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()\nBidang-bidang spasial terdiri dari deretan dari jenis bidang geometri dan satu jenis bidang raster. Setiap jenis-jenis bidang geometri sesuai pada spesifikasi OpenGIS Simple Features [1]. Tidak ada standar untuk data raster.
\nGeometryFieldLink to this heading#Kelas dasar untuk bidang geometri.
\nPointFieldLink to this heading#Menyimpan Point.
LineStringFieldLink to this heading#Menyimpan LineString.
PolygonFieldLink to this heading#Menyimpan Polygon.
MultiPointFieldLink to this heading#Menyimpan MultiPoint.
MultiLineStringFieldLink to this heading#Menyimpan MultiLineString.
MultiPolygonFieldLink to this heading#Menyimpan MultiPolygon.
GeometryCollectionFieldLink to this heading#Menyimpan GeometryCollection.
RasterFieldLink to this heading#Menyimpan GDALRaster.
RasterField saat ini hanya diterapkan untuk backend PostGIS.
Sebagai tambahan pada Pilihan bidang biasa tersedia untuk bidang model Django, bidang-bidang spasial mempunyai pilihan tambahan berikut. Semua adalah pilihan.
\nsridLink to this heading#Setel SRID [2] (Spatial Reference System Identity) dari bidang geometri pada nilai diberikan. Awalan pada 4326 (juga dikenal sebagai WGS84, satuan-satuan adalah dalam derajat dari garis bujur dan garis lintang).
\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\nSumber daya Tambahan:
\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.
Ada pilihan tambahan tersedia untuk bidang Geometri. Semua pilihan berikut adalah pilihan.
\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).
Karena perhitungan geografis melibatkan matematika lebih, hanya bagian dari pencarian spasial PostGIS tersedia untuk jenis geografi. Secara praktik, ini berarti bahwa sebagai tambahan pada distance lookups hanya berikut tambahan spatial lookups tersedia untuk kolom geografi:
\nJika anda butuh menggunakan pencarian spasial atau pengumpulan yang tidak mendukung jenis geografi sebagai masukan, anda dapat menggunakan fungsi basisdata Cast untuk merubah kolom geografi menjadi jenis geometri dalam permintaan:
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.
\nCatatan kaki
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