[OSRS-PROJ] cs2cs question: Gauss-Boaga

[Markus Neteler]

On Thu, May 30, 2002 at 09:05:23AM -0400, Frank Warmerdam wrote:
> Markus Neteler wrote:
> > Dear list,
> > 
> > sorry for a sort of "newbie" question: Today I tried to
> > investigate if cs2cs does datum transformation for
> > the Italian Gauss-Boaga system:
> > 
> > [btw: there is no +towgs84 present for the Gauss-Boaga related 
> > EPSG code 26591 in nad/epsg, any reason for that?]
> 
> Markus,
> 
> The code that looks up datums in the EPSG database and tries to derive
> a towgs84 value for them fails if their are no ways to translate the
> datum directly to wgs84 or if there is more than one way.  I have looked
> into the datum in question but I imagine there is more than one transformation
> available (presumably applicable to different areas) so the code has no way to
> know which is most apppropriate.
> 
> > I struggled here: While this is doing calculations:
[...]
> 
> It would appear the problem is use of +datum=wgs84 instead of +datum=WGS84.
> To my mind it would be better if all proj comparisons to internal names was
> case insensitive, but this does not appear to be the established practice.

ah, I see.

> > ...although both should be the same (or not?). Beside that I would like
> > to know if I am right in my assumption, that this "cs2cs" with additional
> > +towgs84 parameter is the way to reproject from lat/long to Gauss-Boaga
> > *with* datum transformation from WGS84 to Rome40. Say, at least with
> > less error than using the 'proj' command?
> 
> Your approach seems good, though the transformation will only be accurate
> as the +towgs84 values you gave approximates the datum shift in the area you
> are working.

I started an investigation. So far I have 2m E-W precision and 9m N-S
precision for known points in our area (Northern Italy). As soon I
get more points elsewhere in Italy, I'll post a summary. It looks
promising since the 'proj' is 25m/70m shifted.

More soon,

 Markus
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