following output:
-PROJ_INFO-------------------------------------------------
name : Universal Transverse Mercator
proj : utm
datum : nad27
ellps : clark66
zone : 6
no_defs : defined
-PROJ_UNITS------------------------------------------------
unit : meter
units : meters
meters : 1
+proj=utm +no_defs +zone=6 +a=6378206.4 +rf=294.9786982
+towgs84=-22.000,157.000,176.000 +to_meter=1
Here it is maintaining the correct ellipsoid, but it does not maintain the
towgs84 datum transformation parameters.
I'm not 100% sure of this (since the string is being interpreted by GDAL
and not GRASS nor PROJ) but specifying +datum overrides both +ellps and
+towgs84. You are correctly getting Clarke 66 since it is associated with
NAD27, but specifying NAD27 seems to be overriding your towgs84
parameters. I suppose that isn't ideal - but seeing you can work around it
by not specifying the datum, I guess it's OK.
Paul
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