How to check if points are within a circular sector using PostGIS?

Question

I want to get the stars within the black region as shown above. To find this I have the centre of circle (lat, long), radius and the angle of region (45 degree).

I am new to postgres. I worked on code to check if points exist within the circle of a given radius.

How I can find points within this black region?

Answer 1

Use ST_Azimuth to get the angle from the origin point to the point of interest. I've used simple geometry points here since you didn't have any sample data, but you're probably working with geography. The principle is the same:

WITH points(star) AS (VALUES
  (point(0.5, 1)),
  (point(0.2,0.2)),
  (point(0.2, 1)),
  (point(0.8, 0.2))
)
SELECT star 
FROM points 
WHERE degrees(ST_Azimuth( point(0,0)::geometry, star::geometry )) 
      BETWEEN 0 AND 40;
  star   
---------
 (0.5,1)
 (0.2,1)
(2 rows)

You can also limit distance. Here's how to get distance and azimuth from table "points" containing a point-typed column "star" using ST_Distance too:

SELECT
  star, 
  degrees(ST_Azimuth( point(0,0)::geometry, star::geometry )) AS azimuth_deg,
  ST_Distance( point(0,0)::geometry, star::geometry ) AS distance
FROM points;

You can move those into the WHERE clause and filter on them however you like.

If possible work directly in radians, rather than converting to degrees.

If you're working with points on the earth, consider using the geography type instead of geometry. Both functions mentioned here support geography.

Answer 2

A circular wedge in a spherical space is just a triangle. You might look at using the geography type for this. However, defining the "radius" in that type might be hard since you're probably wanting to work in angular units, not metric units. I'd suggest that if you're doing astronomy you'll find that http://pgsphere.projects.pgfoundry.org fits your use cases more closely since that is what it was built for.