Finding points 100 miles outside county using PostGIS?

Question

Which way is correct? Use distance or buffer? I am not sure about this.

Here is how I use distance:

cursor.execute(""" SELECT COUNT(tw.user_id)
               FROM tweets tw, ca_census_tract ce
               WHERE ST_Distance(ST_Transform(tw.location,26986),
               ST_Transform(ce.geom,26986))>=160934 
               AND ce.name10='Alameda'; """)

cursor.fetchall()

and here is how I use buffer:

cursor.execute(""" SELECT COUNT(tw.user_id)
               FROM tweets tw, ca_census_tract ce
               WHERE ST_within(ST_Transform(tw.location,26986),
               ST_Buffer(ST_Transform(ce.geom,26986), 160934))=false
               AND ce.name10='Alameda'; """)

cursor.fetchall()

The tw.location are points.

---

There was a small difference in the result and a huge difference in running time. The result from distance was 14818 and the result from buffer was 14899. I don't understand why there was a difference. And, the running time of buffer was much longer. By the way, the total number of the dataset was 100K.

Answer 1

ST_Distance() or ST_DWithin() are much better in this context.

If you use ST_Buffer(), you add the expensive operation of creating a new geometry. Also, you won't be able to use a spatial index that you may have in place. The other two options avoid this.

You should get the same answer, however.

EDIT: See the important qualifier in the comments below, that the spatial index still won't be used if you are nesting a ST_Transform within ST_DWithin().