Afhandeling van geometrie¶

Construeren van geometrie¶

There are several options for creating a geometry:

• uit coördinaten

  gPnt = QgsGeometry.fromPoint(QgsPoint(1,1))
  gLine = QgsGeometry.fromPolyline([QgsPoint(1, 1), QgsPoint(2, 2)])
  gPolygon = QgsGeometry.fromPolygon([[QgsPoint(1, 1), QgsPoint(2, 2),
                                      QgsPoint(2, 1)]])
  

  Coordinates are given using QgsPoint class.

  Polyline (Linestring) is represented by a list of points. Polygon is represented by a list of linear rings (i.e. closed linestrings). First ring is outer ring (boundary), optional subsequent rings are holes in the polygon.

  Geometrieën die bestaan uit meerdere delen gaan een niveau verder: multi-punt is een lijst van punten, multi-lijnen zijn een lijst van lijnen en multi-polygoon is een lijst van polygonen.

• uit bekende tekst (WKT)

  gem = QgsGeometry.fromWkt("POINT(3 4)")
  

• uit bekende binaire (WKB)

  >>> g = QgsGeometry()
  >>> wkb = '010100000000000000000045400000000000001440'.decode('hex')
  >>> g.fromWkb(wkb)
  >>> g.exportToWkt()
  'Point (42 5)'
  

Toegang tot geometrie¶

First, you should find out geometry type, wkbType() method is the one to use — it returns a value from QGis.WkbType enumeration

>>> gPnt.wkbType() == QGis.WKBPoint
True
>>> gLine.wkbType() == QGis.WKBLineString
True
>>> gPolygon.wkbType() == QGis.WKBPolygon
True
>>> gPolygon.wkbType() == QGis.WKBMultiPolygon
False

As an alternative, one can use type() method which returns a value from QGis.GeometryType enumeration. There is also a helper function isMultipart() to find out whether a geometry is multipart or not.

To extract information from geometry there are accessor functions for every vector type. How to use accessors

>>> gPnt.asPoint()
(1, 1)
>>> gLine.asPolyline()
[(1, 1), (2, 2)]
>>> gPolygon.asPolygon()
[[(1, 1), (2, 2), (2, 1), (1, 1)]]

For multipart geometries there are similar accessor functions: asMultiPoint(), asMultiPolyline(), asMultiPolygon().

Predicaten en bewerking voor geometrieën¶

QGIS uses GEOS library for advanced geometry operations such as geometry predicates (contains(), intersects(), ...) and set operations (union(), difference(), ...). It can also compute geometric properties of geometries, such as area (in the case of polygons) or lengths (for polygons and lines)

Here you have a small example that combines iterating over the features in a given layer and performing some geometric computations based on their geometries.

# we assume that 'layer' is a polygon layer
features = layer.getFeatures()
for f in features:
  geom = f.geometry()
  print "Area:", geom.area()
  print "Perimeter:", geom.length()

Areas and perimeters don’t take CRS into account when computed using these methods from the QgsGeometry class. For a more powerful area and distance calculation, the QgsDistanceArea class can be used. If projections are turned off, calculations will be planar, otherwise they’ll be done on the ellipsoid.

d = QgsDistanceArea()
d.setEllipsoid('WGS84')
d.setEllipsoidalMode(True)

print "distance in meters: ", d.measureLine(QgsPoint(10,10),QgsPoint(11,11))

U kunt zoeken naar vele voorbeelden van algoritmen die zijn opgenomen in QGIS en die methoden gebruiken om vectorgegevens te analyseren en te transformeren. Hier zijn enkele koppelingen naar de code van sommige ervan.

Additional information can be found in following sources:

• Geometry transformation: Reproject algorithm
• Distance and area using the QgsDistanceArea class: Distance matrix algorithm
• Multi-part to single-part algorithm