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

[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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-- 
Daniel Vaz
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