[OSRS-PROJ] Projection parameters for Italian old IGM?

Clifford J Mugnier cjmce at lsu.edu
Tue Apr 29 11:38:49 EDT 2003

The systems being referred to are the old Gauss-Boaga Transverse Mercator
Grid and it is based on the Monte Mario Datum of 1940.  Rome is the Prime
Meridian from whence the two zones are measured.  The scale factor at
origin is 0.9996, but is NOT UTM.  Furthermore, it is oft confused with the
military system used in Italy as specified by NATO which is European Datum
of 1950 with UTM measured from Greenwich.  The latitudes and longitudes are
different - different datums in addition to the grids.

Gauss-Boaga is a specific truncation and has characteristic distortion that
is masked by inadvertent substitution with Gauss-Kruger.  For
survey-quality work it DOES make a difference.  For 1:50,000-scale GIS work
it does not make a difference on land - it does offshore.

The datum shift generally quoted is that found in the NIMA TR 8350.2 (WGS84
Datum) which is ONLY for EU50 to WGS84.  The derivative EPSG database gives
no further guidance.  On occasion the Istituto Geografico Militare in
Firenze (Florence) will give out a 7-parameter transformation for a local
area of Italy, but be careful as they use left-handed rotations.

Most cadastral mapping is still based on the aphylactic Cassini-Soldner
Grids.  Italy has only 540 of them.

So now you know.

Cliff Mugnier

The biggest problem for me is the lack of complete information.

1. Italian IGM series? Old or new?  Are they based on UTM?

2. What are the specific geographic coordinates being projected
and the x-y of the results.  What do you feel they should be?

3. Correcting the longitude of Rome by hand?  Mighty big area to define
by one point.

4. The "PS" specification is incompleat and possibly redundant but
Sorry, I can't read Italian so my "translation" may be
suspect but there seems to be two central
meridians.  I would guess that "Ovest" and "Est" are west
and east but no mention of north or south.  Similarly, two
false westings and eastings?  The description seems incomplete.

Lastly, the zone number of 32 is a little suspect and I would have
chosen zone=33 which would put Roma closer to the central meridian.
Lon 12 is the dividing line between zone 32 and 33.

The following script:
./lproj +proj=utm +zone=32 +ellps=intl <<EOF
12d37E 41d52N
./lproj +proj=utm +zone=33 +ellps=intl <<EOF
12d37E 41d52N

produces the results:

800183.96       4641382.04
302191.13       4637801.10

On Tue, 2003-04-29 at 03:45, Bud P. Bruegger wrote:
> Hello everyone,
> I hope someone can help me with what should probably be an easy
> question.
> For an old series Italian IGM 1:25,000 map, I try to project the map
> corners (whose lat/long is known) to UTM (as used in the map).
> I naively tried the following:
> proj +proj=utm +zone=32 +ellps=intl
> and corrected the longitude by hand for the meridian of Rome
> (12d27'8.4"E).  But the UTM coordinates that come out don't seem to
> be correct.  Any idea what I'm doing wrong and how the projection
> should be defined?
> Also, if anyone has a numeric example handy for testing...
> Many thanks for any help I can get!
> --bud
> PS.  I found the following information that is probably for the new
> series IGM that may shed more light on my problem:
>  Transverse_Mercator (ARC/INFO keyword: TRANSVERSE)
>  Unità di Distanza Planare: METRI (ARC/INFO keyword: METERS)
>  Fattore di scala al meridiano centrale: 0.99960
>  Longitudine del meridiano centrale: -3°27'08.4'' per il fuso Ovest /
>                                      +2°32'51.6'' per il fuso Est
>  Falso est: 1500000 per il fuso Ovest /
>             2520000 per il fuso Est
>  Nome del Datum: MOD
>  Nome dell'Ellissoide: INTERNATIONAL1909
> /-----------------------------------------------------------------
> | Bud P. Bruegger, Ph.D.
> | Sistema (www.sistema.it)
> | Via U. Bassi, 54
> | 58100 Grosseto, Italy
> | +39-0564-411682 (voice and fax)
> \-----------------------------------------------------------------
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