Traditionally, breaklines were used in surface models to represent all kinds of linear features. With lidar, they are mostly used for hydro enforcement. The higher resolution the lidar, the less need for breaklines unless the application is water related. Breaklines are important for maintaining the definition of water-related features in an elevation model. Breaklines are used to capture linear discontinuities in the surface, lake shorelines, single-line drains for small rivers, and double-line drains for large rivers. Sometimes, breaklines are also collected to help define and sculpt the surface without necessarily representing discontinuities. Examples of these applications include contour-like form lines and the crests of rounded ridges.
If you have photogrammetric breaklines to go along with your lidar, you can use either the LAS dataset or the terrain dataset to enforce the breaklines in the surface model .
For an overview of the terrain dataset, see What is a terrain dataset?
For an overview of the LAS dataset, see What is a LAS dataset?
The graphic below depicts a surface with breakline enforcement. The left image is a surface made without breaklines along the river banks. The right image has breakline enforcement. Specifically notice how clear the river's edge becomes with the breaklines and how the water surface stays level.
Breaklines, while frequently used in bare earth models, tend to be detrimental when used with first return surfaces because they can conflict with the aboveground points. For example, breaklines capturing road edge of pavement can be coincident in x,y but different in z to points in the tree canopy overhanging the road. Because of this, consider excluding breaklines from your first return surface or at least those where you know there's potential conflict.
The most efficient means of organizing breaklines is to separate them into different feature classes based on surface feature type (SFType). Surface feature types control how the features are enforced in the model and how the natural neighbor interpolator interprets the surface as it crosses over these features. A distinct break in slope will occur across hard features but not across soft features.
The following table provides a few examples of different surface feature types supported in the LAS and terrain dataset:
SFType for LAS datasets and terrain datasets
|Measurement type||Feature class type||SFType|
Edge of pavement, single- and double-line drains for rivers, sharp ridgelines
3D line feature class
2D polygon feature class with z-value stored as attribute
Hardline or hardreplace
Eroded/Rounded ridgelines, contour-like form lines
3D line feature class
Study area boundary
2D polygon feature class; no z-value
When using LAS datasets, breaklines are referred to as surface constraints and can be either feature classes or shapefiles. Breaklines are referred to as surface constraints in a LAS dataset as they are only enforced when the LAS dataset is displayed as a surface and not when points are displayed. With terrain datasets, breaklines are represented as feature classes that reside in a feature dataset alongside the terrain dataset surface model.