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PostGIS 1.5 allows distance queries on non-point geographic geometry columns with ST_Distance_Sphere, enabled this functionality.

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Justin Bronn 2010-03-30 23:15:43 +00:00
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24
docs/ref/contrib/gis/db-api.txt
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@ -160,21 +160,26 @@ to be in the units of the field.
.. note::
For PostGIS users, the routine ``ST_distance_sphere``
For users of PostGIS 1.4 and below, the routine ``ST_Distance_Sphere``
is used by default for calculating distances on geographic coordinate systems
-- which may only be called with point geometries [#fndistsphere]_. Thus,
geographic distance lookups on traditional PostGIS geometry columns are
(e.g., WGS84) -- which may only be called with point geometries [#fndistsphere14]_.
Thus, geographic distance lookups on traditional PostGIS geometry columns are
only allowed on :class:`PointField` model fields using a point for the
geometry parameter.
.. note::
PostGIS 1.5 introduced :ref:`geography columns <geography-type>`, which
is limited on what geometry types distance queries are performed with. In
other words, if you have ``geography=True`` in your geometry field
definition you'll be allowed to peform arbitrary distance queries with your
data in geodetic units of WGS84.
In PostGIS 1.5, ``ST_Distance_Sphere`` does *not* limit the geometry types
geographic distance queries are performed with. [#fndistsphere15]_ However,
these queries may take a long time, as great-circle distances must be
calculated on the fly for *every* row in the query. This is because the
spatial index on traditional geometry fields cannot be used.
For much better performance on WGS84 distance queries, consider using
:ref:`geography columns <geography-type>` in your database instead because
they are able to use their spatial index in distance queries.
You can tell GeoDjango to use a geography column by setting ``geography=True``
in your field definition.
For example, let's say we have a ``SouthTexasCity`` model (from the
`GeoDjango distance tests`__ ) on a *projected* coordinate system valid for cities
@ -300,5 +305,6 @@ Method PostGIS Oracle SpatiaLite
.. [#fnwkt] *See* Open Geospatial Consortium, Inc., `OpenGIS Simple Feature Specification For SQL <http://www.opengis.org/docs/99-049.pdf>`_, Document 99-049 (May 5, 1999), at Ch. 3.2.5, p. 3-11 (SQL Textual Representation of Geometry).
.. [#fnewkb] *See* `PostGIS EWKB, EWKT and Canonical Forms <http://postgis.refractions.net/documentation/manual-1.5/ch04.html#EWKB_EWKT>`_, PostGIS documentation at Ch. 4.1.2.
.. [#fngeojson] *See* Howard Butler, Martin Daly, Allan Doyle, Tim Schaub, & Christopher Schmidt, `The GeoJSON Format Specification <http://geojson.org/geojson-spec.html>`_, Revision 1.0 (June 16, 2008).
.. [#fndistsphere] *See* PostGIS 1.5 ``ST_distance_sphere`` `documentation <http://postgis.refractions.net/documentation/manual-1.5/ST_Distance_Sphere.html>`_.
.. [#fndistsphere14] *See* `PostGIS 1.4 documentation <http://postgis.refractions.net/documentation/manual-1.4/ST_Distance_Sphere.html>`_ on ``ST_distance_sphere``.
.. [#fndistsphere15] *See* `PostGIS 1.5 documentation <http://postgis.refractions.net/documentation/manual-1.5/ST_Distance_Sphere.html>`_ on ``ST_distance_sphere``.
.. [#] MySQL only supports bounding box operations (known as minimum bounding rectangles, or MBR, in MySQL). Thus, spatial lookups such as :lookup:`contains <gis-contains>` are really equivalent to :lookup:`bbcontains`.

22
docs/ref/contrib/gis/model-api.txt
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@ -107,10 +107,11 @@ a flat surface is a straight line, the shortest path between two points on a cur
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, PostGIS does 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. [#fndist]_
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.
[#fndist]_
Portions of the earth's surface may projected onto a two-dimensional, or
Cartesian, plane. Projected coordinate systems are especially convenient
@ -123,9 +124,10 @@ calculations.
.. note::
If you wish to peform arbitrary distance queries using non-point
geometries, consider using PostGIS 1.5 and enabling the
:attr:`GeometryField.geography` keyword to use the
:ref:`geography database type <geography-type>` instead.
geometries in WGS84, consider upgrading to PostGIS 1.5. For
better performance, enable the :attr:`GeometryField.geography`
keyword so that :ref:`geography database type <geography-type>`
is used instead.
Additional Resources:
@ -182,7 +184,7 @@ three-dimensonal support.
.. attribute:: GeometryField.geography
If set to ``True``, this option will use create a database column of
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.
@ -223,8 +225,6 @@ For more information, the PostGIS documentation contains a helpful section on
determining `when to use geography data type over geometry data type
<http://postgis.refractions.net/documentation/manual-1.5/ch04.html#PostGIS_GeographyVSGeometry>`_.
``GeoManager``
==============
@ -262,5 +262,5 @@ for example::
.. [#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.
.. [#fnharvard] Harvard Graduate School of Design, `An Overview of Geodesy and Geographic Referencing Systems <http://www.gsd.harvard.edu/gis/manual/projections/fundamentals/>`_. This is an excellent resource for an overview of principles relating to geographic and Cartesian coordinate systems.
.. [#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 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.
.. [#fndist] 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.
.. [#fngeography] Please refer to the `PostGIS Geography Type <http://postgis.refractions.net/documentation/manual-1.5/ch04.html#PostGIS_Geography>`_ documentation for more details.