GeoDjango 模型 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()
空间字段类型¶
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。
GeometryField¶
几何字段的基类。
PointField¶
存储一个 Point。
LineStringField¶
存储一个 LineString。
PolygonField¶
存储一个 Polygon。
MultiPointField¶
存储一个 MultiPoint。
MultiLineStringField¶
存储一个 MultiLineString。
MultiPolygonField¶
存储一个 MultiPolygon。
GeometryCollectionField¶
存储一个 GeometryCollection。
RasterField¶
存储一个 GDALRaster。
RasterField 目前仅在 PostGIS 后端实现。
空间字段选项¶
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。
srid¶
- BaseSpatialField.srid¶
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）。
选择一个 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. [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.
Note
如果您希望在 PostGIS 中使用 WGS84 中的非点几何体执行任意距离查询，并且希望获得良好的性能，请启用 GeometryField.geography 关键字，以便使用 地理数据库类型。
其他资源：
spatialreference.org：一个由 Django 驱动的空间参考系统数据库。
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.
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.
几何字段选项¶
几何字段还有其他可用的选项。以下所有选项都是可选的。
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.
Note
目前，对三维的支持仅限于 PostGIS 和 SpatiaLite 后端。
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.
Note
地理支持仅限于 PostGIS，并且会强制将 SRID 设置为 4326。
地理类型¶
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).
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：
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
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
