[GIS] PostGIS – Get a point inside a line or polygon

centroidslinestringpolygonpostgis

I need to get the center point of a line or polygon to make marker for my app. So that when you click a marker the geometry appears (line or polygon). I used ST_Centroid to make it work.

The result is what I expected unnest some polygons or lines whichs centroids are out of the geometry. What i would like for those lines or polygons is to get the "most center point" but inside the geometry.

How can I do this? Is there a solution?

Best Answer

From the doc: ST_PointOnSurface — Returns a POINT guaranteed to lie on the surface.

Related Solutions

[GIS] How to calculate the center point inside a polygon in ArcGIS 9.3

Each instance of geometry (esri.geometry.Geometry) has a method called getExtent() which returns the extent of the geometry (polygon in your example). esri.geometry.Extent has a method called getCenter() that returns a point.

So you could do something like:

var myPolygonCenterLatLon = myPolygon.getExtent().getCenter();

You could alternatively calculate the centroids to polygons beforehand in ArcMap. Then host the layer which has a one-to-one relationship with the polygons.

[GIS] Intersection between line & polygon in PostGIS

As user30184 noted, you have to use ST_Crosses() instead of ST_Intersects().

Additional, your expression works on one line compared to one polygon only: The black line touches Polygon 1 and doesn't cross Polygon 1. In more detail:

SELECT x FROM line, polygons WHERE ...

means: Build a case from every pair of a line and a polygon, so number of cases is n_lines x n_polygons. Do the WHERE clause for every case and keep cases where it is true. Result: blue line, P1: false, blue line, P2: true, black line, P1: true, black line, P2: false.

Maybe you want something like this:

SELECT lines.geom
 FROM lines, polygons
 WHERE ST_Touches(lines.geom, polygons.geom) AND
                 NOT EXISTS (SELECT 1 FROM polygons p2 WHERE ST_Crosses(lines.geom, p2.geom));

Best Answer

Related Solutions

[GIS] How to calculate the center point inside a polygon in ArcGIS 9.3

[GIS] Intersection between line & polygon in PostGIS

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