TPC uses the Dulaney algorithm to triangulate a surface and ensure the best possible representation of the actual shape of the surface. Of course, this all happens very fast, with 10,000 points being triangulated in just one second.
TPC determines which survey points to include in the TIN based on the source you've selected (points in a traverse, points in the border, etc). If you select a border, TPC checks each point to determine if it is inside the border. Points determined to be outside the border are not included in the TIN.
TPC also checks for duplicate topo points using the Coordinate limits you have set for your program. To check or change these limits, choose Tools | Program Settings and left click the Limits tab.
The resulting TIN is a series of adjacent triangles. These triangles model the surface. The assumption is that the plane surfaces of each triangle ARE the surface. So a surface is actually composed of many triangle planes.
When TPC generates a cut sheet, it compares the selected points with these triangle planes to determine the surface elevation above or below the point. These triangles planes are also used to compute the surface volume.
Each triangle has three sides or edges, and most triangles share an edge with another triangle. Only triangles along the border have edges that correspond to a single triangle.
When TPC generates a transect from a surface, it is actually just determining which TIN edges intersect the transect.
Breaklines are also TIN edges. They are fixed by the user and cannot be removed during the computation of the TIN.
In very simplest terms, contour lines are generated by interpolating elevations along each TIN edge, then connecting the dots.
Once TPC has generated the surface lines it can discard the TIN points / triangles / edges to save memory. By default, these objects are retained for use with transects, volumes and surface information reports. See Optimizing Surfaces.
Editing Topo Points
TIN and Breakline Settings