7. Manipularea geometriei


Fragmentele de cod de pe această pagină necesită următoarele importuri, dacă vă aflați în afara consolei pyqgis:

 1from qgis.core import (
 2  QgsGeometry,
 3  QgsGeometryCollection,
 4  QgsPoint,
 5  QgsPointXY,
 6  QgsWkbTypes,
 7  QgsProject,
 8  QgsFeatureRequest,
 9  QgsVectorLayer,
10  QgsDistanceArea,
11  QgsUnitTypes,
12  QgsCoordinateTransform,
13  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.

Uneori, o geometrie poate fi de fapt o colecție de simple geometrii (simple-părți). O astfel de geometrie poartă denumirea de geometrie multi-parte. În cazul în care conține doar un singur tip de geometrie simplă, o denumim multi-punct, multi-linie sau multi-poligon. De exemplu, o țară formată din mai multe insule poate fi reprezentată ca un multi-poligon.

Coordonatele geometriilor pot fi în orice sisteme de referință și coordonate (CRS). Când extragem entitățile dintr-un strat, geometriile asociate vor avea coordonatele în CRS-ul stratului.

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

7.1. Construirea geometriei

PyQGIS provides several options for creating a geometry:

  • din coordonate

    1gPnt = QgsGeometry.fromPointXY(QgsPointXY(1,1))
    3gLine = QgsGeometry.fromPolyline([QgsPoint(1, 1), QgsPoint(2, 2)])
    5gPolygon = QgsGeometry.fromPolygonXY([[QgsPointXY(1, 1),
    6    QgsPointXY(2, 2), QgsPointXY(2, 1)]])

    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.

    Geometriile multi-parte merg cu un nivel mai departe: multi-punctele sunt o listă de puncte, multi-liniile o listă de linii iar multi-poligoanele sunt o listă de poligoane.

  • din well-known text (WKT)

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

    1g = QgsGeometry()
    2wkb = bytes.fromhex("010100000000000000000045400000000000001440")
    5# print WKT representation of the geometry

7.2. Accesarea geometriei

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.

1if gPnt.wkbType() == QgsWkbTypes.Point:
2  print(gPnt.wkbType())
3  # output: 1 for Point
4if gLine.wkbType() == QgsWkbTypes.LineString:
5  print(gLine.wkbType())
6  # output: 2 for LineString
7if gPolygon.wkbType() == QgsWkbTypes.Polygon:
8  print(gPolygon.wkbType())
9  # 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.

2# output: 'Point'
4# output: 'LineString'
6# output: '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:

2# output: <QgsPointXY: POINT(1 1)>
4# output: [<QgsPointXY: POINT(1 1)>, <QgsPointXY: POINT(2 2)>]
6# output: [[<QgsPointXY: POINT(1 1)>, <QgsPointXY: POINT(2 2)>, <QgsPointXY: POINT(2 1)>, <QgsPointXY: POINT(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() 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():
Point (0 0)
Point (1 1)
Point (2 2)
geom = QgsGeometry.fromWkt( 'LineString( 0 0, 10 10 )' )
for part in geom.parts():
LineString (0 0, 10 10)
gc = QgsGeometryCollection()
gc.fromWkt('GeometryCollection( Point(1 2), Point(11 12), LineString(33 34, 44 45))')
Point (11 12)

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

1geom = QgsGeometry.fromWkt( 'MultiPoint( 0 0, 1 1, 2 2)' )
2for part in geom.parts():
3  part.transform(QgsCoordinateTransform(
4    QgsCoordinateReferenceSystem("EPSG:4326"),
5    QgsCoordinateReferenceSystem("EPSG:3111"),
6    QgsProject.instance())
7  )
MultiPoint ((-10334728.12541878595948219 -5360106.25905461423099041),(-10462135.16126426123082638 -5217485.4735023295506835),(-10589399.84444035589694977 -5072021.45942386891692877))

7.3. Predicate și operațiuni geometrice

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.

 1# let's access the 'countries' layer
 2layer = QgsProject.instance().mapLayersByName('countries')[0]
 4# let's filter for countries that begin with Z, then get their features
 5query = '"name" LIKE \'Z%\''
 6features = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query))
 8# now loop through the features, perform geometry computation and print the results
 9for f in features:
10  geom = f.geometry()
11  name = f.attribute('NAME')
12  print(name)
13  print('Area: ', geom.area())
14  print('Perimeter: ', geom.length())
2Area:  62.822790653431014
3Perimeter:  50.65232014052552
5Area:  33.41113559136511
6Perimeter:  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.

 1d = QgsDistanceArea()
 4layer = QgsProject.instance().mapLayersByName('countries')[0]
 6# let's filter for countries that begin with Z, then get their features
 7query = '"name" LIKE \'Z%\''
 8features = layer.getFeatures(QgsFeatureRequest().setFilterExpression(query))
10for f in features:
11  geom = f.geometry()
12  name = f.attribute('NAME')
13  print(name)
14  print("Perimeter (m):", d.measurePerimeter(geom))
15  print("Area (m2):", d.measureArea(geom))
17  # let's calculate and print the area again, but this time in square kilometers
18  print("Area (km2):", d.convertAreaMeasurement(d.measureArea(geom), QgsUnitTypes.AreaSquareKilometers))
2Perimeter (m): 5539361.250294601
3Area (m2): 751989035032.9031
4Area (km2): 751989.0350329031
6Perimeter (m): 2865021.3325076113
7Area (m2): 389267821381.6008
8Area (km2): 389267.8213816008

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

 1d = QgsDistanceArea()
 4# Let's create two points.
 5# Santa claus is a workaholic and needs a summer break,
 6# lets see how far is Tenerife from his home
 7santa = QgsPointXY(25.847899, 66.543456)
 8tenerife = QgsPointXY(-16.5735, 28.0443)
10print("Distance in meters: ", d.measureLine(santa, tenerife))

Puteți căuta mai multe exemple de algoritmi care sunt incluși în QGIS și să folosiți aceste metode pentru a analiza și a transforma datele vectoriale. Mai jos sunt prezente câteva trimiteri spre codul unora dintre ele.