Using a surface as a hide boundary in Civil 3D allows you to exclude certain areas of a surface from being displayed or triangulated. This is useful for removing unwanted portions of a surface, such as areas underneath structures or other overlapping surfaces.

Steps to Use a Surface as a Hide Boundary
1. Prepare the Drawing
Ensure you have a TIN Surface in the drawing.
Identify or create the boundary area that you want to hide. This could be a polyline or feature line representing the boundary.
2. Open Surface Properties
In the Toolspace palette, go to the Prospector tab.
Expand the Surfaces collection and find your surface.
Right-click the surface name and select Surface Properties.
3. Define the Boundary
Access the Definition:

Expand the surface node in the Prospector tab.
Right-click Definition and select Add Boundaries.
Set Boundary Properties:

In the Add Boundaries dialog box:
Type: Choose Hide as the boundary type.
Name: Assign a name to the boundary (e.g., "Structure Hide").
Non-destructive Breakline: Leave unchecked unless you want the boundary to affect triangulation without excluding data.
Click OK.
Select the Boundary Object:

Select the polyline or feature line representing the area to hide.
The boundary is applied, and the selected area is excluded from the surface.
4. Update the Surface
The surface triangulation is updated, and the hidden area no longer contributes to contours or display.
Additional Tips
Boundary Hierarchy: Ensure the hide boundary is applied after any other boundaries (like outer or show boundaries) to avoid conflicts.
Closed Objects: The polyline or feature line used for the boundary must be closed.
Edit the Boundary: To modify the hide boundary, adjust the geometry of the polyline or feature line, and the surface will update dynamically.
Use Case Examples
Hiding Structures: Exclude building footprints or other features from the surface.
Multiple Boundaries: Use multiple hide boundaries to manage complex surfaces with overlapping exclusions.
Refining Visualizations: Clean up the display of surfaces in profile or plan views by removing unnecessary areas.