[Proj] Google maps <3857> datum
Noel Zinn (cc)
ndzinn at comcast.net
Mon Jan 27 08:20:59 EST 2014
There is no ellipsoid-to-sphere transformation involved in the EPSG 3857
(Web Mercator) map projection. EPSG 3857 is a conversion from WGS84
ellipsoidal (lat/lon) to grid Easting/Northing using spherical Mercator
equations, but there is no intervening sphere. It's just another map
projection (though it has its own distortions that are not the same as
either the ellipsoidal Mercator or the Spherical Mercator). It is
completely reversible LL <=> EN.
More about the distortions at:
Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
From: Alain Orsoni
Sent: Monday, January 27, 2014 3:47 AM
To: proj at lists.maptools.org
Subject: [Proj] Google maps datum
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.
Institut Géographique National, FRANCE
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