How to find flat triangles in a TIN

Flat triangles and edges are common artifacts in triangular irregular network (TIN) surfaces created from contour line. They occur where the three nodes of a triangle, or the two nodes of a triangle edge, are derived from the same elevation contour. They are artifacts of the model which do not represent the true elevation at these places.

Flat triangles and edges can be located in a number of ways in ArcGIS.

1. Display the original contour lines or the TIN soft-edges over the TIN model and look for triangles and edges in with nodes derived from contour lines with the same elevation.
2. Use the Information tool to explore how values change within a triangle or along an edge. If all values are the same it is flat.
3. Display the TIN in ArcScene and look for "flat looking" triangles. Confirm with the information tool.
4. Render the TIN to show slope. In the TIN Symbology tab Add... a Face slope with graduated color ramp renderer.  Now change to symbology levels to differentiate triangles with very low slope from those with a slope.

The red triangles are flat or very nearly flat.