[Proj] Question about Italian conversion

Frank Warmerdam warmerdam at pobox.com
Sat Dec 29 14:54:04 EST 2007

massimo at studiodemarchi.net wrote:
>>> What am I doing wrong?
>> Trying to follow the procedure with independent software, I get the
>> impression that you didn't transform the datum from Hayford to WGS84.
>> You simply inverse projected from x,y to lon,lat with the proper transverse
>> mercator projection.
>> With the Roma40 transform parameters found at <http://crs.bkg.bund.de>
>> I arrive at lat,lon:
>> 45d 06m 55.252s,  10d 47m 39.415s
>> assuming a position vector rotation model (ISO) and an initial height of 0.
>> The proj/epsg definitions you mention don't contain any datum transform
>> instructions.

> Thanks for your answer. I tried to do a coordinate transform using the -s_srs
> EPSG:3003 (or 26591) and -t_srs EPSG:4326 parameters. How can I add the
> necessary datum transform? I checked the documentation amd the forum, but
> didn't find a clue.


I followed Oscar's link to http://crs.bkg.bund.de but did not see any easily
identifiable datum transformation parameters for the Monte Mario datum.
However, if you can find some appropriate ones they can be specified in the
PROJ.4 string.  EPSG:3003 expands to:

+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl
+units=m +no_defs

To add datum shift parameters you might use a string like:

+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl
+units=m +towgs84=100,50,25 +no_defs

Insert whatever towgs84 parameter you find are appropriate to the datum.

Best regards,

I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

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