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API Múnla GeodJango

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()

Cineálacha Réimse spású

Is éard atá i réimsí spásúla sraith cineálacha réimse geoiméadrachta agus cineál réimse raster amháin. Freagraíonn gach ceann de na cineálacha réimse geoiméadair do shonraíocht Gnéithe Simple OpenGIS [#fnogc] _. Níl aon chaighdeán den sórt sin ann do shonraí raster.

Réimse Geoiméadraithe

class GeometryField

An bunaicme do réimsí geoiméadrachta.

Pointfield

class PointField

Stórais a: aicme: ~django.contrib.gis.geos.point.

Línestringfield

class LineStringField

Stórais a: aicme: ~django.contrib.gis.geos.lineString.

Polygonfield

class PolygonField

Stórais a: aicme: ~django.contrib.gis.geos.polygon.

Réimse Ilphoint

class MultiPointField

Stórais a: aicme: ~django.contrib.gis.geos.multipoint.

Páirc Illíneach

class MultiLineStringField

Stórais a: aicme: ~django.contrib.gis.geos.multilineString.

Multipolygonfield

class MultiPolygonField

Stórais a: aicme: ~django.contrib.gis.geos.multipolygon.

Réimse Bailiúcháin Geoimetrach

class GeometryCollectionField

Stórais a: aicme: ~django.contrib.gis.geos.geometryCollection.

Rasterfield

class RasterField

Stórais a:aicme: ~django.contrib.gis.gdal.gdalraster.

Ní chuirtear `Rasterfield ``i bhfeidhm faoi láthair ach amháin don chúltaca PostGIS.

Roghanna Réimse spásúla

Chomh maith leis na rialt:ref: common-model-field-options atá ar fáil do réimsí samhail Django, tá na roghanna breise seo a leanas ag réimsí spásúla. Tá gach ceann roghnach.

Srid

BaseSpatialField.srid

Socraíonn an SRID [#fnogcsrid] _ (Aitheantas Córais Tagartha Spásúil) an réimse geoiméadrach don luach a thugtar. Réamhshocraithe go 4326 (ar a dtugtar WGS84 freisin, tá aonaid i gcéimeanna fadmhéad agus domhanleithead).

SRID a roghnú

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.

Acmhainní Breise:

  • spatialreference.org: Bunachar sonraí faoi thiomáint Django de chórais tagartha spásúla.

  • 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.

Innéacsa_spáis

BaseSpatialField.spatial_index

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

Roghanna Réimse Céim

Tá roghanna breise ar fáil do réimsí Geoiméadra. Tá na roghanna seo a leanas go léir roghnach.

`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.

Geografa

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.

Cineál Tíreolaíochta

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).

Toisc go bhfuil níos mó matamaitice i gceist le ríomhanna tíreolaíochta, níl ach fo-thacar de na cuardaigh spásúla PostGIS ar fáil don chineál tíreolaíochta. <spatial-lookups>Go praiticiúil, <distance-lookups>ciallaíonn sé seo nach bhfuil ach na cuardach breise seo a leanas: ref: cuardaigh spásúla `ar fáil do cholúin tíreolaíochta, i dteann:ref: `cuardach achair:

Más gá duit cuardach spásúil nó comhiomlán a úsáid nach dtacaíonn leis an gcineál tíreolaíochta mar ionchur, is féidir leat feidhm bhunachar sonraí: class: `~django.db.Models.functions.cast a úsáid chun an colún tíreolaíochta a thiontú go cineál geoiméadrach san fhiosrúchán:

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.

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