When you generate a digital elevation model (DEM) from vector, you must determine how the values between the vectors and the raster are calculated.
This method is only available when you are generating a DEM from points. The Natural Neighbors algorithm constructs circles using three points based on the Delaunay triangle. OrthoEngine calculates the pixel values by using all the points to form the least number of the largest possible circles. Inside each circle the interpolated values are influenced by the three points forming the Delaunay triangle and all the circles that overlap that circle.
Natural Neighbor Interpolation is recommended for files that contain sparsely distributed or unevenly distributed points since it performs a more intelligent calculation than Finite Difference. However, it is not recommended for files that contain a large number of points since the calculations required will take considerable more time to process. Since it does not extrapolate beyond the bounds of the points, it may also leave areas of the interpolated DEM empty.
This method performs the interpolation in three steps. In the first step, the vector elevation values are encoded into the corresponding pixels in the raster DEM. In the second step, the elevations for the remaining pixels are interpolated using the Distance Transform algorithm, which estimates the values from pixels equidistant from the pixels encoded in the first step. In the third step, the Finite Difference algorithm iteratively smooths the raster DEM. During the iterations, the pixels that were encoded in the first step are not changed, while the interpolated pixel values are updated based on the neighborhood values.
Two parameters determine the completion of the process, No. of Iterations and Tolerance. The No. of Iterations parameter specifies the maximum number of times smoothing is applied to the raster DEM. The default is 10. Tolerance restricts the number of times smoothing is applied according to how it changes the elevation values of the pixels. The default is 1.
For example, you set the No. of Iterations to 10 and the Tolerance to 1 meter. Smoothing can be applied up to 10 times, but smoothing ceases as soon as the smoothing causes a change of less than 1 meter in the elevation values. If the maximum change on the third iteration is only 0.3 meters, then only three iterations of the smoothing are applied.
Finite Difference is recommended for files that contain evenly distributed points. Since it is a simpler technique, it processes the raster DEM much faster, and it is better suited to large input data sets.
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