Why is PostGIS stacking all points on top of each other? (Using ST_DWithin to find all results within 1000m radius) - sql

Question

New to PostGIS/PostgreSQL...any help would be greatly appreciated!
I have two tables in a postgres db aliased as gas and ev. I'm trying to choose a specific gas station (gas.site_id=11949) and locate all EV/alternative fuel charging stations within a 1000m radius. When I run the following though, PostGIS returns a number of ev stations that are all stacked on top of each other in the map (see screenshot).
Anyone have any idea why this is happening? How can I get PostGIS to visualize the points within a 1000m radius of the specified gas station?
with myplace as (
SELECT gas.geom
from nj_gas gas
where gas.site_id = 11949 limit 1)
select myplace.*, ev.*
from alt_fuel ev, myplace
where ST_DWithin(ev.geom1, myplace.geom, 1000)

Answer 1

The function ST_DWithin does not compute distances in meters using geometry typed parameters.
From the documentation:
For geometry: The distance is specified in units defined by the
spatial reference system of the geometries. For this function to make
sense, the source geometries must both be of the same coordinate
projection, having the same SRID.
So, if you want compute distances in meters you have to use the data type geography:
For geography units are in meters and measurement is defaulted to
use_spheroid=true, for faster check, use_spheroid=false to measure
along sphere.
That all being said, you have to cast the data type of your geometries. Besides that your query looks just fine - considering your data is correct :-)
WITH myplace as (
SELECT gas.geom
FROM nj_gas gas
WHERE gas.site_id = 11949 LIMIT 1)
SELECT myplace.*, ev.*
FROM alt_fuel ev, myplace
WHERE ST_DWithin(ev.geom1::GEOGRAPHY, myplace.geom::GEOGRAPHY, 1000)
Sample data:
CREATE TABLE t1 (id INT, geom GEOGRAPHY);
INSERT INTO t1 VALUES (1,'POINT(-4.47 54.22)');
CREATE TABLE t2 (geom GEOGRAPHY);
INSERT INTO t2 VALUES ('POINT(-4.48 54.22)'),('POINT(-4.41 54.18)');
Query
WITH j AS (
SELECT geom FROM t1 WHERE id = 1 LIMIT 1)
SELECT ST_AsText(t2.geom)
FROM j,t2 WHERE ST_DWithin(t2.geom, j.geom, 1000);
st_astext
--------------------
POINT(-4.48 54.22)
(1 Zeile)

Answer 2

You are cross joining those tables and have PostgreSQL return the cartesian product of both when selecting myplace.* & ev.*.
So while there is only one row in myplace, its geom will be merged with every row of alt_fuel (i.e. the result set will have all columns of both tables in every possible combination of both); since the result set thus has two geometry columns, your client application likely chooses either the first, or the one called geom (as opposed to alt_fuel.geom1) to display!
I don't see that you are interested in myplace.geom in the result set anyway, so I suggest to run
WITH
myplace as (
SELECT gas.geom
FROM nj_gas gas
WHERE gas.site_id = 11949
LIMIT 1
)
SELECT ev.*
FROM alt_fuel AS ev
JOIN myplace AS mp
ON ST_DWithin(ev.geom1, mp.geom, 1000) -- ST_DWithin(ev.geom1::GEOGRAPHY, mp.geom::GEOGRAPHY, 1000)
;
If, for some reason, you also want to display myplace.geom along with the stations, you'd have to UNION[ ALL] the above with a SELECT * on myplace; note that you will also have to provide the same column list and structure (same data types!) as alt_fuel.* (or better, the other side of the UNION[ ALL]) in that SELECT!
Note the suggestions made by #JimJones about units; if your data is not projected in a meter based CRS (but in a geographic reference system; 'LonLat'), use the cast to GEOGRAPHY to have ST_DWithin consider the input as meter (and calculate using spheroidal algebra instead of planar (Euclidean))!

Answer 3

Resolved by using:
WITH
myplace as (
SELECT geom as g
FROM nj_gas
WHERE site_id = 11949 OR site_id = 11099 OR site_id = 11679 or site_id = 480522
), myresults AS (
SELECT ev.*
FROM alt_fuel AS ev
JOIN myplace AS mp
ON ST_DWithin(ev.geom, mp.g, 0.1))
select * from myresults```
Thanks so much for your help #ThingumaBob and #JimJones ! Greatly appreciate it.