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 as our example:
from django.contrib.gis.db import models
class Zipcode(models.Model):
code = models.CharField(max_length=5)
poly = models.PolygonField()
objects = models.GeoManager()
Each of the following geometry field types correspond with the OpenGIS Simple Features specification [1].
In addition to the regular Field options available for Django model fields, geometry fields have the following additional options. All are optional.
Sets the SRID [2] (Spatial Reference System Identity) of the geometry field to the given value. Defaults to 4326 (also known as WGS84, units are in degrees of longitude and 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. [4] 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. [5] 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, PostGIS versions 1.4 and below do not have the capability to perform distance calculations between non-point geometries using geographic coordinate systems, e.g., constructing a query to find all points within 5 miles of a county boundary stored as WGS84. [6]
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
If you wish to peform arbitrary distance queries using non-point geometries in WGS84, consider upgrading to PostGIS 1.5. For better performance, enable the GeometryField.geography keyword so that geography database type is used instead.
Additional Resources:
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.
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-dimensonal support.
Note
At this time 3D support requires that GEOS 3.1 be installed, and is limited only to the PostGIS spatial backend.
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
Geography support is limited only to PostGIS 1.5+, and will force the SRID to be 4326.
In PostGIS 1.5, the geography type was introduced – it provides native support for spatial features represented with geographic coordinates (e.g., WGS84 longitude/latitude). [7] 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).
Because geography calculations involve more mathematics, only a subset of the PostGIS spatial lookups are available for the geography type. Practically, this means that in addition to the distance lookups only the following additional spatial lookups are available for geography columns:
For more information, the PostGIS documentation contains a helpful section on determining when to use geography data type over geometry data type.
In order to conduct geographic queries, each geographic model requires a GeoManager model manager. This manager allows for the proper SQL construction for geographic queries; thus, without it, all geographic filters will fail. It should also be noted that GeoManager is required even if the model does not have a geographic field itself, e.g., in the case of a ForeignKey relation to a model with a geographic field. For example, if we had an Address model with a ForeignKey to our Zipcode model:
from django.contrib.gis.db import models
from django.contrib.localflavor.us.models import USStateField
class Address(models.Model):
num = models.IntegerField()
street = models.CharField(max_length=100)
city = models.CharField(max_length=100)
state = USStateField()
zipcode = models.ForeignKey(Zipcode)
objects = models.GeoManager()
The geographic manager is needed to do spatial queries on related Zipcode objects, for example:
qs = Address.objects.filter(zipcode__poly__contains='POINT(-104.590948 38.319914)')
Footnotes
| [1] | OpenGIS Consortium, Inc., Simple Feature Specification For SQL. |
| [2] | See id. at Ch. 2.3.8, p. 39 (Geometry Values and Spatial Reference Systems). |
| [3] | Typically, SRID integer corresponds to an EPSG (European Petroleum Survey Group) identifier. However, it may also be associated with custom projections defined in spatial database’s spatial reference systems table. |
| [4] | Harvard Graduate School of Design, An Overview of Geodesy and Geographic Referencing Systems. This is an excellent resource for an overview of principles relating to geographic and Cartesian coordinate systems. |
| [5] | Terry A. Slocum, Robert B. McMaster, Fritz C. Kessler, & Hugh H. Howard, Thematic Cartography and Geographic Visualization (Prentice Hall, 2nd edition), at Ch. 7.1.3. |
| [6] | This limitation does not apply to PostGIS 1.5. It should be noted that even in previous versions of PostGIS, this isn’t impossible using GeoDjango; you could for example, take a known point in a projected coordinate system, buffer it to the appropriate radius, and then perform an intersection operation with the buffer transformed to the geographic coordinate system. |
| [7] | Please refer to the PostGIS Geography Type documentation for more details. |
Dec 23, 2012