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Gestione della Geometria

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

Alcune volte una geometria é effettivamente una collezione di geometrie (parti singole) piú semplici. Se contiene un tipo di geometria semplice, la chiameremo punti multipli, string multi linea o poligoni multipli. Ad esempio, un Paese formato da piú isole puó essere rappresentato come un poligono multiplo.

Le coordinate delle geometrie possono essere in qualsiasi sistema di riferimento delle coordinate (CRS). Quando si estraggono delle caratteristiche da un vettore, le geometrie associate avranno le coordinate nel CRS del vettore.

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

Costruzione della Geometria

There are several options for creating a geometry:

  • dalle coordinate

    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.

    Le geometrie a parti multiple vanno ad un livello successivo: punti multipli é una lista di punti, una stringa multi linea é una linea di linee ed un poligono multiplo é una lista di poligoni.

  • da well-known text (WKT)

    gem = QgsGeometry.fromWkt("POINT(3 4)")
  • da well-known binary (WKB)

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

Accedere alla Geometria

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
>>> gLine.wkbType() == QGis.WKBLineString
>>> gPolygon.wkbType() == QGis.WKBPolygon
>>> gPolygon.wkbType() == QGis.WKBMultiPolygon

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)]]


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().

Predicati ed Operazioni delle Geometrie

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()

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

É possibile trovare molti esempi di algoritmi che sono inclusi in QGIS ed utilizzare questi metodi per analizzare e trasformare i dati vettoriali. Di seguito i link al codice di alcuni di questi.

Additional information can be found in following sources: