[Proj] towgs84 functionality

[Frank Warmerdam]

Jeff Huber wrote:
> I'm new to this, so here is a, hopefully, simple question.
> Why don't I get a shift when I use towgs84 in the following:
> 
> invproj +proj=utm +ellps=krass +x_0=500000 +y_0=0 +lat_0=0 +lon_0=75 
> +k=.9996
> 701176.9 7550769.7
> 79d48'58.181"E  67d59'57.701"N
> 
> Expected to get +79.8161616,     +67.9993614  so that one is OK
> 
> invproj +proj=tmerc +ellps=krass +towgs84=28,-130,-95 +x_0=500000 +y_0=0 
> +lat_0=0 +lon_0=75 +k=.9996
> 701176.9 7550769.7
> 79d48'58.181"E  67d59'57.701"N
> 
> Expected to get +79.8149530,     +68.0000817  but did I get same answer 
> as above

Jeff,

The invproj command converts between projected coordinates and geographic
coordinates *on the same datum*.  The towgs84 value is only used when you
change datums.   You can do that with the cs2cs command.

eg.

cs2cs +proj=utm +ellps=krass +x_0=500000 +y_0=0 +lat_0=0 +lon_0=75 \
       +k=.9996 +towgs84=28,-130,-95 \
       +to +proj=latlong +datum=WGS84

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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