mirror of
https://github.com/django/django.git
synced 2024-12-24 01:55:49 +00:00
254 lines
8.9 KiB
Plaintext
254 lines
8.9 KiB
Plaintext
===================
|
|
GeoDjango Model API
|
|
===================
|
|
|
|
.. module:: django.contrib.gis.db.models
|
|
:synopsis: GeoDjango model and field API.
|
|
|
|
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()
|
|
|
|
__ https://en.wikipedia.org/wiki/ZIP_code
|
|
__ https://en.wikipedia.org/wiki/Digital_elevation_model
|
|
|
|
Spatial Field Types
|
|
===================
|
|
|
|
Spatial fields consist of a series of geometry field types and one raster field
|
|
type. Each of the geometry field types correspond to the OpenGIS Simple
|
|
Features specification [#fnogc]_. There is no such standard for raster data.
|
|
|
|
``GeometryField``
|
|
-----------------
|
|
|
|
.. class:: GeometryField
|
|
|
|
``PointField``
|
|
--------------
|
|
|
|
.. class:: PointField
|
|
|
|
``LineStringField``
|
|
-------------------
|
|
|
|
.. class:: LineStringField
|
|
|
|
``PolygonField``
|
|
----------------
|
|
|
|
.. class:: PolygonField
|
|
|
|
``MultiPointField``
|
|
-------------------
|
|
|
|
.. class:: MultiPointField
|
|
|
|
``MultiLineStringField``
|
|
------------------------
|
|
|
|
.. class:: MultiLineStringField
|
|
|
|
``MultiPolygonField``
|
|
---------------------
|
|
|
|
.. class:: MultiPolygonField
|
|
|
|
``GeometryCollectionField``
|
|
---------------------------
|
|
|
|
.. class:: GeometryCollectionField
|
|
|
|
``RasterField``
|
|
---------------
|
|
|
|
.. class:: RasterField
|
|
|
|
``RasterField`` is currently only implemented for the PostGIS backend.
|
|
|
|
Spatial Field Options
|
|
=====================
|
|
|
|
In addition to the regular :ref:`common-model-field-options` available for
|
|
Django model fields, spatial fields have the following additional options.
|
|
All are optional.
|
|
|
|
``srid``
|
|
--------
|
|
|
|
.. attribute:: BaseSpatialField.srid
|
|
|
|
Sets the SRID [#fnogcsrid]_ (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).
|
|
|
|
__ https://en.wikipedia.org/wiki/WGS84
|
|
|
|
.. _selecting-an-srid:
|
|
|
|
Selecting an SRID
|
|
~~~~~~~~~~~~~~~~~
|
|
|
|
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. [#fnsrid]_ 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`__. [#fnthematic]_ 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.
|
|
[#fndist]_
|
|
|
|
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 perform arbitrary distance queries using non-point
|
|
geometries in WGS84 in PostGIS and you want decent performance, enable the
|
|
:attr:`GeometryField.geography` keyword so that :ref:`geography database
|
|
type <geography-type>` is used instead.
|
|
|
|
Additional Resources:
|
|
|
|
* `spatialreference.org`__: A Django-powered database of spatial reference
|
|
systems.
|
|
* `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.
|
|
|
|
__ https://en.wikipedia.org/wiki/Geodesy
|
|
__ https://en.wikipedia.org/wiki/Great_circle
|
|
__ http://www.spatialreference.org/ref/epsg/2796/
|
|
__ http://spatialreference.org/
|
|
__ https://web.archive.org/web/20080302095452/http://welcome.warnercnr.colostate.edu/class_info/nr502/lg3/datums_coordinates/spcs.html
|
|
|
|
``spatial_index``
|
|
-----------------
|
|
|
|
.. attribute:: BaseSpatialField.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.
|
|
|
|
.. _geometry-field-options:
|
|
|
|
Geometry Field Options
|
|
======================
|
|
|
|
There are additional options available for Geometry fields. All the following
|
|
options are optional.
|
|
|
|
``dim``
|
|
-------
|
|
|
|
.. attribute:: GeometryField.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::
|
|
|
|
At this time 3D support is limited to the PostGIS spatial backend.
|
|
|
|
``geography``
|
|
-------------
|
|
|
|
.. attribute:: GeometryField.geography
|
|
|
|
If set to ``True``, this option will create a database column of
|
|
type geography, rather than geometry. Please refer to the
|
|
:ref:`geography type <geography-type>` section below for more
|
|
details.
|
|
|
|
.. note::
|
|
|
|
Geography support is limited to PostGIS and will force the SRID to be 4326.
|
|
|
|
.. _geography-type:
|
|
|
|
Geography Type
|
|
~~~~~~~~~~~~~~
|
|
|
|
The geography type provides native support for spatial features represented
|
|
with geographic coordinates (e.g., WGS84 longitude/latitude). [#fngeography]_
|
|
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 :ref:`distance lookups <distance-lookups>`
|
|
only the following additional :ref:`spatial lookups <spatial-lookups>` are
|
|
available for geography columns:
|
|
|
|
* :lookup:`bboverlaps`
|
|
* :lookup:`coveredby`
|
|
* :lookup:`covers`
|
|
* :lookup:`intersects`
|
|
|
|
If you need to use a spatial lookup or aggregate that doesn't support the
|
|
geography type as input, you can use the
|
|
:class:`~django.db.models.functions.Cast` database function to convert the
|
|
geography column to a geometry type in the query::
|
|
|
|
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
|
|
<https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_GeographyVSGeometry>`_.
|
|
|
|
.. rubric:: Footnotes
|
|
.. [#fnogc] OpenGIS Consortium, Inc., `Simple Feature Specification For SQL <http://www.opengeospatial.org/standards/sfs>`_.
|
|
.. [#fnogcsrid] *See id.* at Ch. 2.3.8, p. 39 (Geometry Values and Spatial Reference Systems).
|
|
.. [#fnsrid] Typically, SRID integer corresponds to an EPSG (`European Petroleum Survey Group <http://www.epsg.org>`_) identifier. However, it may also be associated with custom projections defined in spatial database's spatial reference systems table.
|
|
.. [#fnthematic] 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.
|
|
.. [#fndist] This limitation does not apply to PostGIS.
|
|
.. [#fngeography] Please refer to the `PostGIS Geography Type <https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography>`_ documentation for more details.
|