[Proj] cs2cs how to use

[José Alberto Gonçalves]

Frank Warmerdam wrote:

> As Cliff mentions, the above does not include information on the datum
> shift to be applied between WGS84 and "datum 73".  I believe the 
> following
> commandline would use the above parameters and Cliff's suggested datum
> shift.
>
> cs2cs +proj=wgs84 +datum=WGS84 +to \
>       +proj=tmerc +k=1 +lon_0=-8.131906111111112 +lat_0=39d40 \
>                     +x_0=180.598 +y_0=-86.66 \
>                     +ellps=intl +towgs84=223.116,-106.825,-36.87
>
> As Cliff mentions your false easting and northing seem pretty unusual.
> It is also possible the signs on the towgs84 parameters need to be
> changed.
>
> Best regards,

The shifts x_0 and y_0 are correct (or nearly; y_0 is 86.99), although 
they are in fact unusual. Portugal had an older datum, called "Datum 
Lisboa", which was replaced by "Datum 73" in the 70's. The projection 
was made with the same parameters and the false easting and northing  
would account for the datum difference, yielding cartographic 
coordinates very similar to the older ones. Confusions and mistakes are 
frequent and probably a better choice would have been to choose a 
different central point (lon_0, lat_0).

Anyway at the moment the Portuguese Geographic Institute is in the 
process of adopting a new map projection, based on ETRS-89, for all the 
cartography.

About the command line: I checked with known coordinates and it seems to 
be correct (but with changed signs). Also the syntax should be "cs2cs 
+proj=latlong ..."

Regards

José A. Gonçalves


---------------------------------------
José Alberto Gonçalves
Faculdade de Ciências, Universidade do Porto
Rua Campo Alegre, 687
P-4169-007 Porto
Portugal
Email: jagoncal at fc.up.pt
Phone: 220100837