Gestione della Geometria

The code snippets on this page need the following imports if you’re outside the pyqgis console:

from qgis.core import (
  QgsGeometry,
  QgsPoint,
  QgsPointXY,
  QgsWkbTypes,
  QgsProject,
  QgsFeatureRequest,
  QgsDistanceArea
)

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

PyQGIS provides several options for creating a geometry:

  • dalle coordinate

    gPnt = QgsGeometry.fromPointXY(QgsPointXY(1,1))
print(gPnt)
gLine = QgsGeometry.fromPolyline([QgsPoint(1, 1), QgsPoint(2, 2)])
print(gLine)
gPolygon = QgsGeometry.fromPolygonXY([[QgsPointXY(1, 1),
    QgsPointXY(2, 2), QgsPointXY(2, 1)]])
print(gPolygon)

    Coordinates are given using QgsPoint class or QgsPointXY class. The difference between these classes is that QgsPoint supports M and Z dimensions.

    A Polyline (Linestring) is represented by a list of points.

    A Polygon is represented by a list of linear rings (i.e. closed linestrings). The first ring is the outer ring (boundary), optional subsequent rings are holes in the polygon. Note that unlike some programs, QGIS will close the ring for you so there is no need to duplicate the first point as the last.

    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)

    geom = QgsGeometry.fromWkt("POINT(3 4)")
print(geom)

  • da well-known binary (WKB)

    g = QgsGeometry()
wkb = bytes.fromhex("010100000000000000000045400000000000001440")
g.fromWkb(wkb)

# print WKT representation of the geometry
print(g.asWkt())

Accedere alla Geometria

First, you should find out the geometry type. The wkbType() method is the one to use. It returns a value from the QgsWkbTypes.Type enumeration.

if gPnt.wkbType() == QgsWkbTypes.Point:
  print(gPnt.wkbType())
  # output: 1 for Point
if gLine.wkbType() == QgsWkbTypes.LineString:
  print(gLine.wkbType())
if gPolygon.wkbType() == QgsWkbTypes.Polygon:
  print(gPolygon.wkbType())
  # output: 3 for Polygon

As an alternative, one can use the type() method which returns a value from the QgsWkbTypes.GeometryType enumeration.

You can use the displayString() function to get a human readable geometry type.

print(QgsWkbTypes.displayString(gPnt.wkbType()))
# output: 'Point'
print(QgsWkbTypes.displayString(gLine.wkbType()))
# output: 'LineString'
print(QgsWkbTypes.displayString(gPolygon.wkbType()))
# output: 'Polygon'

Point
LineString
Polygon

There is also a helper function isMultipart() to find out whether a geometry is multipart or not.

To extract information from a geometry there are accessor functions for every vector type. Here’s an example on how to use these accessors:

print(gPnt.asPoint())
# output: <QgsPointXY: POINT(1 1)>
print(gLine.asPolyline())
# output: [<QgsPointXY: POINT(1 1)>, <QgsPointXY: POINT(2 2)>]
print(gPolygon.asPolygon())
# output: [[<QgsPointXY: POINT(1 1)>, <QgsPointXY: POINT(2 2)>, <QgsPointXY: POINT(2 1)>, <QgsPointXY: POINT(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() and asMultiPolygon().

Predicati ed Operazioni delle Geometrie

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

Let’s see an example that combines iterating over the features in a given layer and performing some geometric computations based on their geometries. The below code will compute and print the area and perimeter of each country in the countries layer within our tutorial QGIS project.

The following code assumes layer is a QgsVectorLayer object that has Polygon feature type.

# let's access the 'countries' layer
layer = QgsProject.instance().mapLayersByName('countries')[0]

# let's filter for countries that begin with Z, then get their features
query = '"name" LIKE \'Z%\''
features = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query))

# now loop through the features, perform geometry computation and print the results
for f in features:
  geom = f.geometry()
  name = f.attribute('NAME')
  print(name)
  print('Area: ', geom.area())
  print('Perimeter: ', geom.length())

Now you have calculated and printed the areas and perimeters of the geometries. You may however quickly notice that the values are strange. That is because areas and perimeters don’t take CRS into account when computed using the area() and length() methods from the QgsGeometry class. For a more powerful area and distance calculation, the QgsDistanceArea class can be used, which can perform ellipsoid based calculations:

The following code assumes layer is a QgsVectorLayer object that has Polygon feature type.

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

layer = QgsProject.instance().mapLayersByName('countries')[0]

# let's filter for countries that begin with Z, then get their features
query = '"name" LIKE \'Z%\''
features = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query))

for f in features:
  geom = f.geometry()
  name = f.attribute('NAME')
  print(name)
  print("Perimeter (m):", d.measurePerimeter(geom))
  print("Area (m2):", d.measureArea(geom))

  # let's calculate and print the area again, but this time in square kilometers
  print("Area (km2):", d.convertAreaMeasurement(d.measureArea(geom), QgsUnitTypes.AreaSquareKilometers))

Alternatively, you may want to know the distance and bearing between two points.

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

# Let's create two points.
# Santa claus is a workaholic and needs a summer break,
# lets see how far is Tenerife from his home
santa = QgsPointXY(25.847899, 66.543456)
tenerife = QgsPointXY(-16.5735, 28.0443)

print("Distance in meters: ", d.measureLine(santa, tenerife))

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.