GeoDjango: How to create a circle based on point and radius

Question

I have the following (simplified) Model:

class Zone(gismodels.Model):
    name = gismodels.CharField()
    poly = gismodels.PolygonField()

I want to create and save a polygon that represents a circle, based upon a given point and radius.

The only way I can figure out how to achieve this, is to call the postgis ST_Buffer function using raw SQL. I'm really hoping that there is another way.

Is it possible to access the GEOS buffer methods?

Answer 1

Yes, it is possible to use the geos buffer method:

>>> from django.contrib.gis import geos
>>> center = geos.Point(5, 5)
>>> radius = 2
>>> circle = center.buffer(radius)
>>> circle
<Polygon object at 0x1029d8370>

The radius here is in the same units as the coordinates of the points. This will work for some coordinate systems like UTM, but not as well for others.

Also, while this is appropriate for constructing a circular geometry, the PostGIS documentation notes that for doing radius searches ST_DWithin is more efficient.

Answer 2

I spent a ridiculous amount of time trying to get this working. Since this is the number one google search result, here's what worked for me:

radius_km = radius*1.609 # convert miles to km
point = target.geolocation # a django PointField using SRID 4326
# re-project point to a flat coordinate system 
# so we can use meters instead of degrees below, 
# AND get an actual circle instead of oval    
point.transform(6347) 
poly = point.buffer(radius_km*1000) # get a circular polygon from radius
poly.transform(4326)# re-project the resulting polygon back

Bonus: If you're doing this so you can get a circle on a google static map, grab polyline:

import polyline
import ast

geo = ast.literal_eval(poly.geojson) # turn the text into a dict
points = geo['coordinates'][0]
pl = polyline.encode(points, geojson=True, precision=5) # make a polyline out of the polygon for google
map_url += '&path=color:0x00000000%7Cfillcolor:0x0000AA33%7Cweight:1%7Cenc:' + pl