1

I am working on QGIS 2.2.0.

The PRS I am working on is Monte Mario / Italy zone 1 (EPSG:3003), with the following parameters:

PROJCS["Monte Mario / Italy zone 1",
GEOGCS["Monte Mario",
    DATUM["Monte_Mario",
        SPHEROID["International 1924",6378388,297,
            AUTHORITY["EPSG","7022"]],
        AUTHORITY["EPSG","6265"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.01745329251994328,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4265"]],
UNIT["metre",1,
    AUTHORITY["EPSG","9001"]],
PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",9],
PARAMETER["scale_factor",0.9996],
PARAMETER["false_easting",1500000],
PARAMETER["false_northing",0],
AUTHORITY["EPSG","3003"],
AXIS["X",EAST],
AXIS["Y",NORTH]]

A dataset I need to use on the above PRS is in Monte_Mario_Italy_1 (EPSG:102091), with the following parameters:

PROJCS["Monte_Mario_Italy_1",
GEOGCS["GCS_Monte_Mario",
    DATUM["Monte_Mario",
        SPHEROID["International_1924",6378388,297]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295]],
PROJECTION["Transverse_Mercator"],
PARAMETER["False_Easting",1500000],
PARAMETER["False_Northing",0],
PARAMETER["Central_Meridian",9],
PARAMETER["Scale_Factor",0.9996],
PARAMETER["Latitude_Of_Origin",0],
UNIT["Meter",1],
AUTHORITY["EPSG","102091"]]

Is there any difference between the two PRS? (I see a difference in decimals in parameter UNIT["Degree", ...]).

In other words, do I need a re-projection or datum transformation to analyse the data together (e.g. buffering, calculating distances etc) ?

Thanks in advance. Alessandro

  • Looks same, You can test it in QGIS if it has both srid in system, set onfly projection off , load data and set to correct proj, then see if it looks correct – simplexio Jun 19 '14 at 8:43
2

There is a difference in the proj definition:

EPSG:3003

+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl +towgs84=-104.1,-49.1,-9.9,0.971,-2.917,0.714,-11.68 +units=m +no_defs

EPSG:102091

+proj=tmerc +lat_0=0 +lon_0=9 +k=0.9996 +x_0=1500000 +y_0=0 +ellps=intl +units=m +no_defs

The first one has a datum shift (+towgs84), the second not.

The second is not a defintion by EPSG, but invented by ESRI. They do not write datum shifts into their projection definitions, but keep them separate.

In any case, I suggest to use the first one inside QGIS. But do not reproject the second to the first, instead use Set CRS for Layer, then save to disk under another name.

  • Thanks for that. I have to correct this (sorry I am a newbie of course): the EPSG:102091 is AUTOMATICALLY SELECTED by QGIS. My data is a raster dataset (DTM, Altitude) with the following associated prj.adf: Projection: TRANSVERSE -- Datum: MOD -- Spheroid: INT1909 -- Units: METERS -- Zunits: NO -- Xshift: 0.0 -- Yshift: 0.0 -- Parameters: 0.9996 -- /* scale factor at central meridian: 9 0 0.0 /* longitude of central meridian -- 0 0 0.0 /* latitude of origin -- 1500000.0 /* false easting (meters) -- 0.0 /* false northing (meters). Does this correctly correspond to EPSG:102091? – alemagoo Jun 19 '14 at 12:03
  • This corresponds to both, as no datum shift is specified. I would nethertheless use EPSG:3003. – AndreJ Jun 19 '14 at 13:56
  • 102091 is Esri's. We added that CRS before it was added to the EPSG registry. Disclosure: Esri employee and member of EPSG registry committee. – mkennedy Jun 19 '14 at 21:53
  • @mkennedy: I still favour to call it ESRI:102091, not EPSG:102091. Would ESRI have anything against it? – AndreJ Jun 20 '14 at 3:52
  • @AndreJoost No, of course not. In fact, 102091 should never be identified as EPSG's, but a lot of software (including some of ours) doesn't differentiate. – mkennedy Jun 20 '14 at 17:28
0

You can test the transformation between EPSG:3003 and EPSG:102091 in an unambiguous way with the GDAL tool "gdaltransform" http://www.gdal.org/gdaltransform.html

GDAL_dev>gdaltransform -s_srs epsg:3003 -t_srs epsg:102091
100000 100000
100000.113807246 99999.9986070729 0

Test with your own coordinates. There seems to be a little difference and you are the only one who knows if 10 cm accuracy is good enough.

  • I have a feeling that if you set the target to epsg:3003, it would also show close to the same output--I'm surprised that there's that much difference (just from the different DD/rad value? Must test it). Disclosure: Esri employee – mkennedy Jun 19 '14 at 21:48
  • Oh, the transformation may be occurring as well using the towgs84 values. – mkennedy Jun 19 '14 at 21:55

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