Dan,
You raised some excellent questions, thanks for sharing.
> 1 - Is there anyway in Geodjango to calculate distances between two
> points?
GEOSGeometry objects may be used to calculate the distance between two
points. For example:
>>> from django.contrib.gis.geos import Point
>>> p1 = Point(0, 0)
>>> p2 = Point(1, 1)
>>> print p1.distance(p2)
1.41421356237
However, as can be seen above, this is a simple Pythagorean
calculation and will not take into account the distance between two
points on a sphere. Thus, for spherical coordinates (e.g., WGS84 lat/
lon) great-circle spherical distance formulation is needed -- the
geopy module (as Samuel suggested) contains some implementations, but
none are within GeoDjango at the moment.
> 2 - When using 'dwithin' like below, what is the unit used in
> distance?
>
> object.filter(point__dwithin=(point,distance))
>
The unit of distance is that of the SRID of the field. For example,
if you did not explicitly specify an SRID in your geographic field
(you did not use the `srid` keyword), it defaults to WGS84, and the
units of the `distance` parameter would interpreted as degrees --
which are not optimal for calculating distances.
Because of this I use a projected coordinate systems for my models,
which use linear units of measurement for a particular portion of the
earth. Specifically I use the SRID of 32140 ("NAD83 / Texas South
Central") which has its units in meters. Most of us are not fortunate
enough to encounter data already in our desired coordinate system,
which is why I implemented GeoDjango with implicit SRID
transformation. This means that when a geographic parameter is in a
different SRID than that of the field, it will be transformed within
the spatial database automatically. For example, let's say that I
have a Neighborhood model:
from django.contrib.gis.db import models
class Neighborhood(models.Model):
name = models.CharField(max_length=50)
poly = models.PolygonField(srid=32140)
objects = models.GeoManager()
Now, let's say we have a point of interest in WGS84 and we want to
find all neighborhoods within 5km of it. We would use the `dwithin`
lookup type in the following manner:
>>> from django.contrib.gis.geos import Point
>>> from geoapp.models import Neighborhood
>>> pnt = Point(-95.3631, 29.7633, srid=4326) # notice the srid being set here
>>> qs = Neghborhood.objects.filter(poly__dwithin=(pnt, 5000.0))
Within the SQL, ST_Transform() will be used to convert the WGS84 point
into the SRID of the coordinate system, and return the desired
results.
Happy Thanksgiving,
-Justin
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