Clip polygons

Clip polygons make the boundaries of the TIN smaller by restricting the zone of interpolation, the area of the TIN for which elevation, slope, and aspect are calculated.

Top: The light blue polygon will be added to the TIN as a clip polygon. Middle: The TIN is clipped to the polygon extent.
Bottom: Clipping does not actually change the extent of the triangulated area, only the zone of interpolation. By default, triangles outside the zone are not displayed, but they can be turned on, as they are here.

One common use of clip polygons is to create a rectangular extent for the TIN, to give it a more regular appearance.

Clip polygons also affect analysis. By restricting the zone of interpolation, they change the statistics for area, volume, elevation, slope, and aspect.

Where do I find these statistics?

You can get area and volume for a TIN from the 3D Analyst menu (point to Surface Analysis and click Area and Volume). You can get statistics for elevation, slope, and aspect from the Layer Properties dialog (click the Symbology tab, add and highlight the renderer you want in the Show box, and click the Classify button).

The boundaries of clip polygons become triangle edges, which means that the TIN is retriangulated.

Top: Detail of a TIN before a clip polygon is added.
Bottom: When the clip polygon (symbolized in red) is added, the TIN is retriangulated. The sides of the polygon become triangle edges.