[Qgis-user] ogr2ogr - how to use -s_srs, -t_srs and -a_srs

[Erik Josefsson]

Thank you very much Andre.

I'm still puzzled how I can tell ogr2ogr to convert the coordinates
themselves along with the conversion of the CRS (e.g. from EPSG:3008 to
EPSG:3006).

But maybe that  is because I have not yet managed to retrieve from the
.shp and .prj files the information that a certain shapefile is e.g.
EPSG:3008.

It seems I need an interpreter of the output from ogrinfo that can
translate that rather complex output into a simple EPSG-code.

I imagine that it then becomes much easier to convert both CRS and
coordinate values from any shapefile into anther shapfile with any other
CRS you'd like.

Thanks again!

//Erik


On 06/08/17 07:42, Andre Joost wrote:
> Am 06.08.2017 um 02:22 schrieb Erik Josefsson:
>> Hello,
>>
>> I have a handful of zip archives containing original shapefiles with
>> different coordinate reference systems (e.g. SGER:3008 and SGER:3021).
>>
>> I want to extract and convert them all to SGER:3006 and thought that
>> ogr2ogr would do the job, but I don't find intelligible instructions on
>> how to use the srs-parameters: -s_srs, -t_srs and -a_srs.
> 
> You can either use -s_srs and -t_srs to reproject from one CRS to
> another, or use -a_srs to assign a CRS if the source data has no CRS
> information (like CSV), or has a wrong CRS (it is misplaced compared to
> other data).
> 
> If the source data has a .prj file, the CRS information is stored inside
> that. But beware that EPSG codes and datum shifts (+towgs84) are not
> stored in .prj files. GDAL will try to auto-guess the EPSG code,but
> sometimes fails. In that case, no datum shift will be applied. From your
> example, EPSG:3021 must have a datum shift to WGS84, while the others
> are based on ellipsoids with no shift to the WGS84 definition. So it is
> safe to use -s_srs EPSG:3021 (together with -t_srs) for that data.
> 
>>
>> Do I have to know the coordinate reference system of each original shape
>> file to use ogr2ogr successfully? If so, how do I retrieve that
>> information?
>>
> 
> ogrinfo will report the CRS of every vector file supported by GDAL.
> 
>> Or should I just override the original coordinate reference system like
>> this?
>>
>> ogr2ogr -f 'ESRI Shapefile' -s_srs EPSG:3006 -t_srs EPSG:3006 -a_srs
>> EPSG:3006 A.shp B.shp
>>
> That will be wrong in most cases.
> 
> HTH,
> André Joost
> 
> 
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