Using Breaklines to Model Terrain

This tutorial is the second in a series on how to build, edit, and analyze surface models using Autodesk® Land Desktop. This article focuses specifically on how to use breaklines to more accurately model the terrain. We will look at:

The three different types of breaklines—standard, proximity, and wall
How to create them
Their effect on the surface model

Use breaklines, sometimes known as fault lines, in your drawings to reflect an abrupt change in grade in the topography, for example, the top or toe of a bank, a crown of pavement, curbs, and retaining walls. A breakline forces the interpolation of the surface triangulation to follow the terrain, rather than cutting across it. Surface lines cannot cross breaklines.

Figure 2: Surface built without breaklines.

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Figure 1: Breaklines shortcut menu.

To access the breakline routines, go to the Terrain drop-down menu and choose the Terrain Model Explorer. Here, you must first create a surface as described in the first article in this series, An Introduction to 3D Site Modeling. Once the surface has been created, right-click the Breaklines data type to access a shortcut menu that lists all of your options (see Figure 1).

This menu is broken down into sections for the three different types of breaklines—standard, proximity, and wall—breakline utilities, breakline editing, and an option to export breakline data to an external file that can be read into other surfaces or projects. Breakline types are described in the following sections.

Standard Breaklines
Standard breaklines are lists of Northings, Eastings, and Elevations used to locate critical points in three dimensions. The lines between these points cannot be crossed by surface triangulation. Standard breaklines are typically used when there is survey data at known points along the edge of a change in slope (such as a crown of pavement or top of a bank).

Figure 2 shows an example of a surface built without breaklines. The magenta line indicates the top of a steep embankment. The white surface-triangulation lines that cross this bank have a constant slope and do not account for the change in slope at the top of the bank.

Figure 3: Surface built using breaklines.

Figure 3 shows the same surface with the breaklines taken into account. Note that no surface triangulation lines cross the top of the bank. Because the surface lines follow the top of the bank, the magenta line is completely covered.

Figure 4: Wall breakline listing.

You can define standard breaklines by:

Selecting points on the screen (using Osnaps)
Typing in point numbers from the survey data
Using a previously drawn polyline
Using previously drawn 3D lines
Reading breaklines in from an external data file

When creating standard breaklines using points, you may use the .P, .G, and .N toggles to enter point numbers, select point objects, or enter Northings and Eastings. When creating standard breaklines using point numbers, you can indicate ranges of point numbers using the hyphen (-) or individual point numbers separated by commas (,). Point numbers should be entered following the direction of the breakline, not in point number order.

Proximity Breaklines
Proximity breaklines are identical to standard breaklines in terms of their use, but they differ in how they are defined. Unlike standard breaklines, proximity breaklines only hold Northings and Eastings (no elevations). When the surface is built, the proximity breakline will find the nearest data point containing elevation information and use that for the Z value. Proximity breaklines can be defined by selecting points or from an existing polyline.

The main benefit of using proximity breaklines is that you do not have to draw them as accurately. Rather than snapping to the exact points along the breakline, you can click "close enough." This is very useful when you already have a drawing with 2D polylines representing surface features (such as streams). Simply define those features as proximity breaklines.

Wall Breaklines
Wall breaklines are used in areas where there is a vertical face in the surface such as curbs and retaining walls. Before defining a wall breakline, you must draw a polyline (2D or 3D) along the path of the wall. You cannot define wall breaklines from points.

Once the polyline has been drawn, use the following steps to define the wall breakline.

1. Select the Breaklines data type in the Terrain Model Explorer window and right-click to access the Breaklines shortcut menu. Choose the Define Wall Breaklines command from that menu.

2. At the command prompt, enter a breakline name (it does not have to be unique) and select the polyline.

3. After you select the polyline, the command prompts you to enter an offset side.

Note: The offset side is determined by which part of the wall or curb was surveyed, top or bottom. If you surveyed along the top of the wall, the offset side shows which direction is the bottom of the wall. This will be used to add a small amount of offset to avoid vertical faces.

4. Once you have entered an offset side, the command steps along the polyline, one vertex at a time, prompting you for elevation values on either side of the breakline. Enter the height of the wall either as true elevations or as an elevation and a difference. If you surveyed the top of the wall, the difference should be a negative value.

Note: Autodesk Land Desktop cannot use true vertical faces in surface models, so a wall breakline uses a small amount of offset (as described in the preceding Note) to avoid vertical face errors. Whenever you need to use vertical features, use the wall breakline; do not create vertical faces manually.

Figure 4 shows the listing of a wall breakline. Note that for each Northing and Easting location, there are two elevations (top and bottom of wall).

Once you have defined all the breaklines, build and edit the surface model as usual. If you plan to reuse certain breaklines in future surface modeling, you may want to export them to an external data file for ease of use.

Conclusion
Using breaklines properly results in a much more accurate surface model, which means better contour maps, more precise volume calculations, and more accurate elevations when surface models are used for profile and cross-section extraction.

The next article in this series will discuss how to analyze and edit a surface model once it is built.