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How can i prevent sloped buildings when I extrude building footprints over sloped terrain? I am using FME to drape 2D features over terrain (SurfaceDraper), then using the Extruder transformer to create 3D buildings from OS Building Heights. Unfortunately the height value seems to apply itself to each vertex, which creates sloped building tops (roof geom is not relevant here) - example below:

enter image description here

Is there an extrusion method that uses a single vertex, or is there another method entirely?

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Solved on FME Knowledge Centre - https://knowledge.safe.com/questions/50240/extruding-building-heights-from-3d-terrain-model-c.html

Thanks Takashi!

Steps and output image below:

  1. GeometryExtractor: Save the original geometry (building footprint: 2D polygon) as an attribute.
  2. SurfaceDraper: Drape the polygon by the terrain surface.
  3. BoundsExtractor: Extract the minimum z of the draped polygon.
  4. GeometryReplacer: Restore the original 2D polygon.
  5. 3DForcer: Convert the polygon to 3D with the minimum z as its elevation.
  6. Extruder: Extrude the 3D polygon by the building height.

enter image description here

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