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I am very new to PostGIS. I have a table with sections (multilineString) drawed on a map and a table with observations (point).

Users can make observations and I have to automatically place the point on the section.

My first try was to use ST_Centroid to place the observations in the middle of the section and it technically works.

Unfortunately, it is not usable later. I print all observations on a map, and they all have same positions, so the top observation only is visible and clickable.

I would like to create the furthest new point from existings points that is still in the multilineString (So it will naturally distribute on the multilineString)

So for exemple:

  • 1st point : Center
  • 2nd and 3rd point : begin or end of multilineString (0% or 100% lenght of the section)
  • 4th and 5th : 25% or 75% length of the section
  • etc.

I don't need to find the 1st observation, only the followings.

Desired output

I saw a lot of resources saying that we should use ST_Distance DESC but it's not working as my points are new and not already in the database.

Could someone help me, please?

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I would try to use ST_Line_Interpolate_Point. Count number of points you need to place on a line, and generate them by ST_Line_Interpolate_Point with fraction equal to 0, 1/(n-1), 2/(n-1), ... 1.

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  • Good start but doesn't work unfortunately because I'm using multilineString. It only works with lineString or with connected lineString (with combination of st_linemerge and st_makeline). – Roman Jul 6 '19 at 14:28
  • Maybe try to split it using ST_Dump into linestrings, and distribute points among lines proportionally to their lengths. But it is getting complicated. – Michael Jul 6 '19 at 17:48
  • I finally used ST_GENERATEPOINTS(ST_BUFFER(ST_SetSRID(<Section.geom Column>, 4326)::geography, 10)::geometry, 1). It doesn't distribute proportionally, but it ditribute randomly in a perimeter of 10 meters from my section. Normally, I won't have many observations points on the exact same position (unless there are too much and I'm not lucky). Thank you for your time and answers. And I valide your answer as I think it can help others people as long as they don't use MultilineString :-) – Roman Jul 7 '19 at 9:22
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I suppose your observations are point geometry. If that's the case you can use ST_ClosestPoint to find the closest point on line and plot them on line itself.

    SELECT ST_ClosestPoint(
    ST_GeomFromText('MULTILINESTRING ((10 10, 40 50), (20 20, 50 20, 50 60, 20 20))'),
    ST_GeomFromText('POINT(20 40)')
);
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  • I'm not sure I understood. Yes, my observations are point geometry. But with ClosestPoint, I'll have the opposite about what I need no ? Actually, I'd like to find the Furthest localisation (too bad, function doesn't exist) on my multilineString from the others points. – Roman Jul 4 '19 at 14:23

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