[Proj] Google Mercator Projection

[zagot]

i have copied from the official note definition
cs2cs +proj=latlong +datum=WGS84 \
    +to +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 \
                   +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs

ok.. however with the datum now the result is different:-626172.1357121646, 
6853979.3008168638

but if I transform this point using CS2CS tool, as follows:

echo -5.625 52.4827802220782 | cs2cs -f “%.10f” +init=epsg:4326 +to
+init=epsg:3857

then I'll get the following result instead:

(-626172.1357121646, 6887893.4928337997)

why this difference? what's is correct?

thanks

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