Le jeudi 23 juin 2016 16:01:10, Marco Hugentobler a écrit : > Hi Even > > I agree it will be good to follow the postgres / gdal interpretation. > Reading S2, I somehow thought the segment midpoint would be the circle > midpoint, but it's not the same.
Filed as https://hub.qgis.org/issues/15116 > > Regards, > Marco > > On 06/23/16 15:32, Even Rouault wrote: > > Hi, > > > > I just came into a difference how QGIS and GDAL interpret a > > CircularString made of a 3 points p0, p1, p2 where p0 == p2, which is a > > way of representing a full circle. > > > > GDAL interprets p1 as the symetrical point of p0 with respect to the > > center > > (https://github.com/osgeo/gdal/blob/trunk/gdal/ogr/ogrcircularstring.cpp > > #L640), that is p1 is on the perimeter of the circle, or said otherwise > > the center of the circle is the midpoint of [p0,p1]. > > Whereas QGIS interprets p1 as the center of the circle ( > > https://github.com/qgis/QGIS/blob/master/src/core/geometry/qgsgeometryuti > > ls.cpp#L359 ) > > > > I'd think GDAL interpretation is the right one, since according to > > http://jtc1sc32.org/doc/N1101-1150/32N1107-WD13249-3--spatial.pdf (ISO > > SQL MM Part 3), paragraph 4.1.6 : > > (let's call this sentence S1) > > """ In the case where the segment is a circle, then the center is located > > at the midpoint of the line connecting the start point with the > > intermediate point. """ > > > > But if you look a bit above in the paragraph, there's a sentence (call it > > S2) > > > > : """In the special case where a segment is a complete circle, that is, > > : the > > > > start and end points are coincident, then the intermediate point shall be > > the midpoint of the segment. """, which looks confusing at first. I > > think this sentence must be interpreted considering distances along the > > curve, in which case the "midpoint of the segment" when doing a full > > walk along the circle from p0 back to p0 is the symetrical point of p0 > > with respect to the center. > > > > I think this interpretation is also more logical since, even in that > > particular case, p0,p1,p2 are all on the arc, like in the general case. > > > > I can also see that PostGIS (in > > https://github.com/postgis/postgis/blob/svn- > > trunk/liblwgeom/lwalgorithm.c#L252 ) has the same interpretation as GDAL > > (*) > > > > I'd be happy to read opinions regarding this. > > > > Even > > > > (*) Can also be shown with : > > > > # SELECT > > ST_AsText(ST_Envelope(ST_CurveToLine(ST_GeomFromText('CIRCULARSTRING(-1 > > 0,1 0,-1 0)')))); > > > > st_astext > > > > -------------------------------------- > > > > POLYGON((-1 -1,-1 1,1 1,1 -1,-1 -1)) > > > > And http://postgis.net/docs/manual-2.2/using_postgis_dbmanagement.html > > says "The exception to this is for a closed circle, where the start and > > end points are the same. In this case the second point MUST be the > > center of the arc, ie the opposite side of the circle.", which is > > another way to rephrase S2. -- Spatialys - Geospatial professional services http://www.spatialys.com _______________________________________________ Qgis-developer mailing list Qgis-developer@lists.osgeo.org List info: http://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: http://lists.osgeo.org/mailman/listinfo/qgis-developer
