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I would like to transform Barcelona map coordinates to Latitude, Longitude with Proj.4. I'm using EPSG number 23031 and the parameters are: +proj=utm +zone=31 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs

However, the Y coordinates in the original maps are shifted by -4000000.

For example:
x=438120.5406, y=568084.9572 instead of x=438120.5406, y=4568084.9572.

I realized that I have to modify the +towgs84 values, but I can't find the exact values.

What is the correct method?

  • Can you please add to your question one of the input latitude, longitude coordinates? PROJ.4 is widely used, and it's unlikely that it's not working. I suspect the input data. Are the input values in WGS84 or ED50? – mkennedy Sep 22 '15 at 17:01
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+x_0 and +y_0 parameters can be used to set false origin. See: https://trac.osgeo.org/proj/wiki/GenParms#FalseEastingNorthing

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    I tried +proj=utm +zone=31 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs +y_0=4000000, but the results are not correct: – Valeria Sep 22 '15 at 11:59
  • I tried +proj=utm +zone=31 +ellps=intl +towgs84=-87,-98,-121,0,0,0,0 +units=m +no_defs +y_0=4000000, but the results are not correct: lat=5.250163071319898, lon=2.3653349246780837. The lat value should be around 41. (I have already tried it before my question). In the trac.osgeo.org/proj/wiki/GenParms#FalseEastingNorthing I read: "Some coordinate systems (such as UTM) have implicit false easting and northing values. " I suppose the +towgs84=-87,-98,-121,0,0,0,0 values should be modified, with "implicit" way. – Valeria Sep 22 '15 at 12:07
  • +towgs are to make more precise the transformation, try without +towgs. But the three non-zero parameters in the +towgs are x,y,z offsets. These cannot cause a big change in the coordinates. – Zoltan Sep 24 '15 at 7:45

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