Manejo de Geometría

Points, linestrings and polygons that represent a spatial feature are commonly referred to as geometries. In QGIS they are represented with the QgsGeometry class.

A veces una geometría es realmente una colección simple (partes simples) geométricas. Tal geometría se llama geometría de múltiples partes. Si contiene un tipo de geometría simple, lo llamamos un punto múltiple, lineas múltiples o polígonos múltiples. Por ejemplo, un país consiste en múltiples islas que se pueden representar como un polígono múltiple.

Las coordenadas de las geometrías pueden estar en cualquier sistema de referencia de coordenadas (SRC). Cuando extrae características de una capa, las geometrías asociadas tendrán sus coordenadas en el SRC de la capa.

Description and specifications of all possible geometries construction and relationships are available in the OGC Simple Feature Access Standards for advanced details.

Construcción de Geometría

There are several options for creating a geometry:

  • desde coordenadas

    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.

    Las geometrías multi-parte van un nivel más allá: multi-punto es una lista de puntos, multi-linea es una lista de polilíneas y multi-polígono es una lista de polígonos.

  • desde well-known text (WKT)

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

  • desde well-known binary (WKB)

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

Acceso a Geometría

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)]]

Nota

The tuples (x,y) are not real tuples, they are QgsPoint objects, the values are accessible with x() and y() methods.

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

Geometría predicados y Operaciones

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))

Puede encontrar muchos ejemplos de algoritmos que se incluyen en QGIS y utilizan estos métodos para analizar y transformar los datos vectoriales. Aquí hay algunos enlaces al código de algunos de ellos.

Additional information can be found in following sources: