Originally published at Digital Architecture: http://www.davidmaudlin.com/ArchiCAD/DigitalArchitecture/Extra/Tips/MeshContours/MeshContours.html
The following article will help you to extract contours from a Mesh and apply to the 2D view.
With the Mesh selected, right-click and select Show Selection/Marquee in 3D (so only the mesh appears in the 3D Window).
Set your 3D Cutting Planes: In the “Z” box, enter the height of the contour, click the “Z” icon to lock it, then draw the Z Cutting Plane in the window above, click above the line to remove all above, set the Fill Material & Edge of Pen Cuts to Custom, and set the pen number for a unique pen (this will help in seeing and selecting the lines).
Activate 3D Cutaway
Go to the 3D Window, your Mesh will be cut at the contour height, the cut line will be the pen previously selected
Note: The 3D Window with the Mesh and Cutting Plane can be preserved with a 3D View: after generating the 3D Window, use the Organizer to pull the Generic Axonometry from the 3D List into the View Map, give the view a descriptive name (for example: Contour -10’), click the Settings button and click the Get Current Window’s Settings to save the settings of the 3D Window, including the Cutting Plane. This allows the contours to be regenerated more quickly if the mesh is later altered.
Select the Marquee Tool, flat marquee option, select the area around the contour line and copy.
In the appearing Copy dialog box select Scaled Drawing, Edges, and Remove Redundant Lines
Go to the 2D Window and Paste, the contour lines will come in over the Mesh matching their position from the 3D Window.
Repeat steps 3 to 6 for each contour level
Now you can select the lines, change their layer, line type and other attributes depending on your drafting standards. You can add text for the contour heights. If you have a mesh showing the existing grade and a new mesh showing proposed grades, then lines from both meshes can be extracted to show grading changes.