[Qgis-user] Recognition of SRS in MapInfo-Files

[Andre Joost]

Hi all,

the problem is that QGIS (and GDAL/OGR) mixes projection and datum shift 
together in one EPSG Code. This is not the way intended by EPSG or ESRI. 
They handle projection and datum shift seperately.

As a consequence, shapefiles by ESRI don't have datum shift paramteres, 
while QGIS and Mapinfo write them into the file.

Datum shift can be 3- or 7-parameter Helmert, or a nadgrid file.
For some countries, several transformation parameters were published 
over the years. A 7-parameter transformation does not even have to be 
more exact, if it was derived only from a few reference stations.
Nadgrid is supposed to be the most precise, but it is not available for 
all countries and softwares.

EPSG and ESRI let the user decide which datum shift he wants for a given 
projection; Mapinfo and QGIS don't. And when Mapinfo and QGIS have 
different datum shifts bundled to a EPSG-code, the result will not match 
easily.

HTH,
André Joost

Am 25.05.2013 01:57, schrieb Fischer, Andreas:
> Dear Community, I just had an idea:
>
> If a shape file is created with QGIS a .qpj file is created as well.
> This file is used always when it is existing. QGIS doesn't care of
> settings in .prj file and just uses the definitions in .qpj file.
> That seems to be almost perfect, because all changes of the build in
> SRS definitions are considered in that qpj file. Unfortunately this
> file is without use for MapInfo TAB files.
>
> Wouldn't it be possible to make this going on with other formats as
> well? This might be a solution that does not care about ogr settings
> in first place. If ogr works fine nobody has needs for .qpj files.
> But in other cases, everyone could determine the parameters to use.
> If these are defined exactly like build in SRS matching should not be
> a problem.
>