7. 도형 다루기

힌트

여러분이 PyQGIS 콘솔을 사용하지 않는 경우 이 페이지에 있는 코드 조각들을 다음과 같이 가져와야 합니다:

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.

도형 한 개가 실제로는 단순(단일 영역, single-part) 도형의 집합인 경우가 종종 있습니다. 이런 도형을 다중 영역(multi-part) 도형이라고 합니다. 다중 영역 도형이 한 가지 유형의 단순 도형으로만 이루어져 있을 경우 다중 포인트, 다중 라인스트링, 다중 폴리곤이라 부릅니다. 예를 들어 여러 개의 섬으로 이루어진 국가라면 다중 폴리곤으로 표현할 수 있습니다.

도형의 좌표는 어떤 좌표계(CRS)라도 될 수 있습니다. 레이어에서 피처를 불러올 때, 해당 도형은 레이어의 좌표계를 따르는 좌표를 가지게 될 겁니다.

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

7.1. 도형 작성

PyQGIS provides several options for creating a geometry:

• 좌표로부터

  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.

  다중 영역 도형은 한 단계 심화됩니다. 다중 포인트는 포인트의 목록, 다중 라인스트링은 라인스트링의 목록, 다중 폴리곤은 폴리곤의 목록입니다.

• WKT(well-known text)로부터

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

• WKB(well-known binary)로부터

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

7.2. 도형에 접근

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)>]]

참고

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. 도형 관계계산 및 연산

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))

QGIS에 포함되어 있는, 벡터 데이터를 분석하고 변환하는 데 사용할 수 있는 알고리즘들의 수많은 예시가 있습니다. 다음은 링크들은 그 가운데 몇몇 코드를 보여줍니다.

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

• Lines to polygons algorithm