Visualizing convex hull in QGIS?

Question

There is no way to visualize a convexhull in qgis 1.8 lisboa? I have tried following views in Postgis 1.5

I was told to use an id column to publish it in Qgis. If you not use an id column, u cannot load it anyway because of an error.

1.

create or replace view con_hull_baeume3(bid) as
select '0', ST_ConvexHull(ST_Collect(geom)) FROM baeume;

2.

create or replace view con_hull_baeume4(bid) as
select bid, ST_ConvexHull(ST_Collect(geom)) FROM baeume group by bid;

but this one make a convexhull on each of the geometrypoints. I mean just use 'select ST_ConvexHull(ST_Collect(geom)) FROM baeume;' works as sql perfectly. But i must define it as a view to load it into a postgis layer of QGIS.

Answer 1

To get unique IDs in views so they display in QGIS I use the row_number() window function. In your example it would be:

SELECT row_number() OVER () As id, ST_ConvexHull(ST_Collect(geom)) FROM baeume;

If this is a one-off analysis you can skip creating a view and use the DB Manager plugin to visualize query results. Its SQL Window tool lets you run any SQL Query you want to against Postgres / SQLite databases and if the query returns a unique ID and a spatial field they can be brought into your QGIS map with just a few clicks.

Answer 2

I'm not familiar with the view concept nor st_collect at all so I may tell something stupid...

If the bid refers to the unique id of your points, you must remove the groupby statement so CvxHull gets all the points as an argument; otherwise you will have nbPoints hulls with one point in each! => something like CREATE OR REPLACE VIEW con_hull_baeume3 AS SELECT( some_id serial PRIMARY KEY as oid, ST_ConvexHull(ST_Collect(geom)) as the_geom) FROM baeume;

Grouping by bid is a good advice if bid is not a unique id and if you want to create a convex hull for each different bid (but I think this is not your case...)

Also, ansswers from How to make a spatial view in PostGIS and add it as a layer in QGIS? could help you, and http://hub.qgis.org/wiki/quantum-gis/Using_views_with_the_postgis_provider shows how to create a qgis acceptable view from postgis.

Good luck!