Andrea Aime wrote:
> Mauro Bartolomeoli ha scritto:
>> We are working on an Italy Cadastre importer, and we are thinking to
>> use udig to view the produced data, so we are going to need a CRS to
>> attach to Cassini-Soldner projected data.
>> It's not widely tested and not very accurate (the error is in the
>> order of meters, compared to commercial software we use to convert to
>> the same data).
>
> Interesting. Wondering if it's the math or the towgs84 params.
> Italy has the very sorry situation (not sure how common in other
> countries) that in order to have sub-meter precision on a local area
> transformation you need the TOWGS84 params _for that area_, usually
> contained in a document that can be bought by the militars.
For the math, I think we could investigate if the formulas used by PROJ
are different from those found on the "OGP Surveying and Positioning
Guidance Note number 7, part 2 - August 2006" you can find on the EPSG
site (if I have enough time I'll try to compare the two set of formulas).
>
> The 3 towgs84 params contained in the epsg database do not allow
> for a precision better than 4 meters, and unfortunately geotools
> is just picking one of them, I believe the one valid for Sardinia,
> so if you're reprojecting on the continental part, you'll
> get even bigger errors. (I'm reporting by memory, so I may
> be completely off on the magnitude of the errors or about the
> chosen towgs84 set of params).
I think this could one of the inaccuracy causes (maybe not the only
one). The points I used for testing are in the continental part (Urbino
zone), quite far from Sardinia, so the several meters error is probably
due to these parameters.
The test I did uses the following CRS:
PROJCS["unnamed",
GEOGCS["unknown",
DATUM["unknown",
SPHEROID["Bessel 1841", 6377397.155,
299.1528128,AUTHORITY["EPSG","7004"]]],
PRIMEM["Greenwich", 0.0],
UNIT["degree", 0.0174532925199433],
AXIS["Longitude", EAST],
AXIS["Latitude", NORTH]],
PROJECTION["Cassini_Soldner"],
PARAMETER["latitude_of_origin", 43.72513523],
PARAMETER["central_meridian", 12.63626379],
PARAMETER["false_easting", 0],
PARAMETER["false_northing", 0],
UNIT["m", 1.0],
AXIS["x", EAST],
AXIS["y", NORTH]]
Do you think it's correct? The latitude of origin and central meridian
are the WGS84 coordinates of the Urbino reference point (got from
www.fiduciali.it).
Mauro
>
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