[Qgis-user] What's wrong with PRJ file for CRS 3857?

[Redoute]

Am 16.09.2015 um 12:15 schrieb Bernhard Ströbl:

> hope this helps

It seems my question was not very clear, so in other words (all "as far
as I understand"):

When I create a shapefile with CRS 3857 (Pseudo Mercator), and upload it
to the cartodb web service, the features are shifted. I tried to inspect
this and could reproduce the same behaviour in QGIS itself:

QGIS creates two files for projection with two forms of WKT, which
should express the same projection:

.prj:
PROJCS["WGS_84_Pseudo_Mercator",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Mercator"],PARAMETER["central_meridian",0],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1],PARAMETER["standard_parallel_1",0.0]]

.qpj:
PROJCS["WGS 84 / Pseudo-Mercator",GEOGCS["WGS
84",DATUM["WGS_1984",SPHEROID["WGS
84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Mercator_1SP"],PARAMETER["central_meridian",0],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["X",EAST],AXIS["Y",NORTH],EXTENSION["PROJ4","+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"],AUTHORITY["EPSG","3857"]]

To reproduce the problem with cartodb, I deleted the qpj file (as other
applications are expected to ignore this suffix) and loaded the
shapefile into QGIS. The layer properties then show EPSG 54004 World
Mercator and I see the same shift as in cartodb. The coordinate
774575/6610921, which marks Cologne Cathedral in 3857, is projected into
a lake near Duesseldorf.

So I think there is something wrong either with the shorter WKT, or how
it is interpreted by QGIS or GDAL/OGR. Once you know this, a workaround
is obvious. I hope with your help we can describe the error more
precisely to make an informed bug report.

Am 16.09.2015 um 12:15 schrieb Bernhard Ströbl:
> Hi,
> 
> general: read the documentation on projections, be sure to know what 
> projections are and what they are used for.
> 
> specific: both your project and your data have a projection (spatial 
> reference system). Cases:
> 1) both match => you're ok
> 2) both differ => QGIS can reproject the data on-the-fly => you're ok
> 3) the projection assigned to the data is wrong (I assume this is your 
> case but without more info on what you do nobody can tell). Tell QGIS in 
> layer properties which projection your data is in (the prj and qpj files 
> do just that), then goto either case 1 or case 2
> 
> hope this helps
> 
> Bernhard
> 
> Am 16.09.2015 um 11:46 schrieb Redoute:
>> This question is arising from
>> http://gis.stackexchange.com/questions/162779/why-is-the-city-of-san-francsico-floating-over-point-richmond
>>
>> I create a shapefile in QGIS in CRS 3857. I delete the .qpj file and
>> load the shapefile again. The features then are placed wrong, the
>> Cologne Cathedral shows up in a lake near Duesseldorf :-((. So I reason
>> there is something wrong with the .prj file.
>>
>> What is it? Is it a bug in GDAL/OGR?
>> Tested with QGIS 2.10.1.