[Proj] Google maps <3857> datum

[Alain Orsoni]

Hi,

I have to implement the 3857 projection in a software.
My problem is the following : it seems that when a point on earth having long1 and lat1 coordinates in WGS84 datum is projection in this direct mercator projection based on a spheric ellipsoid (I could say directly a sphere), this point is considered by proj4 to have the same lon1 and lat1 coordinates on the sphere. After the transform to Mercator occurs.
I noticed this because when I transform from WGS84 datum to Google sphere (the +a=6378137 +b=6378137 sphere given in 3857 description) I get lon2 and lat2 coordinates for our point where lon2=lon1 and lat2 not equal lat1 (except for lat=0 or 90). This is normal but doesn't seem to be the process applied by proj4.

When I consider lon1 and lat1 being the coordinates of the point on the sphere (saying that I don"t transform from WGS84 to the sphere), I get the same Marcator coordinates than the ones I get through proj4 transform from WGS84 to 3857.

Could someone confirm or document this particular application of the transform done for 3857 datum.

Hopping my problem is clearly expressed, and waiting to ear from you.

Alain ORSONI
Institut Géographique National, FRANCE