PostGIS does not find intersection between line and polygon

Question

I have a line and polygon:

I would like to programmatically figure out whether they intersect or not. So, I do:

select ST_Intersects (
    (
        select ST_SetSRID(ST_Buffer(ST_Transform(ST_SetSRID((p.geom), 4326), 26986), 50), 4326)
        from point p
        where p.id = 25785
    )
    ,
    (
        select ST_SetSRID(ST_MakeLine(ST_SetSRID(p.geom, 4326)), 4326)
        FROM point p
        where p.trajectory_id = 1455
    )
);

But when I run the query, PostGIS says that they do not intersect!! But on the picture above we definitely see that they intersect (or line crosses the polygon).

What is the problem?

I also tried ST_Crosses function, but it also gives false.

Answer 1

Here is a proposed cleaned-up code. Assuming you need to input the buffer size in the local unit of "EPSG:26986".

To simplify a bit, I propose to intersect geometry directly into the local SRID. This make the query more generic because it will work no matter what is the input SRID of the geometry (if its correctly defined!).

And if the SRID (4326) is already set up in the point table meta-data you could probably drop all the ST_SetSRID() functions because they aren't doing anything and they just make the function less reusable.

select ST_Intersects (
    (
        select ST_Buffer(ST_Transform(ST_SetSRID(p.geom,4326),26986),50))
        from point p
        where p.id = 25785
    )
    ,
    (
        select ST_MakeLine(ST_Transform(ST_SetSRID(p.geom,4326),26986))
        FROM point p
        where p.trajectory_id = 1455
        ORDER BY id
    )
);

Please also notice that you must add an ORDER BY clause to your second query in order to make a clean path object. On your snapshot we can clearly see that there is a false straight line. Hence this could cause false positive later-on in your algorithm. I reused the field id but maybe there is a more appropriate one depending on your data.

Answer 2

That first ST_SetSRID in your subquerry should be ST_Transform.

With ST_SetSRID you just assign new SRID to the geometry without actually transforming your data into the new SRID.