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.
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?