Terrain Modeling
The Grids | Calculate | Calculus terrain modeling option provides additional tools to analyze the surface geometry of a grid file. Results are based on the gradient direction (direction of steepest slope) and not a predefined direction as with Directional Derivatives. For each grid node location on a surface, Surfer determines the magnitude and direction of the steepest slope.
There are five terrain modeling operations:
Threshold
When you choose the Terrain Aspect, Profile Curvature, Plan Curvature, or Tangential Curvature options, you can also specify a Threshold value. In areas where the steepest slope approaches zero (where the surface is nearly horizontal), the gradient direction is somewhat difficult to ascertain (i.e., the definitions "downhill" and "uphill" are rather arbitrary). In these cases, it is preferable to classify the surface as flat. The Threshold value is the minimum value for the slope for which the aspect and curvature values are computed. In locations where the Threshold value is not met, the grid node value in the output grid is set to NoData. The Threshold value is, by default, set to a very small value.