This document explores the details of the GeoDjango Model API. Throughout this section, we’ll be using the following geographic model of a ZIP code and of a Digital Elevation Model as our examples:
from django.contrib.gis.db import models
class Zipcode(models.Model):
code = models.CharField(max_length=5)
poly = models.PolygonField()
class Elevation(models.Model):
name = models.CharField(max_length=100)
rast = models.RasterField()
Les champs spatiaux consistent en une série de types de champs géométriques et en un type de champ matriciel. Chaque type de champ géométrique correspond à la spécification OpenGIS Simple Features [1]. Il n’existe pas de standard équivalent pour les données matricielles.
GeometryField¶La classe de base pour les champs de type géométrique.
PointField¶Stocke un Point.
LineStringField¶Stocke une LineString.
PolygonField¶Stocke un Polygon.
MultiPointField¶Stocke un MultiPoint.
MultiLineStringField¶Stocke une MultiLineString.
MultiPolygonField¶Stocke un MultiPolygon.
GeometryCollectionField¶Stocke une GeometryCollection.
RasterField¶Stocke un GDALRaster.
RasterField n’est actuellement implémenté que pour le moteur PostGIS.
En plus des Options des champs disponibles habituellement pour les champs de modèles Django, les champs spatiaux possèdent les options supplémentaires suivantes. Toutes sont facultatives.
srid¶Définit le SRID [2] (Spatial Reference System Identity) du champ géométrique à la valeur indiquée. La valeur par défaut est 4326 (aussi connue sous le nom WGS84, les unités étant en degrés de longitude et de latitude).
Choosing an appropriate SRID for your model is an important decision that the developer should consider carefully. The SRID is an integer specifier that corresponds to the projection system that will be used to interpret the data in the spatial database. [3] Projection systems give the context to the coordinates that specify a location. Although the details of geodesy are beyond the scope of this documentation, the general problem is that the earth is spherical and representations of the earth (e.g., paper maps, web maps) are not.
Most people are familiar with using latitude and longitude to reference a location on the earth’s surface. However, latitude and longitude are angles, not distances. In other words, while the shortest path between two points on a flat surface is a straight line, the shortest path between two points on a curved surface (such as the earth) is an arc of a great circle. [4]
Thus, additional computation is required to obtain distances in planar units (e.g., kilometers and miles). Using a geographic coordinate system may introduce complications for the developer later on. For example, SpatiaLite does not have the capability to perform distance calculations between geometries using geographic coordinate systems, e.g. constructing a query to find all points within 5 miles of a county boundary stored as WGS84. [5]
Portions of the earth’s surface may projected onto a two-dimensional, or Cartesian, plane. Projected coordinate systems are especially convenient for region-specific applications, e.g., if you know that your database will only cover geometries in North Kansas, then you may consider using projection system specific to that region. Moreover, projected coordinate systems are defined in Cartesian units (such as meters or feet), easing distance calculations.
Note
Si vous souhaitez effectuer des requêtes de distance arbitraires avec des objets géométriques autres que des points en WSG84 avec PostGIS, avec des performances décentes, définissez le mot-clé GeometryField.geography afin que le type de données geography soit utilisé.
Ressources supplémentaires :
spatialreference.org: une base de données programmée en Django de systèmes de références spatiales.
The State Plane Coordinate System: A website covering the various projection systems used in the United States. Much of the U.S. spatial data encountered will be in one of these coordinate systems rather than in a geographic coordinate system such as WGS84.
spatial_index¶Defaults to True. Creates a spatial index for the given geometry
field.
Note
This is different from the db_index field option because spatial
indexes are created in a different manner than regular database
indexes. Specifically, spatial indexes are typically created using
a variant of the R-Tree, while regular database indexes typically
use B-Trees.
Les champs géométriques disposent d’options supplémentaires. Toutes les options suivantes sont facultatives.
dim¶This option may be used for customizing the coordinate dimension of the geometry field. By default, it is set to 2, for representing two-dimensional geometries. For spatial backends that support it, it may be set to 3 for three-dimensional support.
Note
Pour l’instant, seuls les moteurs spatiaux PostGIS et SpatiaLite prennent en charge la 3D.
geography¶If set to True, this option will create a database column of
type geography, rather than geometry. Please refer to the
geography type section below for more
details.
Note
La prise en charge du type géographique nécessite PostGIS et force le SRID à 4326.
The geography type provides native support for spatial features represented
with geographic coordinates (e.g., WGS84 longitude/latitude). [6]
Unlike the plane used by a geometry type, the geography type uses a spherical
representation of its data. Distance and measurement operations
performed on a geography column automatically employ great circle arc
calculations and return linear units. In other words, when ST_Distance
is called on two geographies, a value in meters is returned (as opposed
to degrees if called on a geometry column in WGS84).
Comme les calculs géographiques induisent plus d’opérations mathématiques, seul un sous-ensemble des recherches spatiales PostGIS sont disponibles pour le type géographique. En pratique, cela signifie qu’en plus des recherches de distance, seules les recherches spatiales supplémentaires suivantes sont disponibles pour les colonnes de type géographique :
Si vous avez besoin d’utiliser une requête ou un agrégat spatial qui ne prend pas en charge le type géographique en entrée, il est possible de faire appel à la fonction de base de données Cast pour convertir la colonne géographique en type géométrique dans la requête :
from django.contrib.gis.db.models import PointField
from django.db.models.functions import Cast
Zipcode.objects.annotate(geom=Cast("geography_field", PointField())).filter(
geom__within=poly
)
For more information, the PostGIS documentation contains a helpful section on determining when to use geography data type over geometry data type.
Notes de bas de page
déc. 03, 2025