[Qgis-developer] Python: Union selected polygons in a layer

[Andreas Neumann]

Hi Daniel,

Thanks - this should do the trick! And it explains why I did not find
"Union".

Will try on Monday when I am back in the office.

Andreas

Am 05.10.2013 11:55, schrieb Daniel:
> Try the QgsGeometry::combine() method.
> 
> http://www.qgis.org/api/classQgsGeometry.html#a63244d3435cc99794b82a6cbd0af0eb5
> 
> Quoting class doc
> 
> *Note:*this operation is not called union since its a reserved word in C++.
> 
> 
> On Sat, Oct 5, 2013 at 8:07 AM, gene <martin.laloux at gmail.com> wrote:
> 
>> I also do not see union in  Geometry Handling
>> <http://www.qgis.org/en/docs/pyqgis_developer_cookbook/geometry.html>  :
>>
>> the layer:
>>
>>     for elem in layer.getFeatures():
>>          geom= elem.geometry()
>>          print geom.exportToGeoJSON()
>>     { "type": "LineString", "coordinates": [ [203271.93800293002277613,
>> 89444.43836556003952865],   [204056.96179387057782151,
>> 89149.26942016639804933], [204590.77797171016572975,
>> 89111.58827820124861319], [204823.14501382855814882,
>> 89456.99874621508934069] ] }
>>     { "type": "LineString", "coordinates": [ [204541.79248715547146276,
>> 89757.64295613372814842], [204474.17871974196168594,
>> 90086.05268357080058195] ] }
>>
>>
>> Using the  see <https://github.com/inky/see>   module (alternative to
>> Python's dir()):
>>
>>      from see import see
>>      see(geom)
>>     hash()                 help()                 repr()
>> str()
>>     .Error()               .addPart()             .addRing()
>>     .adjacentVertices()    .area()                .asGeometryCollection()
>>     .asMultiPoint()        .asMultiPolygon()      .asMultiPolyline()
>>     .asPoint()             .asPolygon()           .asPolyline()
>>     .asWkb()               .avoidIntersections()  .boundingBox()
>>     .buffer()              .centroid()
>> .closestSegmentWithContext()
>>     .closestVertex()       .closestVertexWithContext()
>>     .combine()             .contains()            .convertToMultiType()
>>     .convexHull()          .crosses()             .deletePart()
>>     .deleteRing()          .deleteVertex()        .difference()
>>     .disjoint()            .distance()            .equals()
>>     .exportToGeoJSON()     .exportToWkt()         .fromMultiPoint()
>>     .fromMultiPolygon()    .fromMultiPolyline()   .fromPoint()
>>     .fromPolygon()         .fromPolyline()        .fromRect()
>>     .fromWkb()             .fromWkt()             .insertVertex()
>>     .interpolate()         .intersection()        .intersects()
>>     .isGeosEmpty()         .isGeosEqual()         .isGeosValid()
>>     .isMultipart()         .length()              .makeDifference()
>>     .moveVertex()          .overlaps()            .reshapeGeometry()
>>     .simplify()            .splitGeometry()       .sqrDistToVertexAt()
>>     .symDifference()       .touches()             .transform()
>>     .translate()           .type()                .validateGeometry()
>>     .vertexAt()            .within()              .wkbSize()
>>     .wkbType()
>>
>> I see .intersection(), .touches() but no .union()
>>
>> Whereas with Shapely:
>> --------------------------
>>
>>     from shapely.geometry import LineString, shape
>>     from json import loads
>>     # empty shapely geometry
>>     myunion = LineString()
>>     for elem in layer.getFeatures():
>>            geom = elem.geometry()
>>            # transformation of the QGIS geometry to shapely geometry with
>> the geo_interface
>>            geoms = shape(jloads(elem.geometry().exportToGeoJSON()))
>>            myunion = myunion.union(geoms)
>>    print myunion
>>    MULTILINESTRING ((203271.9380029300227761 89444.4383655600395286,
>> 204056.9617938705778215 89149.2694201663980493, 204590.7779717101657297
>> 89111.5882782012486132, 204823.1450138285581488 89456.9987462150893407),
>> (204541.7924871554714628 89757.6429561337281484, 204474.1787197419616859
>> 90086.0526835708005819))
>>
>> and
>>       gem = QgsGeometry.fromWkt(myunion.wkt)
>>       gem
>>       <qgis.core.QgsGeometry object at 0x12986fef0>
>>        print gem.exportToGeoJSON()
>>       { "type": "MultiLineString", "coordinates": [ [
>> [203271.93800293002277613, 89444.43836556003952865],
>> [204056.96179387057782151, 89149.26942016639804933],
>> [204590.77797171016572975, 89111.58827820124861319],
>> [204823.14501382855814882, 89456.99874621508934069] ], [
>> [204541.79248715547146276, 89757.64295613372814842],
>> [204474.17871974196168594, 90086.05268357080058195] ] ] }
>>
>> or with ogr (same result):
>> -------------
>>     from osgeo import ogr
>>     # empty ogr geometry
>>     myunion = ogr.Geometry(ogr.wkbLineString)
>>     for elem in layer.getFeatures():
>>            geom = elem.geometry()
>>            # transformation of the QGIS geometry to ogr geometry
>>            geoms = ogr.CreateGeometryFromWkb(geom.asWkb())
>>            myunion = myunion.Union(geoms)
>>
>> Is there a comparable solution with PyQGIS ?
>>
>>
>>
>>
>> --
>> View this message in context:
>> http://osgeo-org.1560.x6.nabble.com/Python-Union-selected-polygons-in-a-layer-tp5081862p5081959.html
>> Sent from the Quantum GIS - Developer mailing list archive at Nabble.com.
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> 
> 
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