[GRASS-user] projection issue

[Vincent Bain]

Le lundi 24 mars 2014 à 13:24 -0700, Hamish a écrit :
> 
> Hi,
> 
> First of all, avoid using Google's "spherical mercator" projection for anything other than necessary data import/export if you can possibly help it. But sometimes we don't have a choice, so..
> 

I wish I could never have heard about it... till I had to cope with
french geographical web services : their wmts service is based on this
projection system, so I need to define a spherical mercator location to
correctly retrieve tilesets.


> Google's spherical Mercator projection (formerly the unofficial
> epsg-ish code 900913 from the esri.extra file) isn't really WGS84, it just borrows the major axis of the Earth used in WGS84 for both the major and minor axes of the ellipsoid, making it a sphere larger than Earth really is. So it's just a simple Mercator on a sphere, which happens to have a particular radius.
> 
> Different ellipsoids (and a sphere being a type of ellipsoid) squash the Earth north-south, so the northing value changes the most. Since Mercator doesn't deform east-west (lines of longitude are all parallel and perfectly vertical), in that projection changing the ellipsoid does not change the easting value at all.
> 

Yes, in my case I noticed a n-s 30 km gap

> Next thing to know is the +nadgrids=@null hack to get around the datum transform. See
> http://trac.osgeo.org/proj/wiki/GenParms#ThenullGrid
> 
> and this basically explains the rest:
> 
> http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator

I believed the nadgrid was a shifting matrix applying to ellipsoid
center (in cartesian geocentric coordinates). Does this mean
+nadgrids=@null implicitly tells cs2cs : keep the from-ellipsoid (ie
ignore the to-sphere), and shift it (of 0,0,0) ?
> 
> 
> good luck,
> Hamish
> 
Your explanations and links on this topic were very welcome !
Thank you,

Vincent