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

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

7. ジオメトリの操作

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

時には1つのジオメトリは実際に単純な(シングルパート)ジオメトリの集合です。このような幾何学的形状は、マルチパートジオメトリと呼ばれています。単純ジオメトリが1種類だけ含まれている場合は、マルチポイント、マルチラインまたはマルチポリゴンと呼んでいます。例えば、複数の島からなる国は、マルチポリゴンとして表現できます。

ジオメトリの座標値はどの座標参照系(CRS)も利用できます。レイヤーから地物を持ってきたときに、ジオメトリの座標値はレイヤーのCRSのものを持つでしょう。

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

7.1. ジオメトリの構成

PyQGIS provides several options for creating a geometry:

  • 座標から

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    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.

    マルチパートジオメトリはさらに上のレベルです: マルチポイントはポイントのリストで、マルチラインストリングはラインストリングのリストで、マルチポリゴンはポリゴンのリストです。

  • well-knownテキスト(WKT)から

    geom = QgsGeometry.fromWkt("POINT(3 4)")
    print(geom)
    
  • Well-Knownバイナリ(WKB)から

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

7.2. ジオメトリにアクセス

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.

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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.

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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:

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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)>]]

注釈

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().

7.3. ジオメトリの述語と操作

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.

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# 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())
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Zambia
Area:  62.822790653431205
Perimeter:  50.65232014052552
Zimbabwe
Area:  33.41113559136521
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.

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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))
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Zambia
Perimeter (m): 5539361.250080013
Area (m2): 752000605894.2937
Area (km2): 752000.6058942936
Zimbabwe
Perimeter (m): 2865021.3323912495
Area (m2): 389250992553.95465
Area (km2): 389250.99255395465

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

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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))

QGISに含まれているアルゴリズムの多くの例を見つけて、これらのメソッドをベクターデータを分析し変換するために使用できます。ここにそれらのコードのいくつかへのリンクを記載します。