django/docs/ref/contrib/gis/tutorial.txt

789 lines
27 KiB
Plaintext
Raw Normal View History

==================
GeoDjango Tutorial
==================
Introduction
============
GeoDjango is an included contrib module for Django that turns it into a
world-class geographic Web framework. GeoDjango strives to make it as simple
as possible to create geographic Web applications, like location-based services.
Its features include:
* Django model fields for `OGC`_ geometries.
* Extensions to Django's ORM for querying and manipulating spatial data.
* Loosely-coupled, high-level Python interfaces for GIS geometry operations and
data formats.
* Editing geometry fields from the admin.
This tutorial assumes familiarity with Django; thus, if you're brand new to
Django, please read through the :doc:`regular tutorial </intro/tutorial01>` to
familiarize yourself with Django first.
.. note::
GeoDjango has additional requirements beyond what Django requires --
please consult the :ref:`installation documentation <ref-gis-install>`
for more details.
This tutorial will guide you through the creation of a geographic web
application for viewing the `world borders`_. [#]_ Some of the code
used in this tutorial is taken from and/or inspired by the `GeoDjango
basic apps`_ project. [#]_
.. note::
Proceed through the tutorial sections sequentially for step-by-step
instructions.
.. _OGC: http://www.opengeospatial.org/
.. _world borders: http://thematicmapping.org/downloads/world_borders.php
.. _GeoDjango basic apps: http://code.google.com/p/geodjango-basic-apps/
Setting Up
==========
Create a Spatial Database
-------------------------
.. note::
MySQL and Oracle users can skip this section because spatial types
are already built into the database.
First, create a spatial database for your project.
If you are using PostGIS, create the database from the :ref:`spatial database
template <spatialdb_template>`:
.. code-block:: bash
$ createdb -T template_postgis geodjango
.. note::
This command must be issued by a database user with enough privileges to
create a database. To create a user with ``CREATE DATABASE`` privileges in
PostgreSQL, use the following commands:
.. code-block:: bash
$ sudo su - postgres
$ createuser --createdb geo
$ exit
Replace ``geo`` with your Postgres database user's username.
(In PostgreSQL, this user will also be an OS-level user.)
If you are using SQLite and SpatiaLite, consult the instructions on how
to create a :ref:`SpatiaLite database <create_spatialite_db>`.
Create a New Project
------------------------
Use the standard ``django-admin.py`` script to create a project called
``geodjango``:
.. code-block:: bash
$ django-admin.py startproject geodjango
This will initialize a new project. Now, create a ``world`` Django application
within the ``geodjango`` project:
.. code-block:: bash
$ cd geodjango
$ python manage.py startapp world
Configure ``settings.py``
-------------------------
The ``geodjango`` project settings are stored in the ``geodjango/settings.py``
file. Edit the database connection settings to match your setup::
DATABASES = {
'default': {
'ENGINE': 'django.contrib.gis.db.backends.postgis',
'NAME': 'geodjango',
'USER': 'geo',
}
}
In addition, modify the :setting:`INSTALLED_APPS` setting to include
:mod:`django.contrib.admin`, :mod:`django.contrib.gis`,
and ``world`` (your newly created application)::
INSTALLED_APPS = (
Simplified default project template. Squashed commit of: commit 508ec9144b35c50794708225b496bde1eb5e60aa Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 22:50:55 2013 +0100 Tweaked default settings file. * Explained why BASE_DIR exists. * Added a link to the database configuration options, and put it in its own section. * Moved sensitive settings that must be changed for production at the top. commit 6515fd2f1aa73a86dc8dbd2ccf512ddb6b140d57 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 14:35:21 2013 +0100 Documented the simplified app & project templates in the changelog. commit 2c5b576c2ea91d84273a019b3d0b3b8b4da72f23 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 13:59:27 2013 +0100 Minor fixes in tutorials 5 and 6. commit 55a51531be8104f21b3cca3f6bf70b0a7139a041 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 13:51:11 2013 +0100 Updated tutorial 2 for the new project template. commit 29ddae87bdaecff12dd31b16b000c01efbde9e20 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 11:58:54 2013 +0100 Updated tutorial 1 for the new project template. commit 0ecb9f6e2514cfd26a678a280d471433375101a3 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 11:29:13 2013 +0100 Adjusted the default URLconf detection to account for the admin. It's now enabled by default. commit 5fb4da0d3d09dac28dd94e3fde92b9d4335c0565 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 10:36:55 2013 +0100 Added security warnings for the most sensitive settings. commit 718d84bd8ac4a42fb4b28ec93965de32680f091e Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 23:24:06 2013 +0100 Used an absolute path for the SQLite database. This ensures the settings file works regardless of which directory django-admin.py / manage.py is invoked from. BASE_DIR got a +1 from a BDFL and another core dev. It doesn't involve the concept of a "Django project"; it's just a convenient way to express relative paths within the source code repository for non-Python files. Thanks Jacob Kaplan-Moss for the suggestion. commit 1b559b4bcda622e10909b68fe5cab90db6727dd9 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 23:22:40 2013 +0100 Removed STATIC_ROOT from the default settings template. It isn't necessary in development, and it confuses beginners to no end. Thanks Carl Meyer for the suggestion. commit a55f141a500bb7c9a1bc259bbe1954c13b199671 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 23:21:43 2013 +0100 Removed MEDIA_ROOT/URL from default settings template. Many sites will never deal with user-uploaded files, and MEDIA_ROOT is complicated to explain. Thanks Carl Meyer for the suggestion. commit 44bf2f2441420fd9429ee9fe1f7207f92dd87e70 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 22:22:09 2013 +0100 Removed logging config. This configuration is applied regardless of the value of LOGGING; duplicating it in LOGGING is confusing. commit eac747e848eaed65fd5f6f254f0a7559d856f88f Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 22:05:31 2013 +0100 Enabled the locale middleware by default. USE_I18N is True by default, and doesn't work well without LocaleMiddleware. commit d806c62b2d00826dc2688c84b092627b8d571cab Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 22:03:16 2013 +0100 Enabled clickjacking protection by default. commit 99152c30e6a15003f0b6737dc78e87adf462aacb Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 22:01:48 2013 +0100 Reorganized settings in logical sections, and trimmed comments. commit d37ffdfcb24b7e0ec7cc113d07190f65fb12fb8a Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:54:11 2013 +0100 Avoided misleading TEMPLATE_DEBUG = DEBUG. According to the docs TEMPLATE_DEBUG works only when DEBUG = True. commit 15d9478d3a9850e85841e7cf09cf83050371c6bf Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:46:25 2013 +0100 Removed STATICFILES_FINDERS/TEMPLATE_LOADERS from default settings file. Only developers with special needs ever need to change these settings. commit 574da0eb5bfb4570883756914b4dbd7e20e1f61e Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:45:01 2013 +0100 Removed STATICFILES/TEMPLATES_DIRS from default settings file. The current best practice is to put static files and templates in applications, for easier testing and deployment. commit 8cb18dbe56629aa1be74718a07e7cc66b4f9c9f0 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:24:16 2013 +0100 Removed settings related to email reporting from default settings file. While handy for small scale projects, it isn't exactly a best practice. commit 8ecbfcb3638058f0c49922540f874a7d802d864f Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 18:54:43 2013 +0100 Documented how to enable the sites framework. commit 23fc91a6fa67d91ddd9d71b1c3e0dc26bdad9841 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:28:59 2013 +0100 Disabled the sites framework by default. RequestSite does the job for single-domain websites. commit c4d82eb8afc0eb8568bf9c4d12644272415e3960 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Tue Jan 29 00:08:33 2013 +0100 Added a default admin.py to the application template. Thanks Ryan D Hiebert for the suggestion. commit 4071dc771e5c44b1c5ebb9beecefb164ae465e22 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 10:59:49 2013 +0100 Enabled the admin by default. Everyone uses the admin. commit c807a31f8d89e7e7fd97380e3023f7983a8b6fcb Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 10:57:05 2013 +0100 Removed admindocs from default project template. commit 09e4ce0e652a97da1a9e285046a91c8ad7a9189c Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:32:52 2013 +0100 Added links to the settings documentation. commit 5b8f5eaef364eb790fcde6f9e86f7d266074cca8 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 11:06:54 2013 +0100 Used a significant example for URLconf includes. commit 908e91d6fcee2a3cb51ca26ecdf12a6a24e69ef8 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 16:22:31 2013 +0100 Moved code comments about WSGI to docs, and rewrote said docs. commit 50417e51996146f891d08ca8b74dcc736a581932 Author: Aymeric Augustin <aymeric.augustin@m4x.org> Date: Mon Jan 28 15:51:50 2013 +0100 Normalized the default application template. Removed the default test that 1 + 1 = 2, because it's been committed way too many times, in too many projects. Added an import of `render` for views, because the first view will often be: def home(request): return render(request, "mysite/home.html")
2013-01-28 14:51:50 +00:00
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'django.contrib.gis',
'world'
)
Geographic Data
===============
.. _worldborders:
World Borders
-------------
The world borders data is available in this `zip file`__. Create a ``data``
directory in the ``world`` application, download the world borders data, and
unzip. On GNU/Linux platforms, use the following commands:
.. code-block:: bash
$ mkdir world/data
$ cd world/data
$ wget http://thematicmapping.org/downloads/TM_WORLD_BORDERS-0.3.zip
$ unzip TM_WORLD_BORDERS-0.3.zip
$ cd ../..
The world borders ZIP file contains a set of data files collectively known as
an `ESRI Shapefile`__, one of the most popular geospatial data formats. When
unzipped, the world borders dataset includes files with the following
extensions:
* ``.shp``: Holds the vector data for the world borders geometries.
* ``.shx``: Spatial index file for geometries stored in the ``.shp``.
* ``.dbf``: Database file for holding non-geometric attribute data
(e.g., integer and character fields).
* ``.prj``: Contains the spatial reference information for the geographic
data stored in the shapefile.
__ http://thematicmapping.org/downloads/TM_WORLD_BORDERS-0.3.zip
__ http://en.wikipedia.org/wiki/Shapefile
Use ``ogrinfo`` to examine spatial data
---------------------------------------
The GDAL ``ogrinfo`` utility allows examining the metadata of shapefiles or
other vector data sources:
.. code-block:: bash
$ ogrinfo world/data/TM_WORLD_BORDERS-0.3.shp
INFO: Open of `world/data/TM_WORLD_BORDERS-0.3.shp'
using driver `ESRI Shapefile' successful.
1: TM_WORLD_BORDERS-0.3 (Polygon)
``ogrinfo`` tells us that the shapefile has one layer, and that this
layer contains polygon data. To find out more, we'll specify the layer name
and use the ``-so`` option to get only the important summary information:
.. code-block:: bash
$ ogrinfo -so world/data/TM_WORLD_BORDERS-0.3.shp TM_WORLD_BORDERS-0.3
INFO: Open of `world/data/TM_WORLD_BORDERS-0.3.shp'
using driver `ESRI Shapefile' successful.
Layer name: TM_WORLD_BORDERS-0.3
Geometry: Polygon
Feature Count: 246
Extent: (-180.000000, -90.000000) - (180.000000, 83.623596)
Layer SRS WKT:
GEOGCS["GCS_WGS_1984",
DATUM["WGS_1984",
SPHEROID["WGS_1984",6378137.0,298.257223563]],
PRIMEM["Greenwich",0.0],
UNIT["Degree",0.0174532925199433]]
FIPS: String (2.0)
ISO2: String (2.0)
ISO3: String (3.0)
UN: Integer (3.0)
NAME: String (50.0)
AREA: Integer (7.0)
POP2005: Integer (10.0)
REGION: Integer (3.0)
SUBREGION: Integer (3.0)
LON: Real (8.3)
LAT: Real (7.3)
This detailed summary information tells us the number of features in the layer
(246), the geographic bounds of the data, the spatial reference system
("SRS WKT"), as well as type information for each attribute field. For example,
``FIPS: String (2.0)`` indicates that the ``FIPS`` character field has
a maximum length of 2. Similarly, ``LON: Real (8.3)`` is a floating-point
field that holds a maximum of 8 digits up to three decimal places.
Geographic Models
=================
Defining a Geographic Model
---------------------------
Now that you've examined your dataset using ``ogrinfo``, create a GeoDjango
model to represent this data::
from django.contrib.gis.db import models
class WorldBorder(models.Model):
# Regular Django fields corresponding to the attributes in the
# world borders shapefile.
name = models.CharField(max_length=50)
area = models.IntegerField()
pop2005 = models.IntegerField('Population 2005')
fips = models.CharField('FIPS Code', max_length=2)
iso2 = models.CharField('2 Digit ISO', max_length=2)
iso3 = models.CharField('3 Digit ISO', max_length=3)
un = models.IntegerField('United Nations Code')
region = models.IntegerField('Region Code')
subregion = models.IntegerField('Sub-Region Code')
lon = models.FloatField()
lat = models.FloatField()
# GeoDjango-specific: a geometry field (MultiPolygonField), and
# overriding the default manager with a GeoManager instance.
mpoly = models.MultiPolygonField()
objects = models.GeoManager()
# Returns the string representation of the model.
# On Python 3: def __str__(self):
def __unicode__(self):
return self.name
Please note two important things:
1. The ``models`` module is imported from ``django.contrib.gis.db``.
2. You must override the model's default manager with
:class:`~django.contrib.gis.db.models.GeoManager` to perform spatial queries.
The default spatial reference system for geometry fields is WGS84 (meaning
the `SRID`__ is 4326) -- in other words, the field coordinates are in
longitude, latitude pairs in units of degrees. To use a different
coordinate system, set the SRID of the geometry field with the ``srid``
argument. Use an integer representing the coordinate system's EPSG code.
__ http://en.wikipedia.org/wiki/SRID
Run ``migrate``
---------------
After defining your model, you need to sync it with the database. First,
let's look at the SQL that will generate the table for the
``WorldBorder`` model::
$ python manage.py sqlall world
This command should produce the following output:
.. code-block:: sql
BEGIN;
CREATE TABLE "world_worldborder" (
"id" serial NOT NULL PRIMARY KEY,
"name" varchar(50) NOT NULL,
"area" integer NOT NULL,
"pop2005" integer NOT NULL,
"fips" varchar(2) NOT NULL,
"iso2" varchar(2) NOT NULL,
"iso3" varchar(3) NOT NULL,
"un" integer NOT NULL,
"region" integer NOT NULL,
"subregion" integer NOT NULL,
"lon" double precision NOT NULL,
"lat" double precision NOT NULL
)
;
SELECT AddGeometryColumn('world_worldborder', 'mpoly', 4326, 'MULTIPOLYGON', 2);
ALTER TABLE "world_worldborder" ALTER "mpoly" SET NOT NULL;
CREATE INDEX "world_worldborder_mpoly_id" ON "world_worldborder" USING GIST ( "mpoly" GIST_GEOMETRY_OPS );
COMMIT;
If this looks correct, run :djadmin:`migrate` to create this table in the database::
$ python manage.py migrate
Creating table world_worldborder
Installing custom SQL for world.WorldBorder model
The :djadmin:`migrate` command may also prompt you to create an admin user.
Either do so now, or later by running ``django-admin.py createsuperuser``.
Importing Spatial Data
======================
This section will show you how to import the world borders
shapefile into the database via GeoDjango models using the
:ref:`ref-layermapping`.
There are many different ways to import data into a spatial database --
besides the tools included within GeoDjango, you may also use the following:
* `ogr2ogr`_: A command-line utility included with GDAL that
can import many vector data formats into PostGIS, MySQL, and Oracle databases.
* `shp2pgsql`_: This utility included with PostGIS imports ESRI shapefiles into
PostGIS.
.. _ogr2ogr: http://www.gdal.org/ogr2ogr.html
.. _shp2pgsql: http://postgis.refractions.net/documentation/manual-1.5/ch04.html#shp2pgsql_usage
.. _gdalinterface:
GDAL Interface
--------------
Earlier, you used ``ogrinfo`` to examine the contents of the world borders
shapefile. GeoDjango also includes a Pythonic interface to GDAL's powerful OGR
library that can work with all the vector data sources that OGR supports.
First, invoke the Django shell:
.. code-block:: bash
$ python manage.py shell
If you downloaded the :ref:`worldborders` data earlier in the
tutorial, then you can determine its path using Python's built-in
``os`` module::
>>> import os
>>> import world
>>> world_shp = os.path.abspath(os.path.join(os.path.dirname(world.__file__),
... 'data/TM_WORLD_BORDERS-0.3.shp'))
Now, open the world borders shapefile using GeoDjango's
:class:`~django.contrib.gis.gdal.DataSource` interface::
>>> from django.contrib.gis.gdal import DataSource
>>> ds = DataSource(world_shp)
>>> print(ds)
/ ... /geodjango/world/data/TM_WORLD_BORDERS-0.3.shp (ESRI Shapefile)
Data source objects can have different layers of geospatial features; however,
shapefiles are only allowed to have one layer::
>>> print(len(ds))
1
>>> lyr = ds[0]
>>> print(lyr)
TM_WORLD_BORDERS-0.3
You can see the layer's geometry type and how many features it contains::
>>> print(lyr.geom_type)
Polygon
>>> print(len(lyr))
246
.. note::
Unfortunately, the shapefile data format does not allow for greater
specificity with regards to geometry types. This shapefile, like
many others, actually includes ``MultiPolygon`` geometries, not Polygons.
It's important to use a more general field type in models: a
GeoDjango ``MultiPolygonField`` will accept a ``Polygon`` geometry, but a
``PolygonField`` will not accept a ``MultiPolygon`` type geometry. This
is why the ``WorldBorder`` model defined above uses a ``MultiPolygonField``.
The :class:`~django.contrib.gis.gdal.Layer` may also have a spatial reference
system associated with it. If it does, the ``srs`` attribute will return a
:class:`~django.contrib.gis.gdal.SpatialReference` object::
>>> srs = lyr.srs
>>> print(srs)
GEOGCS["GCS_WGS_1984",
DATUM["WGS_1984",
SPHEROID["WGS_1984",6378137.0,298.257223563]],
PRIMEM["Greenwich",0.0],
UNIT["Degree",0.0174532925199433]]
>>> srs.proj4 # PROJ.4 representation
'+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs '
This shapefile is in the popular WGS84 spatial reference
system -- in other words, the data uses longitude, latitude pairs in
units of degrees.
In addition, shapefiles also support attribute fields that may contain
additional data. Here are the fields on the World Borders layer:
>>> print(lyr.fields)
['FIPS', 'ISO2', 'ISO3', 'UN', 'NAME', 'AREA', 'POP2005', 'REGION', 'SUBREGION', 'LON', 'LAT']
The following code will let you examine the OGR types (e.g. integer or
string) associated with each of the fields:
>>> [fld.__name__ for fld in lyr.field_types]
['OFTString', 'OFTString', 'OFTString', 'OFTInteger', 'OFTString', 'OFTInteger', 'OFTInteger', 'OFTInteger', 'OFTInteger', 'OFTReal', 'OFTReal']
You can iterate over each feature in the layer and extract information from both
the feature's geometry (accessed via the ``geom`` attribute) as well as the
feature's attribute fields (whose **values** are accessed via ``get()``
method)::
>>> for feat in lyr:
... print(feat.get('NAME'), feat.geom.num_points)
...
Guernsey 18
Jersey 26
South Georgia South Sandwich Islands 338
Taiwan 363
:class:`~django.contrib.gis.gdal.Layer` objects may be sliced::
>>> lyr[0:2]
[<django.contrib.gis.gdal.feature.Feature object at 0x2f47690>, <django.contrib.gis.gdal.feature.Feature object at 0x2f47650>]
And individual features may be retrieved by their feature ID::
>>> feat = lyr[234]
>>> print(feat.get('NAME'))
San Marino
Boundary geometries may be exported as WKT and GeoJSON::
>>> geom = feat.geom
>>> print(geom.wkt)
POLYGON ((12.415798 43.957954,12.450554 ...
>>> print(geom.json)
{ "type": "Polygon", "coordinates": [ [ [ 12.415798, 43.957954 ], [ 12.450554, 43.979721 ], ...
``LayerMapping``
----------------
To import the data, use a LayerMapping in a Python script.
Create a file called ``load.py`` inside the ``world`` application,
with the following code::
import os
from django.contrib.gis.utils import LayerMapping
from models import WorldBorder
world_mapping = {
'fips' : 'FIPS',
'iso2' : 'ISO2',
'iso3' : 'ISO3',
'un' : 'UN',
'name' : 'NAME',
'area' : 'AREA',
'pop2005' : 'POP2005',
'region' : 'REGION',
'subregion' : 'SUBREGION',
'lon' : 'LON',
'lat' : 'LAT',
'mpoly' : 'MULTIPOLYGON',
}
world_shp = os.path.abspath(os.path.join(os.path.dirname(__file__), 'data/TM_WORLD_BORDERS-0.3.shp'))
def run(verbose=True):
lm = LayerMapping(WorldBorder, world_shp, world_mapping,
transform=False, encoding='iso-8859-1')
lm.save(strict=True, verbose=verbose)
A few notes about what's going on:
* Each key in the ``world_mapping`` dictionary corresponds to a field in the
``WorldBorder`` model. The value is the name of the shapefile field
that data will be loaded from.
* The key ``mpoly`` for the geometry field is ``MULTIPOLYGON``, the
geometry type GeoDjango will import the field as. Even simple polygons in
the shapefile will automatically be converted into collections prior to
insertion into the database.
* The path to the shapefile is not absolute -- in other words, if you move the
``world`` application (with ``data`` subdirectory) to a different location,
the script will still work.
* The ``transform`` keyword is set to ``False`` because the data in the
shapefile does not need to be converted -- it's already in WGS84 (SRID=4326).
* The ``encoding`` keyword is set to the character encoding of the string
values in the shapefile. This ensures that string values are read and saved
correctly from their original encoding system.
Afterwards, invoke the Django shell from the ``geodjango`` project directory:
.. code-block:: bash
$ python manage.py shell
Next, import the ``load`` module, call the ``run`` routine, and watch
``LayerMapping`` do the work::
>>> from world import load
>>> load.run()
.. _ogrinspect-intro:
Try ``ogrinspect``
------------------
Now that you've seen how to define geographic models and import data with the
:ref:`ref-layermapping`, it's possible to further automate this process with
use of the :djadmin:`ogrinspect` management command. The :djadmin:`ogrinspect`
command introspects a GDAL-supported vector data source (e.g., a shapefile)
and generates a model definition and ``LayerMapping`` dictionary automatically.
The general usage of the command goes as follows:
.. code-block:: bash
$ python manage.py ogrinspect [options] <data_source> <model_name> [options]
``data_source`` is the path to the GDAL-supported data source and
``model_name`` is the name to use for the model. Command-line options may
be used to further define how the model is generated.
For example, the following command nearly reproduces the ``WorldBorder`` model
and mapping dictionary created above, automatically:
.. code-block:: bash
$ python manage.py ogrinspect world/data/TM_WORLD_BORDERS-0.3.shp WorldBorder \
--srid=4326 --mapping --multi
A few notes about the command-line options given above:
* The ``--srid=4326`` option sets the SRID for the geographic field.
* The ``--mapping`` option tells ``ogrinspect`` to also generate a
mapping dictionary for use with
:class:`~django.contrib.gis.utils.LayerMapping`.
* The ``--multi`` option is specified so that the geographic field is a
:class:`~django.contrib.gis.db.models.MultiPolygonField` instead of just a
:class:`~django.contrib.gis.db.models.PolygonField`.
The command produces the following output, which may be copied
directly into the ``models.py`` of a GeoDjango application::
# This is an auto-generated Django model module created by ogrinspect.
from django.contrib.gis.db import models
class WorldBorder(models.Model):
fips = models.CharField(max_length=2)
iso2 = models.CharField(max_length=2)
iso3 = models.CharField(max_length=3)
un = models.IntegerField()
name = models.CharField(max_length=50)
area = models.IntegerField()
pop2005 = models.IntegerField()
region = models.IntegerField()
subregion = models.IntegerField()
lon = models.FloatField()
lat = models.FloatField()
geom = models.MultiPolygonField(srid=4326)
objects = models.GeoManager()
# Auto-generated `LayerMapping` dictionary for WorldBorder model
worldborders_mapping = {
'fips' : 'FIPS',
'iso2' : 'ISO2',
'iso3' : 'ISO3',
'un' : 'UN',
'name' : 'NAME',
'area' : 'AREA',
'pop2005' : 'POP2005',
'region' : 'REGION',
'subregion' : 'SUBREGION',
'lon' : 'LON',
'lat' : 'LAT',
'geom' : 'MULTIPOLYGON',
}
Spatial Queries
===============
Spatial Lookups
---------------
GeoDjango adds spatial lookups to the Django ORM. For example, you
can find the country in the ``WorldBorder`` table that contains
a particular point. First, fire up the management shell:
.. code-block:: bash
$ python manage.py shell
Now, define a point of interest [#]_::
>>> pnt_wkt = 'POINT(-95.3385 29.7245)'
The ``pnt_wkt`` string represents the point at -95.3385 degrees longitude,
29.7245 degrees latitude. The geometry is in a format known as
Well Known Text (WKT), a standard issued by the Open Geospatial
Consortium (OGC). [#]_ Import the ``WorldBorder`` model, and perform
a ``contains`` lookup using the ``pnt_wkt`` as the parameter::
>>> from world.models import WorldBorder
>>> qs = WorldBorder.objects.filter(mpoly__contains=pnt_wkt)
>>> qs
[<WorldBorder: United States>]
Here, you retrieved a ``GeoQuerySet`` with only one model: the border of
the United States (exactly what you would expect).
Similarly, you may also use a :ref:`GEOS geometry object <ref-geos>`.
Here, you can combine the ``intersects`` spatial lookup with the ``get``
method to retrieve only the ``WorldBorder`` instance for San Marino instead
of a queryset::
>>> from django.contrib.gis.geos import Point
>>> pnt = Point(12.4604, 43.9420)
>>> sm = WorldBorder.objects.get(mpoly__intersects=pnt)
>>> sm
<WorldBorder: San Marino>
The ``contains`` and ``intersects`` lookups are just a subset of the
available queries -- the :ref:`ref-gis-db-api` documentation has more.
Automatic Spatial Transformations
---------------------------------
When doing spatial queries, GeoDjango automatically transforms
geometries if they're in a different coordinate system. In the following
example, coordinates will be expressed in `EPSG SRID 32140`__,
a coordinate system specific to south Texas **only** and in units of
**meters**, not degrees::
>>> from django.contrib.gis.geos import Point, GEOSGeometry
>>> pnt = Point(954158.1, 4215137.1, srid=32140)
Note that ``pnt`` may also be constructed with EWKT, an "extended" form of
WKT that includes the SRID::
>>> pnt = GEOSGeometry('SRID=32140;POINT(954158.1 4215137.1)')
GeoDjango's ORM will automatically wrap geometry values
in transformation SQL, allowing the developer to work at a higher level
of abstraction::
>>> qs = WorldBorder.objects.filter(mpoly__intersects=pnt)
>>> print(qs.query) # Generating the SQL
SELECT "world_worldborder"."id", "world_worldborder"."name", "world_worldborder"."area",
"world_worldborder"."pop2005", "world_worldborder"."fips", "world_worldborder"."iso2",
"world_worldborder"."iso3", "world_worldborder"."un", "world_worldborder"."region",
"world_worldborder"."subregion", "world_worldborder"."lon", "world_worldborder"."lat",
"world_worldborder"."mpoly" FROM "world_worldborder"
WHERE ST_Intersects("world_worldborder"."mpoly", ST_Transform(%s, 4326))
>>> qs # printing evaluates the queryset
[<WorldBorder: United States>]
__ http://spatialreference.org/ref/epsg/32140/
.. admonition:: Raw queries
When using :doc:`raw queries </topics/db/sql>`, you should generally wrap
your geometry fields with the ``asText()`` SQL function (or ``ST_AsText``
for PostGIS) so that the field value will be recognized by GEOS::
City.objects.raw('SELECT id, name, asText(point) from myapp_city')
This is not absolutely required by PostGIS, but generally you should only
use raw queries when you know exactly what you are doing.
Lazy Geometries
---------------
GeoDjango loads geometries in a standardized textual representation. When the
geometry field is first accessed, GeoDjango creates a `GEOS geometry object
<ref-geos>`, exposing powerful functionality, such as serialization properties
for popular geospatial formats::
>>> sm = WorldBorder.objects.get(name='San Marino')
>>> sm.mpoly
<MultiPolygon object at 0x24c6798>
>>> sm.mpoly.wkt # WKT
MULTIPOLYGON (((12.4157980000000006 43.9579540000000009, 12.4505540000000003 43.9797209999999978, ...
>>> sm.mpoly.wkb # WKB (as Python binary buffer)
<read-only buffer for 0x1fe2c70, size -1, offset 0 at 0x2564c40>
>>> sm.mpoly.geojson # GeoJSON (requires GDAL)
'{ "type": "MultiPolygon", "coordinates": [ [ [ [ 12.415798, 43.957954 ], [ 12.450554, 43.979721 ], ...
This includes access to all of the advanced geometric operations provided by
the GEOS library::
>>> pnt = Point(12.4604, 43.9420)
>>> sm.mpoly.contains(pnt)
True
>>> pnt.contains(sm.mpoly)
False
``GeoQuerySet`` Methods
-----------------------
Putting your data on the map
============================
Geographic Admin
----------------
GeoDjango extends :doc:`Django's admin application </ref/contrib/admin/index>`
with support for editing geometry fields.
Basics
^^^^^^
GeoDjango also supplements the Django admin by allowing users to create
and modify geometries on a JavaScript slippy map (powered by `OpenLayers`_).
Let's dive right in. Create a file called ``admin.py`` inside the
``world`` application with the following code::
from django.contrib.gis import admin
from models import WorldBorder
admin.site.register(WorldBorder, admin.GeoModelAdmin)
Next, edit your ``urls.py`` in the ``geodjango`` application folder as follows::
from django.conf.urls import patterns, url, include
from django.contrib.gis import admin
admin.autodiscover()
urlpatterns = patterns('',
url(r'^admin/', include(admin.site.urls)),
)
Start up the Django development server:
.. code-block:: bash
$ python manage.py runserver
Finally, browse to ``http://localhost:8000/admin/``, and log in with the admin
user created after running :djadmin:`migrate`. Browse to any of the ``WorldBorder``
entries -- the borders may be edited by clicking on a polygon and dragging
the vertexes to the desired position.
.. _OpenLayers: http://openlayers.org/
.. _Open Street Map: http://openstreetmap.org/
.. _Vector Map Level 0: http://earth-info.nga.mil/publications/vmap0.html
.. _OSGeo: http://www.osgeo.org
.. _osmgeoadmin-intro:
``OSMGeoAdmin``
^^^^^^^^^^^^^^^
With the :class:`~django.contrib.gis.admin.OSMGeoAdmin`, GeoDjango uses
a `Open Street Map`_ layer in the admin.
This provides more context (including street and thoroughfare details) than
available with the :class:`~django.contrib.gis.admin.GeoModelAdmin`
(which uses the `Vector Map Level 0`_ WMS dataset hosted at `OSGeo`_).
First, there are some important requirements:
* :class:`~django.contrib.gis.admin.OSMGeoAdmin` requires that the
:ref:`spherical mercator projection be added <addgoogleprojection>`
to the ``spatial_ref_sys`` table (PostGIS 1.3 and below, only).
* The PROJ.4 datum shifting files must be installed (see the
:ref:`PROJ.4 installation instructions <proj4>` for more details).
If you meet these requirements, then just substitute the ``OSMGeoAdmin``
option class in your ``admin.py`` file::
admin.site.register(WorldBorder, admin.OSMGeoAdmin)
.. rubric:: Footnotes
.. [#] Special thanks to Bjørn Sandvik of `thematicmapping.org
<http://thematicmapping.org>`_ for providing and maintaining this
dataset.
.. [#] GeoDjango basic apps was written by Dane Springmeyer, Josh Livni, and
Christopher Schmidt.
.. [#] This point is the `University of Houston Law Center
<http://www.law.uh.edu/>`_.
.. [#] Open Geospatial Consortium, Inc., `OpenGIS Simple Feature Specification
For SQL <http://www.opengeospatial.org/standards/sfs>`_.