# Manejo de Geometría¶

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.

A veces una geometría es realmente una colección simple (partes simples) geométricas. Tal geometría se llama geometría de múltiples partes. Si contiene un tipo de geometría simple, lo llamamos un punto múltiple, lineas múltiples o polígonos múltiples. Por ejemplo, un país consiste en múltiples islas que se pueden representar como un polígono múltiple.

Las coordenadas de las geometrías pueden estar en cualquier sistema de referencia de coordenadas (SRC). Cuando extrae características de una capa, las geometrías asociadas tendrán sus coordenadas en el SRC de la capa.

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

## Construcción de Geometría¶

PyQGIS provides several options for creating a geometry:

```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.

Las geometrías multi-parte van un nivel más allá: multi-punto es una lista de puntos, multi-linea es una lista de polilíneas y multi-polígono es una lista de polígonos.

• desde well-known text (WKT)

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

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

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

## Acceso a Geometría¶

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()`.

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))
```

Puede encontrar muchos ejemplos de algoritmos que se incluyen en QGIS y utilizan estos métodos para analizar y transformar los datos vectoriales. Aquí hay algunos enlaces al código de algunos de ellos.