Web Mercator

[kbevers]

Reported by @jmckenna - se details below.

Transformations to and from the Google's pseudo/web Mercator returns incorrect results.
The problem was introduced in #743 which fixed #22 but unfortunately also broke the web Mercator. This happens since the web Mercator handles ellipsoids incorrectly (calculating coordinates on the ellipsoid, but projecting them on a sphere).

So, on hand one cs2cs/pj_transform can behave geodetically correct OR on the other hand it can behave in a slightly wrong manner but return the expected coordinates when the web Mercator is used. We want the latter option to make sure that PROJ behaves similarly to previous version.

Until a new version is release the a work around for the problem is to remove +nadgrids=@null from the web Mercator proj-string. This might be necessary to do in the epsg file, where the system is known as:

<3857> +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 +wktext +no_defs <>

Change it to

<3857> +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 +wktext +no_defs <>

and it should work again.

I wanted to test PROJ v5 by using a point 
(487147.594520173,4934316.46263998) in EPSG:26915 (UTM Zone 15) and 
reprojecting that to EPSG:3857 (Google's Mercator).

with PROJ 4.9.3
---------------

cs2cs -v +proj=utm +zone=15 +datum=NAD83 +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
# ---- From Coordinate System ----
#Universal Transverse Mercator (UTM)
#       Cyl, Sph
#       zone= south
# +proj=utm +zone=15 +datum=NAD83 +ellps=GRS80 +towgs84=0,0,0
# ---- To Coordinate System ----
#Mercator
#       Cyl, Sph&Ell
#       lat_ts=
# +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
487147.594520173 4934316.46263998

RESULT POINT: -10370728.80    5552839.74 -0.00

with PROJ 5.0.0-RC6
-------------------

 >cs2cs -v +proj=utm +zone=15 +datum=NAD83 +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
# ---- From Coordinate System ----
#Universal Transverse Mercator (UTM)
#       Cyl, Sph
#       zone= south
# +proj=utm +zone=15 +datum=NAD83 +ellps=GRS80 +towgs84=0,0,0
# ---- To Coordinate System ----
#Mercator
#       Cyl, Sph&Ell
#       lat_ts=
# +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
487147.594520173 4934316.46263998

RESULT POINT: -10370728.80    5522830.15 -10484.03

More here: http://lists.maptools.org/pipermail/proj/2018-February/008082.html

[kbevers]

fixed with #830