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I am trying to interpolate (TIN) in QGIS to produce surface models. I need to interpolate within a polygon area extent. The options available in QGIS do not allow this as far as I am aware. Is there any way of doing this, including utilising GRASS, and if so, how do you do it?

The image below shows an example of a desired extent with the points lying within it.

area to interpolate

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One way to achieve this is to run (Vector geometry tools/Delaunay triangulation) in QGIS. You can then delete the triangles that fall outside of your desired boundary. This will produce a shapefile which you can rasterize if that is the desired output. You can use any of the three fields to rasterize because each point is shared by multiple triangles (each point will be in different column for each triangle). So you are using all you points to rasterize. See the series of screen shots below.

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