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I added a shapefile as a vector layer in qGIS 1.7 so that I could join in additional attribute data (not geodata) from another DBF file. The Shapefile that I added has this CRS (as far as I can tell using ogrinfo):

PROJCS["WGS_1984_Web_Mercator",
GEOGCS["GCS_WGS_1984_Major_Auxiliary_Sphere",
    DATUM["WGS_1984_Major_Auxiliary_Sphere",
        SPHEROID["WGS_1984_Major_Auxiliary_Sphere",6378137.0,0.0]],
    PRIMEM["Greenwich",0.0],
    UNIT["Degree",0.0174532925199433]],
PROJECTION["Mercator_1SP"],
PARAMETER["False_Easting",0.0],
PARAMETER["False_Northing",0.0],
PARAMETER["Central_Meridian",0.0],
PARAMETER["latitude_of_origin",0.0],
UNIT["Meter",1.0]]

After joining in the extra attribute data in, I exported the original shapefile by right-clicking on the layer and choosing 'Save as.' I kept the CRS as 'Original CRS' in the 'Save vector layer as ...' dialog. However, the exported shapefile has the following CRS defined:

PROJCS["Mercator",
GEOGCS["GCS_unnamed ellipse",
    DATUM["unknown",
        SPHEROID["Unknown",6378137,0]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295]],
PROJECTION["Mercator_1SP"],
PARAMETER["central_meridian",0],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["Meter",1],
PARAMETER["latitude_of_origin",0.0]]

Are the CRS' equivalent? If so, does that mean that some of the strings in the WKS definition of the PRJ file are just identifiers? If not, how can I make qGIS preserve the CRS of the original shapefile when exporting it?

Thanks in advance for any help anyone can offer.

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3  
You can simply copy and rename the original .prj file. – underdark♦ Jul 4 '11 at 20:15
@underdark is there an easy way to tell if the geodata was reprojected? – Geoffrey Hing Jul 5 '11 at 18:43

2 Answers

up vote 4 down vote accepted

Since the numbers are all the same I would say they are the same projection. Technically the web mercator projections do use an unknown ellipsoid so I think that that is right.

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I agree, I think "Aux Web Mercator" just isn't recognized by all the software yet. As so in my Autocad Map. – Brad Nesom Jul 5 '11 at 13:44
@iant, @Brad Nesom, so for the Shapefile projection WKT, do the numbers matter or the names of projection, datum, etc? – Geoffrey Hing Jul 5 '11 at 18:44
yes and yes, the numbers are of course the part that gets applied. The name is usually how the software recognizes. Unless it is EPSG:then the numeric name is how it recognizes. – Brad Nesom Jul 5 '11 at 19:32
Just the numbers matter inside the file – iant♦ Jul 5 '11 at 20:34
up vote 1 down vote

I've seen the same issue in recent versions. 1.6 seems to be ok, so if you are doing a lot of this type of work, just use 1.6 until they get this fixed up!

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