djangodocs

Django 6.1
日本語
Documentation contents

GeoDjango モデル 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:

Code
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()

空間フィールドのタイプ

空間フィールドは一連のジオメトリフィールド型と1つのラスターフィールド型で構成されます。各ジオメトリフィールド型は OpenGIS Simple Features 仕様 [1] に対応しています。ラスターデータに対するこのような標準は存在しません。

GeometryField

class GeometryField

ジオメトリフィールドの基本クラス。

PointField

class PointField

Point を格納します。

LineStringField

class LineStringField

LineString を格納します。

PolygonField

class PolygonField

Polygon を格納します。

MultiPointField

class MultiPointField

MultiPoint を格納します。

MultiLineStringField

class MultiLineStringField

MultiLineString を格納します。

MultiPolygonField

class MultiPolygonField

MultiPolygon を格納します。

GeometryCollectionField

class GeometryCollectionField

GeometryCollection を格納します。

RasterField

class RasterField

GDALRaster を格納します。

RasterField は現在、PostGIS バックエンドのみで実装されています。

空間フィールドのオプション

Django のモデルフィールドで利用できる通常の フィールドオプション に加えて、空間フィールドには以下の追加オプションがあります。すべてオプションです。

srid

BaseSpatialField.srid

ジオメトリフィールドの SRID [2] (Spatial Reference System Identity) を指定した値に設定します。デフォルトは 4326 (別名 WGS84 、単位は緯度経度)。

SRID の選択

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.

追加のリソース:

  • spatialreference.org: A 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.

spatial_index

BaseSpatialField.spatial_index

Defaults to True. Creates a spatial index for the given geometry field.

ジオメトリフィールドのオプション

ジオメトリフィールドには追加オプションがあります。以下のすべてのオプションは任意です。

dim

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.

geography

GeometryField.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.

max_geom_collections

GeometryField.max_geom_collections

This option is forwarded to the form field generated for this model field, bounding how many geometry collections may be contained in submitted WKB/WKT inputs before raising ValueError. Since spatial field assignments are lazy, it is also checked when values are accessed, e.g. when saving an instance, but not when read from a database. It defaults to 198.

ジオグラフィ (Geography) 型

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).

地理計算にはより多くの数学が関わるため、地理型に対して使用できる PostGIS 空間ルックアップのサブセットが限られています。具体的には、距離ルックアップ に加えて、ジオグラフィカラム用に利用可能な追加の 空間ルックアップ は以下のみです。

入力としてジオグラフィ型をサポートしていない空間ルックアップや集計を使用する必要がある場合、 Cast データベース関数を使用して、クエリの中でジオグラフィカラムをジオメトリ型に変換できます:

Code
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.

脚注

View as Markdown Edit this page on GitHub Official version