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The goal is to create a 3D model of imaginary planes in space that I can interrogate to see where other objects in the vicinity of the model cut through any part of that plane. For example, an airspace surrounding an airport and a tall tree that extends into the lower surface of the airspace.

The model will consist various horizontal planes and sloped planes that exist between the horizontal planes (kind of like a dinner plate shape).

I thought I could try creating a DEM that I could analyse. I tried to make polygons around a point (buffer), then converted them to lines, then to points - because the interpolation (average) tool required scattered data (i.e. points) with z values). It didn't work - the output raster wasn't to scale. I also tried using the Interpolation Plugin and the output was to scale but it's not precise enough for the purpose. The dimensions of the airspace are mathematically defined using gradients, distances and elevations.

I created polygons easily enough with the Z values appended to it that represent horizontal surfaces. I just can't figure out how to create the sloped surfaces with varying Z values (which depend on the distance from some point in space, like the edge of a horizontal surface polygon or a point).

Z values need to be based on height above an earth reference point, such as metres above Australian Height Datum (AHD). So, I'm not sure if Sketchup will be useful here (???). I need to be able to interrogate the model to see if an object in Earth is going to penetrate the surface.

Does anyone have any suggestions regarding this issue - or generally some other way to go about this problem using another program?

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    You will need to triangulate or grid your ground. Polygons only have Z values on their boundaries, to make them sit right they have to have lots of boundaries - the easiest way to do this is to intersect with a grid and then get the Z values from the surface DEM, the normal way of doing this is to create a TIN (Triangular Irregular Network). I haven't worked with QGIS 3d yet but that's how I'd do it in ArcScene or Microstation. – Michael Stimson Jul 24 '15 at 1:41

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