The boundary types outer, show, and hide all have the option to be destructive (i.e., the Nondestructive Breakline check box is cleared) or non-destructive (i.e., the Non-destructive Breakline option is checked). What Civil 3D is “destroying” are the triangles that underlie the boundary.
The following image shows a schematic of a surface model before a boundary is added. For illustration purposes, triangle vertices have been highlighted with different shapes. The stars represent surface data points inside the boundary, and the circles represent points outside of the boundary.
Use the Non-destructive Breakline option when the data you are working with is valid right up to the boundary edge. A boundary added as a non-destructive breakline will retriangulate the surface and create triangles up to the boundary, as shown in the following image. The squares show locations where Civil 3D has interpolated a surface data point at the boundary. The portion of the surface that would be hidden by adding a boundary is being shown in light gray for illustration purposes.
Use a destructive boundary if the line you are adding as a boundary is an approximation of the area you want to rein in. A boundary added as destructive will remove any triangle lines it crosses. The following image shows the illustration surface with the same boundary as before but added as destructive. The portion of the surface that would be hidden by adding a boundary is being shown in gray for illustration purposes. Notice how the outermost triangle legs of the surface areall inside the boundary.
Consider a surface that needs an outer boundary, for example. If you create a rough polyline that encloses the points you want to include as your surface model, add the polyline to the surface as an outer boundary with the non-destructive option cleared.
If you deliberately created the polyline by snapping each vertex of the boundary to a surveyed point, for example, you could add it as a non-destructive breakline.
You will need to use your professional judgment to know when to use the Non-destructive Breakline option and when to use the Destructive Breakline option, but a good rule of thumb is that outer boundaries are usually destructive. Hide and show boundaries are usually non-destructive.
With the exception of data clip boundaries, you’ll want to have your boundaries among the last operations in your surface-building process. Therefore, as future edits are made, you may want to move the Add Boundary build operation back to the bottom of the Data Operations list on the Definition tab in the Surface Properties dialog, which will be discussed later in this blog.
