7. Geometry Handling

Dica

Os <i>snippets</i> de código nesta página precisam das seguintes importações se estiver fora da console pyqgis:

from qgis.core import (
  QgsGeometry,
  QgsGeometryCollection,
  QgsPoint,
  QgsPointXY,
  QgsWkbTypes,
  QgsProject,
  QgsFeatureRequest,
  QgsVectorLayer,
  QgsDistanceArea,
  QgsUnitTypes,
  QgsCoordinateTransform,
  QgsCoordinateReferenceSystem
)

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

Sometimes one geometry is actually a collection of simple (single-part) geometries. Such a geometry is called a multi-part geometry. If it contains just one type of simple geometry, we call it multi-point, multi-linestring or multi-polygon. For example, a country consisting of multiple islands can be represented as a multi-polygon.

The coordinates of geometries can be in any coordinate reference system (CRS). When fetching features from a layer, associated geometries will have coordinates in CRS of the layer.

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

7.1. Geometry Construction

PyQGIS provides several options for creating a geometry:

• from coordinates

  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.

  Multi-part geometries go one level further: multi-point is a list of points, multi-linestring is a list of linestrings and multi-polygon is a list of polygons.

• from well-known text (WKT)

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

• from well-known binary (WKB)

  g = QgsGeometry()
  wkb = bytes.fromhex("010100000000000000000045400000000000001440")
  g.fromWkb(wkb)
  
  # print WKT representation of the geometry
  print(g.asWkt())
  

7.2. Access to Geometry

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())
  # output: 2 for LineString
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().

It is possible to iterate over all the parts of a geometry, regardless of the geometry’s type. E.g.

geom = QgsGeometry.fromWkt( 'MultiPoint( 0 0, 1 1, 2 2)' )
for part in geom.parts():
  print(part.asWkt())

Point (0 0)
Point (1 1)
Point (2 2)

geom = QgsGeometry.fromWkt( 'LineString( 0 0, 10 10 )' )
for part in geom.parts():
  print(part.asWkt())

LineString (0 0, 10 10)

gc = QgsGeometryCollection()
gc.fromWkt('GeometryCollection( Point(1 2), Point(11 12), LineString(33 34, 44 45))')
print(gc[1].asWkt())

Point (11 12)

It’s also possible to modify each part of the geometry using QgsGeometry.parts() method.

geom = QgsGeometry.fromWkt( 'MultiPoint( 0 0, 1 1, 2 2)' )
for part in geom.parts():
  part.transform(QgsCoordinateTransform(
    QgsCoordinateReferenceSystem("EPSG:4326"),
    QgsCoordinateReferenceSystem("EPSG:3111"),
    QgsProject.instance())
  )

print(geom.asWkt())

MultiPoint ((-10334728.12541878595948219 -5360106.25905461423099041),(-10462135.16126426123082638 -5217485.4735023295506835),(-10589399.84444035589694977 -5072021.45942386891692877))

7.3. Geometry Predicates and Operations

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())

Zambia
Area:  62.822790653431014
Perimeter:  50.65232014052552
Zimbabwe
Area:  33.41113559136511
Perimeter:  26.608288555013935

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))

Zambia
Perimeter (m): 5539361.250294601
Area (m2): 751989035032.9031
Area (km2): 751989.0350329031
Zimbabwe
Perimeter (m): 2865021.3325076113
Area (m2): 389267821381.6008
Area (km2): 389267.8213816008

Alternatively, you may want to know the distance 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))

You can find many example of algorithms that are included in QGIS and use these methods to analyze and transform vector data. Here are some links to the code of a few of them.

• Distance and area using the QgsDistanceArea class: Distance matrix algorithm

• Lines to polygons algorithm