cut out a piece of the terrain See image. I'm trying to cut out a small piece of the terrain to use for particle system (scatter trees, grass etc). I used the knife tool to draw the area (orange line). How can I then delete the rest of the terrain and just keep the portion within the orange line? (In Max I just use the shapemerge modifier, not sure what to do here in blender).
Create a desired shape from 2D curves, below is an example, and close it enabling cyclic option. Model the plane to shape, convert to mesh and knife project it onto the terrain. Then invert the selection and delete it. Or, having knife projected, separate the selection and delete or keep the portion that's left. NB: to knife project correctly you need to be in ortho view.