[Proj] Incorrect translations to the Gauss-Boaga system

[Frank Warmerdam]

Gino Lucrezi wrote:
> I am currently using version 4.4.8 of the proj4 libraries. I mostly use them from within PostGIS, but I get the same results calling them on a stand-alone basis.
> 
> One widely used coordinate system here in Italy is Gauss-Boaga,
> corresponding to EPSG codes 26591 and 26592.
> 
> So, I often have to convert between Gauss Boaga and UTM 33 (epsg:23033)
> 
> Here are the results for two known points:
> cs2cs +init=epsg:26592 +to +init=epsg:23033 -v <<EOF
> 2413000.0000 4669000.0
> 2389238.48 4690935.27
> EOF
> 
> # ---- From Coordinate System ----
> #Transverse Mercator
> #       Cyl, Sph&Ell
> # +init=epsg:26592 +proj=tmerc +lat_0=0 +lon_0=2.54766666666667 
> +k=0.999600
> # +x_0=2520000 +y_0=0 +ellps=intl +pm=rome +units=m +no_defs
> # ---- To Coordinate System ----
> #Universal Transverse Mercator (UTM)
> #       Cyl, Sph
> #       zone= south
> # +init=epsg:23033 +proj=utm +zone=33 +ellps=intl +units=m +no_defs
> 393000.00       4669000.00 0.00
> 369238.48       4690935.27 0.00
> 
> The first point I could check on a map, and it should have been
> 393070 4669190 I used a good ruler, and I am sure I got this point with an error smaller than 10 metres.
> 
> The second point was converted using another (interactive) program, which is usually regarded as authoritative, and it resulted in 369301 4691118
> 
> In either case, the resulting point is about 200 metres off from the correct result.
> 
> I didn't modify the epsg file in any way.

Gino,

It seems that there is no datum shifting information available for either
coordinate system.  The Guass Boaga is using the Monte Mario datum while the
UTM coordinate system is using ED50.

I *think* the ED50 is relatively close to WGS84, but you may need to find
some approximation for the datum shift needed for ED50.  You might even be
able to derive a reasonable approximation from your observed errors.

PS. The proj mailing list isn't really working yet.  You and I are currently
the only subscribes.  Hopefully by late monday it will be back fully populated.

Best regards,

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