API Model 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:

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()

Jenis Bidang Spasial

Bidang-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.

GeometryField

class GeometryField[sumber]

Kelas dasar untuk bidang geometri.

PointField

class PointField[sumber]

Menyimpan Point.

LineStringField

class LineStringField[sumber]

Menyimpan LineString.

PolygonField

class PolygonField[sumber]

Menyimpan Polygon.

MultiPointField

class MultiPointField[sumber]

Menyimpan MultiPoint.

MultiLineStringField

class MultiLineStringField[sumber]

Menyimpan MultiLineString.

MultiPolygonField

class MultiPolygonField[sumber]

Menyimpan MultiPolygon.

GeometryCollectionField

class GeometryCollectionField[sumber]

Menyimpan GeometryCollection.

RasterField

class RasterField[sumber]

Menyimpan GDALRaster.

RasterField saat ini hanya diterapkan untuk backend PostGIS.

Pilihan Bidang Spasial

Sebagai tambahan pada Pilihan bidang biasa tersedia untuk bidang model Django, bidang-bidang spasial mempunyai pilihan tambahan berikut. Semua adalah pilihan.

srid

BaseSpatialField.srid

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).

Memilih sebuah 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.

Catatan

Jika anda berharap melakukan permintaan jarak berubah-ubah menggunakan geometri bukan-titik dalam WGS84 di PostGIS dan anda ingin penampilan layak, adakan kata kunci GeometryField.geography sehingga geography database type digunakan sebagai gantinya.

Sumber daya Tambahan:

• spatialreference.org: Sebuah basisdata ditenagai-Django dari sistem acuan spasial.

• 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.

Catatan

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.

Pilihan Bidang Geometri

Ada pilihan tambahan tersedia untuk bidang Geometri. Semua pilihan berikut adalah pilihan.

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.

Catatan

At this time 3D support is limited to the PostGIS and SpatiaLite backends.

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.

Catatan

Dukungan geografi adalah terbatas pada PostGIS dan akan memaksa SRID menjadi 4326.

Jenis Geografi

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).

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:

• bboverlaps

• coveredby

• covers

• intersects

Jika 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
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.

Catatan kaki

[1]

OpenGIS Consortium, Inc., Simple Feature Specification For SQL.

[2]

Lihat id. pada Ch. 2.3.8, p. 39 (Nilai Geometri dan Sustem Acuan Spasial).

[3]

Typically, SRID integer corresponds to an EPSG (European Petroleum Survey Group) identifier. However, it may also be associated with custom projections defined in spatial database's spatial reference systems table.

[4]

Terry A. Slocum, Robert B. McMaster, Fritz C. Kessler, & Hugh H. Howard, Thematic Cartography and Geographic Visualization (Prentice Hall, edisi ke 2), pada Bab 7.1.3.

[5]

Batasan ini tidak berlaku pada PostGIS.

[6]

Harap mengacu ke dokumentasi PostGIS Geography Type untuk rincian lebih.