Surface Definition

A surface definition is a collection of a surface build, data, and edit properties, as well as a list of the operations that you have performed on the surface. The type of definition data items that a surface can have depends on the surface type.

The Definition collection under a surface in the Prospector tree provides access to the surface possible data and edit definition items.

Data Definition

The data definition for a surface comprises the data components that you can add to a surface.

The following table illustrates the data categories that are supported for each surface type. If the surface type does not support a data type, the data type is not available in the surface Definition collection in the Prospector tree.

Category	TIN Surface	TIN Volume Surface	Grid Surface	Grid Volume Surface
Boundaries	Yes	Yes	Yes	Yes
Breaklines	Yes	No	No	No
Contours	Yes	No	No	No
DEM Files	Yes	No	Yes	No
Drawing Objects	Yes	No	No	No
Point Files	Yes	No	No	No
Point groups	Yes	No	No	No

Edits

Data edits are operations that are not added to any of the existing data components; rather, they are added to a surface definition as edit operations.

Operation List

The surface build process is incremental. Every time when you add data to a surface or edit the surface, the surface is updated. When data is removed, the surface is rebuilt.

To support the incremental build process, a surface has an operation list, which is a sequential list of all operations performed on the surface in its current state.

Order of Data in a Data Definition Operation

When you add point data to a TIN surface, the result can be affected by the order in which Autodesk Civil 3D processes the points. You cannot control the order in which Autodesk Civil 3D processes points in a single operation; Autodesk Civil 3D adds points to the surface in an order that minimizes the time to perform the operation. The following are two ways the processing order can affect the resulting TIN surface:

• The order can affect the outcome when four points are co-circular (the circle determined by any three of them passes through the fourth). In this situation there are two ways to make a Delaunay triangulation of the four points, and the two triangulations could make the surface contours noticeably different. You can use the Swap Edge command to correct any problems of this sort.
• A second way is when two or more points are close together (less than .0001 units apart). Autodesk Civil 3D treats two such points as the same point, and whichever one gets processed second does not get added to the TIN. If the two points have different elevations, the order will therefore affect the resulting surface; in such a situation Autodesk Civil 3D will post a message to the Event Viewer. You can use the Modify Point command to assign the correct elevation to the point.

Effect of the Order of the Operation List

The order in which surface operations are performed can impact the resulting surface. Because of the incremental way a surface is built or rebuilt (the operations are performed one at a time in the order that they appear in the operation list) the order of the operation list is very important. You determine the order by the sequence of the commands you use to define the surface data. You can later modify the order in the Surface Properties dialog box. The following is a suggested order:

1. Add breakline and contour data.
2. Add point data.
3. Add any necessary boundaries, outermost first.
4. Perform any necessary edit operations.