How Delineate TIN Data Area (3D Analyst) works
Delineate TIN Data Area is used to define the data area, or interpolation zone, of a triangulated irregular network (TIN) based on triangle edge length. Triangles that are evaluated based on rules discussed below and have an edge exceeding the Maximum Edge Length value will be masked out as NoData. These triangles will not be rendered in maps nor used in surface analysis such as interpolation.
The Method determines which triangles are evaluated. The PERIMETER_ONLY option, which is the default, works in an iterative outward-in progression. Triangles on the outer extent of the TIN with edges exceeding the Maximum Edge Length value are masked as NoData. The neighbors of these triangles are put on a stack and evaluated in turn. This process continues until no more triangles are found. This method generally processes triangles on the exterior of the TIN, leaving those with long edges on the interior of the TIN intact (assuming they are surrounded by triangles with shorter edges). The ALL option evaluates all triangles in the TIN, both exterior and interior.
All triangles are marked as inside before edge length classification begins. This will effectively undo any preexisting data area classification.
Often you have data to construct a TIN but no data area boundary to use as a clip polygon in the TIN's definition. Without the use of a clip polygon, the resulting TIN will have a convex data area. If the distribution of data used to build the TIN is not convex, there will be an undesirable discrepancy between what you know is the correct data area and what the TIN thinks it is. This tool is used to improve the definition of the TIN's data area.
The most successful use of this tool is based on knowledge of the data used to create the TIN. Lidar data is typically collected with a known average point spacing. Photogrammetric surveys often require mass points to be collected at a minimal distance. Use a Maximum Edge Length value that is longer than this spacing.
This tool modifies the input TIN. If you are unhappy with the classification results, you can use the tool repeatedly to find the best Maximum Edge Length value to use. If you're concerned about altering the input TIN, copy it first.