Getting multipolygon vertexes using PostGIS

Getting multipolygon vertexes using PostGIS

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Today I needed to create a view in PostGIS that returned the vertexes of a multi-polygon layer. Besides, I needed that they were numerically ordered starting in 1, and with the respective XY coordinates.

Screenshot from 2015-11-05 23:58:19

It seemed to be a trivial task – all I would need was to use the ST_DumpPoints() function to get all vertexes – if it wasn’t for the fact that PostGIS polygons have a duplicate vertex (the last vertex must be equal to the first one) that I have no interess in showing.

After some try-and-fail, I came up with the following query:

CREATE OR REPLACE VIEW public.my_polygons_vertexes AS
WITH t AS -- Transfor polygons in sets of points
    (SELECT id_polygon,
            st_dumppoints(geom) AS dump
     FROM public.my_polygons),
f AS -- Get the geometry and the indexes from the sets of points 
    (SELECT t.id_polygon,
           (t.dump).path[1] AS part,
           (t.dump).path[3] AS vertex,
           (t.dump).geom AS geom
     FROM t)
-- Get all points filtering the last point for each geometry part
SELECT row_number() OVER () AS gid, -- Creating a unique id
       f.id_polygon,
       f.part,
       f.vertex,
       ST_X(f.geom) as x, -- Get point's X coordinate
       ST_Y(f.geom) as y, -- Get point's Y coordinate
       f.geom::geometry('POINT',4326) as geom -- make sure of the resulting geometry type
FROM f 
WHERE (f.id_polygon, f.part, f.vertex) NOT IN
      (SELECT f.id_polygon,
              f.part,
              max(f.vertex) AS max
       FROM f
       GROUP BY f.id_polygon,
                f.part);

The interesting part occurs in the WHERE clause, basically, from the list of all vertexes, only the ones not included in the list of vertexes with the maximum index by polygon part are showed, that is, the last vertex of each polygon part.

Here’s the result:

Screenshot from 2015-11-05 23:58:40

The advantage of this approach (using PostGIS) instead of using “Polygons to Lines” and “Lines to points” processing tools is that we just need to change the polygons layer, and save it, to see our vertexes get updated automatically. It’s because of this kind of stuff that I love PostGIS.

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Labels leading lines with QGIS and Postgis

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Recently I had the need to add labels to features with very close geometries, resulting in their collision.

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Using data-defined override for label’s position (I have used layer to labeled layer plugin to set this really fast) and the QGIS tool to move labels, it was quite easy to relocate them to better places. However, in same cases, it was difficult to understand to which geometry they belonged.

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I needed some kind of leading lines to connect, whenever necessary, label and feature. I knew another great plugin called “Easy Custom Labeling“, by Regis Haubourg, that did what I needed, but it would create a memory duplicate of the original layer, wish meant that any edition on the original layer wouldn’t be updated in the labels.

Since the data were stored in a PostgreSQL/Postgis database, I have decided to create a query that would return a layer with leading lines. I used the following query in DB manager:

SELECT
  gid,
  label,
  ST_Makeline(St_setSRID(ST_PointOnSurface(geom),27493), St_setSRID(St_Point(x_label::numeric, y_label::numeric),27493))
FROM
  epvu.sgev
WHERE
  x_label IS NOT NULL AND
  y_label IS NOT NULL AND
  NOT ST_Within(ST_Makeline(St_setSRID(ST_PointOnSurface(geom),27493), St_setSRID(St_Point(x_label::numeric, y_label::numeric),27493)),geom))

This query creates a line by using the feature centroid as starting point and the label coordinate as end point. The last condition on the WHERE statement assures that the lines are only created for labels outside the feature.

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With the resulting layer loaded in my project, all I need is to move my labels and save the edition (and press refresh) to show a nice leading line.

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