This document explores the details of the GeoDjango Model API. Throughout this\nsection, we'll be using the following geographic model of a ZIP code and\nof a Digital Elevation Model as our examples:
\nfrom django.contrib.gis.db import models\n\n\nclass Zipcode(models.Model):\n code = models.CharField(max_length=5)\n poly = models.PolygonField()\n\n\nclass Elevation(models.Model):\n name = models.CharField(max_length=100)\n rast = models.RasterField()\n空间字段包括一系列几何字段类型和一个栅格字段类型。每种几何字段类型对应于 OpenGIS Simple Features 规范 [1]。对于栅格数据，没有类似的标准。
\nGeometryFieldLink to this heading#几何字段的基类。
\nPointFieldLink to this heading#存储一个 Point。
LineStringFieldLink to this heading#存储一个 LineString。
PolygonFieldLink to this heading#存储一个 Polygon。
MultiPointFieldLink to this heading#存储一个 MultiPoint。
MultiLineStringFieldLink to this heading#存储一个 MultiLineString。
MultiPolygonFieldLink to this heading#存储一个 MultiPolygon。
GeometryCollectionFieldLink to this heading#存储一个 GeometryCollection。
RasterFieldLink to this heading#存储一个 GDALRaster。
RasterField 目前仅在 PostGIS 后端实现。
除了 Django 模型字段可用的常规 常见模型字段选项 外，空间字段还有以下附加选项。所有这些选项都是可选的。
\nsridLink to this heading#将几何字段的 SRID [2] （空间参考系统标识）设置为给定值。默认值为 4326（也称为 WGS84，单位为经度和纬度的度数）。
\nChoosing an appropriate SRID for your model is an important decision that the\ndeveloper should consider carefully. The SRID is an integer specifier that\ncorresponds to the projection system that will be used to interpret the data in\nthe spatial database. [3] Projection systems give the context to the\ncoordinates that specify a location. Although the details of geodesy are\nbeyond the scope of this documentation, the general problem is that the earth\nis spherical and representations of the earth (e.g., paper maps, web maps) are\nnot.
\nMost people are familiar with using latitude and longitude to reference a\nlocation on the earth's surface. However, latitude and longitude are angles,\nnot distances. In other words, while the shortest path between two points on a\nflat surface is a straight line, the shortest path between two points on a\ncurved surface (such as the earth) is an arc of a great circle.\n[4]
\nThus, additional computation is required to obtain distances in planar units\n(e.g., kilometers and miles). Using a geographic coordinate system may\nintroduce complications for the developer later on. For example, SpatiaLite\ndoes not have the capability to perform distance calculations between\ngeometries using geographic coordinate systems, e.g. constructing a query to\nfind all points within 5 miles of a county boundary stored as WGS84. [5]
\nPortions of the earth's surface may projected onto a two-dimensional, or\nCartesian, plane. Projected coordinate systems are especially convenient for\nregion-specific applications, e.g., if you know that your database will only\ncover geometries in North Kansas, then you may consider using projection\nsystem specific to that region. Moreover, projected coordinate systems are\ndefined in Cartesian units (such as meters or feet), easing distance\ncalculations.
\n\n其他资源：
\nspatialreference.org: A database of spatial reference systems.
The State Plane Coordinate System: A website covering the various\nprojection systems used in the United States. Much of the U.S. spatial\ndata encountered will be in one of these coordinate systems rather than\nin a geographic coordinate system such as WGS84.
spatial_indexLink to this heading#Defaults to True. Creates a spatial index for the given geometry\nfield.
几何字段还有其他可用的选项。以下所有选项都是可选的。
\ndimLink to this heading#This option may be used for customizing the coordinate dimension of the\ngeometry field. By default, it is set to 2, for representing two-dimensional\ngeometries. For spatial backends that support it, it may be set to 3 for\nthree-dimensional support.
\n\ngeographyLink to this heading#If set to True, this option will create a database column of\ntype geography, rather than geometry. Please refer to the\ngeography type section below for more\ndetails.
max_geom_collectionsLink to this heading#This option is forwarded to the form field generated for this model\nfield, bounding how many geometry collections may be contained in submitted\nWKB/WKT inputs before raising ValueError. Since spatial field\nassignments are lazy, it is also checked when values are accessed, e.g. when\nsaving an instance, but not when read from a database. It defaults to 198.
The geography type provides native support for spatial features represented\nwith geographic coordinates (e.g., WGS84 longitude/latitude). [6]\nUnlike the plane used by a geometry type, the geography type uses a spherical\nrepresentation of its data. Distance and measurement operations\nperformed on a geography column automatically employ great circle arc\ncalculations and return linear units. In other words, when ST_Distance\nis called on two geographies, a value in meters is returned (as opposed\nto degrees if called on a geometry column in WGS84).
由于地理计算涉及更多的数学运算，因此地理类型仅支持 PostGIS 空间查询的子集。实际上，这意味着除了 距离查询 之外，地理列仅支持以下其他 空间查询：
\n如果您需要使用不支持地理类型作为输入的空间查询或聚合函数，您可以在查询中使用 Cast 数据库函数将地理列转换为几何类型：
from django.contrib.gis.db.models import PointField\nfrom django.db.models.functions import Cast\n\nZipcode.objects.annotate(geom=Cast("geography_field", PointField())).filter(\n geom__within=poly\n)\nFor more information, the PostGIS documentation contains a helpful section on\ndetermining when to use geography data type over geometry data type.
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