[Qgis-developer] script to update the srs.db

[Tim Sutton]

Hi Marco

Sorry for not replying to the thread - I was away for a few days. Your
suggestion seems reasonable to me. Since we are matching on proj4
string it should have no impact if the srs_id changes I guess. We
should do a comparison of the new Maciej generated srs table and the
one prior to it and check we havent lost any srses - I recall some
that were contributed for Paris (I think it was) and Sweden that were
incorporated into the db to augment / replace erroneous ones, and thus
wont have EPSG nos.

Regards

Tim

2008/6/23 Marco Hugentobler <marco.hugentobler at karto.baug.ethz.ch>:
> Hi Maciej,
>
> Since no one else replied, I give it a try. Keep in mind crs is not my special
> area of competence, so others please feel free to correct me where I'm wrong.
>
>> <SpatialRefSys>
>>   <ProjectSRSProj4String type="QString" >+proj=tmerc +lat_0=0 (snip...)
>>   <ProjectSRSID type="int" >181</ProjectSRSID>
>> </SpatialRefSys>
>>
>> Which tag does QGIS depend on to recognize the project's SRS? Does it
>> consider the <srsid> or <ProjectSRSID type="int" > at all?>
>>
>
> From what I have seen in the code, if a srs is read from XML file, import is
> done from proj4 string using OGR.
>
> qgsspatialrefsys.cpp: 665
>
> void QgsSpatialRefSys::setProj4String (QString theProj4String)
> {
>  const char *oldlocale = setlocale(LC_ALL, NULL);
>  setlocale(LC_ALL, "C");
>  mIsValidFlag = OSRImportFromProj4(mSRS, theProj4String.toLatin1
> ().constData() )==OGRERR_NONE;
>  setlocale(LC_ALL, oldlocale);
> }
>
> and for transformation, the proj4 string is exported from OGR and passed to
> proj4 library.
>
>
> It seems to me that the QGIS srsid is only used to create a default srs if the
> <spatialrefsys> tag is not present:
>
> qgsspatialrefsys.cpp: 908
>
> QDomNode srsNode  = theNode.namedItem( "spatialrefsys" );
>
>
> if ( ! srsNode.isNull() )
>  {
> ...
> }
> else
>  {
>    // Return default SRS if none was found in the XML.
>    createFromSrsId(GEOSRS_ID);
> }
>
> where GEOSRS_ID is:
>
> qgis.h:123:  const long GEOSRS_ID = 2585;
>
> I wonder if we just could replace that with epsg number 4326 and then use
>
> const long DEFAULT_EPGS = 4326;
>
> createFromEPSG(DEFAULT_EPSG);
>
> ?
>
> That way, it would be possible to change the qgis srs ids without affecting
> project file support.
>
>
> Regards,
> Marco
>
>
> Am Mittwoch 11 Juni 2008 22:55:36 schrieb Maciej Sieczka:
>> Tim Sutton pisze:
>> > Hi Richard
>> >
>> > One thing I would like to do if you assemble small test datasets is
>> > to create some unit tests for srs stuff. One basic form of test I am
>> > thinking is to simply have a directory with a bunch of shapefiles
>> > with a single feature each, each shp being in a different srs. The
>> > test would load each one in turn and verify that it gets the correct
>> >  epsg etc for it. So if you so start getting little test files in,
>> > bear this in mind so that we can use them for more formal unit
>> > testing too.
>> >
>> > Secondly if I recall correctly, the reason we originally opted to use
>> >  our own srs identifier was that the srs db did inclide some srs
>> > definitions that did not have an epsg code yet.
>>
>> Please anybody who knows details speak up!
>>
>> > It would be good if we could maintain the original srs id pkeys in
>> > any updates you do
>>
>> This could be done with some extra work, but I don't think
>> QGIS should depend on it's own SRS numbering scheme, when EPSG provides
>> an universal, stable scheme for that. That's what EPSG database is good
>> at and let's make use of it if possible.
>>
>> > since changing them may introduce regressions for people loading .qgs
>> >  project files.
>>
>> In a .qgs the layer's and on the <mapcanvas> SRS part looks like this:
>>
>> <spatialrefsys>
>>   <proj4>+proj=tmerc +lat_0=0 +lon_0=19 +k=0 +x_0=500000 (snip...)
>>   <srsid>181</srsid>
>>   <srid>2180</srid>
>>   <epsg>2180</epsg>
>>   <description>ETRS89 / Poland CS92</description>
>>   <projectionacronym>tmerc</projectionacronym>
>>   <ellipsoidacronym>GRS80</ellipsoidacronym>
>>   <geographicflag>false</geographicflag>
>> </spatialrefsys>
>>
>> The <properties> SRS (how does differ from the <mapcanvas> one???) is eg.:
>>
>> <SpatialRefSys>
>>   <ProjectSRSProj4String type="QString" >+proj=tmerc +lat_0=0 (snip...)
>>   <ProjectSRSID type="int" >181</ProjectSRSID>
>> </SpatialRefSys>
>>
>> Which tag does QGIS depend on to recognize the project's SRS? Does it
>> consider the <srsid> or <ProjectSRSID type="int" > at all?
>>
>> Cheers,
>> Maciek
>
>
>
> --
> Dr. Marco Hugentobler
> Institute of Cartography
> ETH Zurich
> Technical Advisor QGIS Project Steering Committee
>



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Tim Sutton
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