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I am currently working on a solar potential estimation tool for existing buildings. The idea is to use buildings (=Polygon) shape, their height, and create the resulting shade at a given hour. I am only going to do the test for a few position of the sun. I know about the grass function r.sun.mask but it is overpowered for what I am trying to do.

For now I only want to get the shade when the sun is south, at 18° in the sky. The building should therefore cast a shadow about three times its height.

What I am trying to get:

What I am trying to get

I have been looking for a while but I did not find any tool in postgis to do this. I was thinking about buffering and then cutting the polygon but could not make it work. Do you know any tool or have any idea how to do it?

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Simplest way to do this is with an

ST_Extrude(geom,x-direction,y-direction,0)

You will have to calculate the extrude factor yourself of course and it assumes your whole polygon is the same height.(in your example, extrude would be like: ST_Extrude(geom, 0, 3.0*height, 0) )

Here is the manual on ST_Extrude. Keep in mind that you need postgis with SFCGAL for this, check the installation docs on how to get this if you don't already have it.

  • The output shape is the one I want, however the Polyhedral Surface cannot be intersected with a Polygon. I tried the ST_Force2D on the polyhedral and then intersect it with my polygon but it does not work. Is there another function with a more practical output format, or a way to convert this polyhedral surface to polygon? – aeboq Jun 2 '17 at 14:12
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    Yes, you can dump it to normal polygons. Example: SELECT ST_AsText((ST_Dump(ST_Extrude(ST_MakeEnvelope(10,10,100,100,28992),0,8,0))).geom) – tilt Jun 2 '17 at 14:17
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It might be worth considering exporting from QGIS to Blender as this would allow you to place the sun as a light and animate through the day and through the year. I did find a workflow here that looks relevant.

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