[GRASS-user] Custom Projection Problems

[Paul Kelly]

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