Math Operations on Surface
1. Create a contour map.
2. Draw the closed line. The "C" key closed the line during a drawings. Click the name of that line or double-click a vertex of line on the map. The selected line draws in red color.
3. Choose the Math operation command from context menu for the Grid tree item.
4. Draw the second closed line and select it.
5. Again choose the Math operation command from context menu for the Grid tree item..
6. OpenGL 3D view.
Without a selected line an operations applied over all surface.