Kegunaan impor data LayerMapping

Kelas LayerMapping menyediakan sebuah cara untuk memetakan isi dari berkas-berkas data spasial vektor (misalnya, shapefiles) kedalam model GeoDjango.

Kegunaan ini tumbuh dari kebutuhan pribadi penulis untuk menhilangkan perulangan kode yang masuk menarik geometri dan bidang diluar lapisan vektor, merubah ke sistem kordinat lain (misalnya WGS84), dan kemudian memasukkan kedalam model GeoDjango.

Catatan

Penggunaan LayerMapping membutuhkan GDAL.

Peringatan

GIS data sources, like shapefiles, may be very large. If you find that LayerMapping is using too much memory, set DEBUG to False in your settings. When DEBUG is set to True, Django automatically logs every SQL query -- and when SQL statements contain geometries, this may consume more memory than is typical.

Contoh

1. You need a GDAL-supported data source, like a shapefile (here we're using a simple polygon shapefile, test_poly.shp, with three features):

>>> from django.contrib.gis.gdal import DataSource
>>> ds = DataSource("test_poly.shp")
>>> layer = ds[0]
>>> print(layer.fields)  # Exploring the fields in the layer, we only want the 'str' field.
['float', 'int', 'str']
>>> print(len(layer))  # getting the number of features in the layer (should be 3)
3
>>> print(layer.geom_type)  # Should be 'Polygon'
Polygon
>>> print(layer.srs)  # WGS84 in WKT
GEOGCS["GCS_WGS_1984",
    DATUM["WGS_1984",
        SPHEROID["WGS_1984",6378137,298.257223563]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295]]

1. Sekarang kami menentukan model Django yang sesuai (pastikan menggunakan migrate):

   from django.contrib.gis.db import models
   
   
   class TestGeo(models.Model):
       name = models.CharField(max_length=25)  # corresponds to the 'str' field
       poly = models.PolygonField(srid=4269)  # we want our model in a different SRID
   
       def __str__(self):
           return "Name: %s" % self.name
   

2. Use LayerMapping to extract all the features and place them in the database:

>>> from django.contrib.gis.utils import LayerMapping
>>> from geoapp.models import TestGeo
>>> mapping = {
...     "name": "str",  # The 'name' model field maps to the 'str' layer field.
...     "poly": "POLYGON",  # For geometry fields use OGC name.
... }  # The mapping is a dictionary
>>> lm = LayerMapping(TestGeo, "test_poly.shp", mapping)
>>> lm.save(verbose=True)  # Save the layermap, imports the data.
Saved: Name: 1
Saved: Name: 2
Saved: Name: 3

Here, LayerMapping transformed the three geometries from the shapefile in their original spatial reference system (WGS84) to the spatial reference system of the GeoDjango model (NAD83). If no spatial reference system is defined for the layer, use the source_srs keyword with a SpatialReference object to specify one.

API LayerMapping

class LayerMapping(model, data_source, mapping, layer=0, source_srs=None, encoding=None, transaction_mode='commit_on_success', transform=True, unique=True, using='default')[sumber]

Berikut adalah argumen dan kata kunci yang mungkin digunakan selama penginstansiasi dari obyek LayerMapping.

Argument	Deskripsi
model	Model geografis, bukan sebuah instance.
data_source	The path to the OGR-supported data source file (e.g., a shapefile). Also accepts django.contrib.gis.gdal.DataSource instances.
mapping	Sebuah dictionary: kunci-kunci adalah string berhubungan pada bidang model, dan nilai-nilai sesuai pada nama-nama bidang string untuk fitur OGR, atau jika bidang model adalah sebuah geografik lalu itu harus sesuai pada jenis geometri OGR, misalnya, 'POINT', 'LINESTRING', 'POLYGON'.

Argumen Katakunci
layer	Indeks dari lapisan untuk digunaan dari Data Source (awalan adalah 0)
source_srs	Use this to specify the source SRS manually (for example, some shapefiles don't come with a '.prj' file). An integer SRID, WKT or PROJ strings, and django.contrib.gis.gdal.SpatialReference objects are accepted.
encoding	Specifies the character set encoding of the strings in the OGR data source. For example, 'latin-1', 'utf-8', and 'cp437' are all valid encoding parameters.
transaction_mode	Mungkin 'commit_on_success' (awalan) atau 'autocommit'.
transform	Setting this to False will disable coordinate transformations. In other words, geometries will be inserted into the database unmodified from their original state in the data source.
unik	Setting this to the name, or a tuple of names, from the given model will create models unique only to the given name(s). Geometries from each feature will be added into the collection associated with the unique model. Forces the transaction mode to be 'autocommit'.
menggunakan	Setel basisdata yang digunakan ketika mengimpor data spasial. Awalan adalah 'default'.

Argumen Kata Kunci save()

LayerMapping.save(verbose=False, fid_range=False, step=False, progress=False, silent=False, stream=sys.stdout, strict=False)[sumber]

The save() method also accepts keywords. These keywords are used for controlling output logging, error handling, and for importing specific feature ranges.

Simpan Argumen Kata Kunci	Deskripsi
fid_range	May be set with a slice or tuple of (begin, end) feature ID's to map from the data source. In other words, this keyword enables the user to selectively import a subset range of features in the geographic data source.
progress	When this keyword is set, status information will be printed giving the number of features processed and successfully saved. By default, progress information will be printed every 1000 features processed, however, this default may be overridden by setting this keyword with an integer for the desired interval.
silent	Secara awalan, pemberitahuan kesalahan bukan-penting dicetak pada sys.stdout, tetapi kata kunci ini mungkin disetel untuk meniadakan pemberitahuan ini.
step	Jika disetel dengan integer, transaksi akan muncul pada setiap jangka waktu langkah. Sebagai contoh, jika step=1000, commit akan muncul setelah fitur ke 1000, fitur ke 2000 dll.
stream	Status information will be written to this file handle. Defaults to using sys.stdout, but any object with a write method is supported.
strict	Execution of the model mapping will cease upon the first error encountered. The default value (False) behavior is to attempt to continue.
verbose	Jika disetel, informasi akan dicetak selanjutnya pada setiap model simpan dijalankan pada basisdata.

Menyelesaikan masalah

Kehabisan memori

As noted in the warning at the top of this section, Django stores all SQL queries when DEBUG=True. Set DEBUG=False in your settings, and this should stop excessive memory use when running LayerMapping scripts.

MySQL: kesalahan max_allowed_packet

If you encounter the following error when using LayerMapping and MySQL:

OperationalError: (1153, "Got a packet bigger than 'max_allowed_packet' bytes")

Then the solution is to increase the value of the max_allowed_packet setting in your MySQL configuration. For example, the default value may be something low like one megabyte -- the setting may be modified in MySQL's configuration file (my.cnf) in the [mysqld] section:

max_allowed_packet = 10M