Breaklines are TIN edges that cannot be replaced as the TIN is being computed. They force the TIN and resulting contour lines to be computed such that the contour lines do not easily cross the breakline. Breaklines, then, can be used to model both grade breaks that you expect to follow a contour like the toe of a slope and lines of constant grade like the bottom of a ditch.
You can import breaklines from a traverse or manually add and remove them from a surface. You will probably do both as you use surfaces in TPC. See Editing Breaklines.
Breaklines are always associated with Survey Points. If the position of a survey point referenced by a breakline changes, the breakline changes with it. Breaklines do not store the end point coordinates of the breakline; they store the survey points that are referenced. This allows even the breaklines in a surface to be dynamic.
Breaklines are stored with the surface in the survey file. Once entered, you do not need re-enter the breaklines when you next open the survey.
If you have coded your breakline points in the field and used TPC's Point Code Table to sort the survey points into traverses, your breaklines may already be defined by traverses. TPC lets you import any traverse as breaklines. So a traverse with points 1, 2, 3, 4, 5, 6 will add the breaklines 1-2, 2-3, 3-4, 4-5 and 5-6. Only the control points in a traverse create breaklines. The surface checks each traverse line to see if it matches an existing breakline. If not, a new breakline is added.
You can also use traverses to remove breaklines from a surface. This makes it real easy to say, "What will my surface look like if I use this edge of road traverse as a breakline". You can try it by importing that traverse as breaklines. If you don't like it or don't need it, just remove the breaklines in that traverse and you're right back where you started.
You can also write a breakline file which includes all the breaklines currently in a surface and/or import a breakline file into your surface.
Breaklines can be included in the triangulation or not included. Not including breaklines does not remove them from the surface – they are still available to be included. Turning breaklines on or off can give you a better feel for how the surface is modeled by your survey.
Editing Topo Points
TIN and Breakline Settings
Creating a TIN
Get Breaklines From Traverse command
Remove Breaklines In Traverse command