Geodjango: How to Buffer From Point

问题

I want to have a radius based distance search. To do this, I want to create a buffer around a point object in order to filter objects that are inside it.

Here is where I am at with it:

>>> lat = 37.7762179974
>>> lon = -122.411562492
>>> from django.contrib.gis.geos import Point
>>> pnt = Point(lat, lon)
>>> buf = pnt.buffer(0.0001)

But I am having problems filtering the Thing objects based on whether they are inside the buffer:

>>> z = Thing.objects.filter(pnt__intersects=buf)

(I know that the above is incorrect, but I use it to elaborate what I am trying to do.)

How can I create a buffer around the Point and then filter Things that are inside the buffer?

EDIT: models.py

class Thing(models.Model):
    lat = models.FloatField()
    lon = models.FloatField()

How can I filter based on a point comprised of a combination of these two model fields?
This cannot work obviously, because I do not have a pnt field in my model:

>>> pnt = Point(lat, lon)
>>> z = Thing.objects.filter(pnt__intersects=buf)

But how can I do something similar?

回答1:

I'm not sure what you mean by "buffer", but if you want a radius based distance search you might try using the... distance!

Here is an example with a kilometer metric:

from django.contrib.gis.measure import D

Thing.objects.filter(pnt__distance_lte=(pnt,D(km=distance)))

You should certainly have a look at the docs: https://docs.djangoproject.com/en/dev/ref/contrib/gis/db-api/#distance-lookups

回答2:

PostGIS has a method called ST_MakePoint which can create a 2D, 3D or 4D point.

In a previous answer we saw that it is possible to create a custom database function from an existing PostGIS function:

from django.contrib.gis.db.models.functions import GeoFunc

class MakePoint(GeoFunc):
    function='ST_MakePoint'

Now we can create a point by annotate()ing and apply the intersects on it:

z = Thing.objects.annotate(pnt=MakePoint('lat', 'lon'))
                 .filter(pnt__intersects=buf)

You can achieve the same effect by utilizing GeoFunc() and F() expressions:

from django.contrib.gis.db.models.functions import GeoFunc

z = Thing.objects.annotate(pnt=GeoFunc(
        F('lat'), F('lon'),
        function='ST_MakePoint'
    ).filter(pnt__intersects=buf)

Note: You could consider adding a pnt field in your Thing model and avoid the above.

来源：https://stackoverflow.com/questions/19144834/geodjango-how-to-buffer-from-point

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python

django

filter

buffer

geodjango