creating wrong points values

Question

I'm working with SRID=32636

I have a point = (2076155.32235105 4828109.18280588)

I'm using this post: How to add 2 points with distance between them (SRID = 32636)? to create 3 points with different distances.

I created the 3 points (source point + 3 different points with 15, 30, 70 meters from source point) with the following queries:

select st_asText(st_project('POINT(2076155.32235105 4828109.18280588)', 15, radians(30)))

I got: POINT(35.322429431595 30.8173112915666)

select st_asText(st_project('POINT(2076155.32235105 4828109.18280588)', 30, radians(30)))

I got: POINT(35.3225078132829 30.8174284630566)

select st_asText(st_project('POINT(2076155.32235105 4828109.18280588)', 70, radians(30)))

I got: POINT(35.3227168320474 30.8177409201211)

when I'm checking the distances between the source point and each other I got incorrect or strange distances:

select st_distance('POINT(2076155.32235105 4828109.18280588)','POINT (2076155.32235105 4828109.18280588)')

got: 0 (seems ok - no distance between source point to same source point)

select st_distance('POINT(2076155.32235105 4828109.18280588)','POINT(35.322429431595 30.8173112915666)')

got 5255531.84344186 (I expect to get 15)

select st_distance('POINT(2076155.32235105 4828109.18280588)','POINT(35.3225078132829 30.8174284630566)')

got 5255531.84330326 (I expect to get 30)

What is wrong ?

why the points I got don't give me the points with the distance I want from source point ?

How can I fix it ?

are the `15` and `30` values are supposed to be meters? – Jim Jones Sep 23 '20 at 10:46 — Jim Jones, Sep 23 '20 at 10:46

Jim Jones · Accepted Answer · 2020-09-24T05:53:01.173

You're mixing spatial reference systems and data types. If you take a look at this query of yours, one can notice that the points are clearly in two distinct SRS:

select st_distance('POINT(2076155.32235105 4828109.18280588)',
                   'POINT(35.3225078132829 30.8174284630566)')

In order to get the result of ST_Distance in meters you have to first convert the geometries to geography - there are other more complex ways. Since we cannot convert from a projected SRS to a lon/lat one, we need to first ST_Transform your coordinates to a lon/lat SRS and from there convert it to geography. After that all distance operations will be calculated in metres. The following query shows exactly what I am talking about:

WITH j (start_point,geo15,geo30,geo75) AS (
  VALUES (ST_Transform('SRID=32636;POINT(2076155.32235105 4828109.18280588)'::geometry,4326)::geography,
          ST_Project(ST_Transform('SRID=32636;POINT(2076155.32235105 4828109.18280588)'::geometry,4326)::geography, 15, radians(30)),
          ST_Project(ST_Transform('SRID=32636;POINT(2076155.32235105 4828109.18280588)'::geometry,4326)::geography, 30, radians(30)),
          ST_Project(ST_Transform('SRID=32636;POINT(2076155.32235105 4828109.18280588)'::geometry,4326)::geography, 75, radians(30)))
)
SELECT 
  ST_Distance(start_point,geo15),
  ST_Distance(start_point,geo30),
  ST_Distance(start_point,geo75)  
FROM j;

 st_distance | st_distance | st_distance 
-------------+-------------+-------------
          15 |          30 |          75

creating wrong points values

1 Answers1