Does anyone know If it’s possible to edit the control points of a surface but keep the boundary fixed?

Trying to model a hill landsape with a fixed border level. When I edit points the boundary also moves also because the control points are not on the boundary curve.

Hello - not directly but you can trick Rhino as in the attached file: Patch the point objects and the boundary curve with History turned on - the points are extracted from your original surface. Because History is on, you can move the points up and down and the surface will update while trying, at least, to keep the boundary.

The general rule of thumb is if you have a Degree=3 surface, leave the outside 2 rows of points alone. You can move the points in the third row (and closer to the interior), without disturbing the edge.

Here is how the math works for untrimmed edges of surfaces. Trimmed edges such as in the example in the first post are different. Using Patch as Pascal suggested is a good method with trimmed edges.

The position of the edge is affected only by the control points along the edge.

The tangency of the surface at the edge is affected by the control points along the edge and the adjacent row of control points.

The curvatue of the surface at the edge is affected by the control points along the edge and the two adjacent rows of control points, (Degree 2 and higher in the direction normal to the edge only.)