Click on any control point of the surface you want copy.

A new surface is created, with the same control point locations with a 1 metre offset in x, y and z coordinates, followed by x-coordinate signs reversed.

Note that the orientation of the new surface is reversed (so that the normals point in the opposite direction).

Use REVERSE_SURFACE_ROWS to reverse the orientation of the new surface.

Switch between orthogonal projection views and continue mirror copying surfaces.