3d distance calculations with GeoDjango

Question

I am using

• python 2.7.12
• django 1.10.6
• postgreSQL 9.5.6
• postGIS 2.2.2

First question

I need to use GeoDjango to calculate distance between two points. When I checked the documentation it says that GeoQuerySet.distance() is deprecated and instead use Distance() from django.contrib.gis.db.models.functions.

The following code works OK:

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

p1 = Instrument.objects.get(pk=151071000).coordinates
p2 = Instrument.objects.filter(pk=151071008)

for i in p2.annotate(distance=Distance('coordinates', p1)):
    print i.distance
    print i.distance.__class__

Output:

461.10913945 m
<class 'django.contrib.gis.measure.Distance'>

My model:

class Instrument(models.Model):
    ...
    coordinates = gis_models.PointField(null=True, blank=True, dim=3)

But I have only two points so when I try to use Distance() without annotate() it returns instance of class django.contrib.gis.db.models.functions.Distance() rathen than django.contrib.gis.measure.Distance():

p1 = Instrument.objects.get(pk=151071000).coordinates
p2 = Instrument.objects.get(pk=151071008).coordinates
print Distance(p1, p2)

Output:

Distance(Value(SRID=4326;POINT Z (-76.48623600000001 44.260223 0)), GeomValue(SRID=4326;POINT Z (-76.490923 44.262658 0)))

How do I receive the same result as with using annotate()?

Second question

I have to calculate 3d distance that is taking into account depth/elevation. But when I try to do it I receive the same result as with 2d. Below I changed elevation to 200 in the first object:

p1 = Instrument.objects.get(pk=151071000)
p1.coordinates = 'SRID=4326;POINT Z (-76.48623600000001 44.260223 200)'
p2 = Instrument.objects.filter(pk=151071008)

for i in p2.annotate(distance=Distance('coordinates', p1.coordinates)):
    print i.distance

Output:

461.10913945 m

Answer 1

Let's break the problem down:

1. In the Distance class documentation, we can read the following:

   Accepts two geographic fields or expressions and returns the distance between them, as a Distance object.

   So the Distance(p1, p2) returns a Distance object.
   If you do:

   p1 = Instrument.objects.get(pk=151071000).coordinates
   p2 = Instrument.objects.get(pk=151071008).coordinates
   d = Distance(m=p1.distance(p2))
   print d.m
   

   You will get the measurement in meters.
   

   I would stick with the annotate solution, which seems more solid! (opinionated response)

2. Distance calculates the 2D distance between two point. In order to get a 3D calculation you need to create one yourself.
   You can have a look at my method from this question: Calculating distance between two points using latitude longitude and altitude (elevation)

   • Let polar_point_1 = (long_1, lat_1, alt_1) and polar_point_2 = (long_2, lat_2, alt_2)

   • Translate each point to it's Cartesian equivalent by utilizing this formula:

     x = alt * cos(lat) * sin(long)
     y = alt * sin(lat)
     z = alt * cos(lat) * cos(long)
     

     and you will have p_1 = (x_1, y_1, z_1) and p_2 = (x_2, y_2, z_2) points respectively.

   • Finally use the Euclidean formula:

     dist = sqrt((x_2-x_1)**2 + (y_2-y_1)**2 + (z_2-z_1)**2)