Available with 3D Analyst license.
To add breaklines to an editable TIN, use the Add TIN Line interactive tool 
from the TIN Editing toolbar.
When the tool is clicked, the Add TIN Line dialog window appears to let you specify properties for the breakline(s) to be created. These properties include the breakline Line type, Height source, and whether digitized vertices should snap to existing nodes. A single click will enter each vertex, then double-click to finish entering a breakline.
To add a breakline to a TIN interactively following the steps outlined below.
- Click the TIN Editing drop-down menu and click Start Editing TIN.
- Click the Add TIN Line tool from the TIN Editing toolbar. The Add TIN Line dialog window appears.
- Set the desired properties for the breaklines to be created on the TIN surface.
Line type—The type of breakline to be created. Choose either hard or soft as the line type. Hard and soft qualifiers for line and polygon feature types are used to indicate whether a distinct break in slope occurs on the surface at their location. A hard line is a distinct break in slope, while a soft line will be represented on the surface as a more gradual change in slope.
Height source—The height source to be used to generate the breakline on the TIN surface. Choose either from surface, max z on surface, min z on surface, or as specified.Height source Description from surface The elevation of the digitized line is interpolated from the selected locations on the surface.
max z on surface The elevation of the digitized line is interpolated from the maximum height value that is encountered along the line.
min z on surface The elevation of the digitized line is interpolated from the minimum height value that is encountered along the line.
as specified The elevation of the digitized line is added to the surface at a constant specified elevation value set in the height field.
Height—Shows the elevation of the surface interactively while moving the cursor over the surface.
When as specified is set as the Height source, the elevation value is taken from the height added to the Height source window.
Vertex snap—The selected vertex locations for the new line will snap to the closest TIN node based on the user-specified snap distance. Click to turn vertex snapping on/off.
Snap distance—The snap distance is the distance to use for vertex snapping. While adding the digitized points to the surface you will see a highlighted node to snap the selection point with. If you do not see a highlighted point on the TIN surface, then the closest node is farther away than the specified snap distance and no snapping occurs. Snap units are xy units of the TIN.
- Select the TIN surface where the breakline will start; the first vertex will be digitized on the surface. Continue to add each vertex to the breakline by a clicking in the desired locations. Double-click to finish entering a new breakline.