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I have a point on a linestring geometry (found by an intersection). What is the best way to get two points along the line in both directions using postgis?

It does not really matter if the points represent a vertex or are just a given distance or fraction away from the intersection point. I just need a short line to calculate the azimuth between those two points.

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  • Why don't you just use the end point vertices of that initial linestring if the only purpose is to calculate the azimuth? I mean the azimuth of that short line you are expecting and the initial linestring must be same!
    – Zia
    Mar 17, 2015 at 9:51
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    Certainly not! If the linestring represents an objects with a lot of curves your endpoint (or startpoint for the same matter) can be in a totally different direction (thus different degree of azimuth!) then s short fraction of the linestring around the given point of interest.
    – LarsVegas
    Mar 17, 2015 at 11:00
  • Hmm... So basically you want azimuth of the tangent along that point.
    – Zia
    Mar 17, 2015 at 13:14

1 Answer 1

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Following SQL could be used to obtain tangent at a given point on a curve (although not rigorously tested). Here poly is the linestring table name and (30.7011637,33.1748235),4326) is the point of intersection where we wanna calculate the tangent's azimuth. The query will give you the tangent line. Let us know if this worked fine for you.

with points1 as (
SELECT (ST_DumpPoints(p.geom)).geom as geompt1 from poly as p
),
points2 as (
SELECT (ST_DumpPoints(p.geom)).geom as geompt2 from poly as p
),
lines1 as (
select st_makeline(geompt1, geompt2) as geomln from points1, points2 where ST_Disjoint(geompt1, geompt2)
)
select geomln as geom from lines1 order by st_distance(geomln, ST_SetSRID(ST_MakePoint(30.7011637,33.1748235),4326)) limit 1
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  • points1 and points2 are the same so only one would do. Also connecting on path would be faster than on ST_Distjoint. And currently it might give wrong results with complicated linestrings. Mar 23, 2015 at 21:11

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