Take the 2-minute tour ×
Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. It's 100% free, no registration required.

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.

share|improve this question
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
add comment

2 Answers 2

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.

share|improve this answer
    
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
add comment

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!

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.