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

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

# 7. Gestione della Geometria¶

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

Alcune volte una geometria é effettivamente una collezione di geometrie (parti singole) piú semplici. Se contiene un tipo di geometria semplice, la chiameremo punti multipli, string multi linea o poligoni multipli. Ad esempio, un Paese formato da piú isole puó essere rappresentato come un poligono multiplo.

Le coordinate delle geometrie possono essere in qualsiasi sistema di riferimento delle coordinate (CRS). Quando si estraggono delle caratteristiche da un vettore, le geometrie associate avranno le coordinate nel CRS del vettore.

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

## 7.1. Costruzione della Geometria¶

PyQGIS provides several options for creating a geometry:

• dalle coordinate

 ```1 2 3 4 5 6 7``` ```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.

Le geometrie a parti multiple vanno ad un livello successivo: punti multipli é una lista di punti, una stringa multi linea é una linea di linee ed un poligono multiplo é una lista di poligoni.

• da well-known text (WKT)

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

 ```1 2 3 4 5 6``` ```g = QgsGeometry() wkb = bytes.fromhex("010100000000000000000045400000000000001440") g.fromWkb(wkb) # print WKT representation of the geometry print(g.asWkt()) ```

## 7.2. Accedere alla Geometria¶

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.

 ```1 2 3 4 5 6 7 8 9``` ```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.

 ```1 2 3 4 5 6``` ```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:

 ```1 2 3 4 5 6``` ```print(gPnt.asPoint()) # output: print(gLine.asPolyline()) # output: [, ] print(gPolygon.asPolygon()) # output: [[, , , ]] ```

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()`.

## 7.3. Predicati ed Operazioni delle Geometrie¶

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 2 3 4 5 6 7 8 9 10 11 12 13 14``` ```# 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 \'Zu%\'' 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()) ```
 ``` 1 2 3 4 5 6 7 8 9 10 11 12``` ```Zubin Potok Area: 0.040717371293465573 Perimeter: 0.9406133328077781 Zulia Area: 3.708060762610232 Perimeter: 17.172123598311487 Zuid-Holland Area: 0.4204687950359031 Perimeter: 4.098878517120812 Zug Area: 0.027573510374275363 Perimeter: 0.7756605461489624 ```

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.

 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18``` ```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 \'Zu%\'' 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)) ```
 ``` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16``` ```Zubin Potok Perimeter (m): 87581.40256396442 Area (m2): 369302069.18814206 Area (km2): 369.30206918814207 Zulia Perimeter (m): 1891227.0945423362 Area (m2): 44973645460.19726 Area (km2): 44973.64546019726 Zuid-Holland Perimeter (m): 331941.8000214341 Area (m2): 3217213408.4100943 Area (km2): 3217.213408410094 Zug Perimeter (m): 67440.22483063207 Area (m2): 232457391.52097562 Area (km2): 232.45739152097562 ```

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

 ``` 1 2 3 4 5 6 7 8 9 10``` ```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)) ```

É possibile trovare molti esempi di algoritmi che sono inclusi in QGIS ed utilizzare questi metodi per analizzare e trasformare i dati vettoriali. Di seguito i link al codice di alcuni di questi.