LP360's Adaptive TIN Ground Filter is designed to classify a ground surface from point data. There are multiple parameters within the filter, especially when using the Advanced Options. The following chart and additional graphics are a summary and general guidelines for setting the parameters.
Seed Sample Distance | Grid size for initial seed surface. | Smaller is more aggressive, higher fidelity, but more false ground points in initial surface. |
Advanced Option: Angular Threshold (degrees) | Maximum value at which a point will be added to the ground surface. | Higher is more aggressive; typical is 4 – 10°. |
Advanced Option: Maximum Distance | Maximum height above surface at which a point will be added to the ground surface. | Higher is more aggressive; typical is 0.5 – 1.5 m. |
Advanced Option: Direction/Edge Control | Turn on/off the algorithm for adding points along the edges of the triangles | You generally want this unchecked. Check this option when you do not want objects such as railroad cross-ties and street curbs included in the Destination class. |
Iteration Control | Hard stop; no more points are added to surface after this point The process also stops automatically when no new points are added to the surface. | Higher is more aggressive; typical is 4-10. Classify Only Seed Points to get a feel for the initial surface being generated from the chosen Seed Sample Distance. |
Seed Sample Distance
The filter starts by creating a seed point surface based off a grid defined by the seed sample distance. This distance should be at least as big as the largest structure in the dataset.
Angular Threshold
The maximum allowed angle in degrees between a candidate point and each of the triangle facet nodes. a0, a1, a2 are angles that are calculated between the candidate point (c), a triangle node (n0, n1, n2), and the closest 3d point between c and the triangle facet (c0). The segment c to c0 is the vertical leg (opposite side) of the right triangle, where n0 to c0 is the adjacent side, and n0 to c is the hypotenuse. Higher angles will allow points that are further away from a tin facet or closer to the triangle nodes to be inserted into the surface. Higher angles generally produce more slope in the resulting surface. Higher angular values may need to be used in rolling terrain as opposed to flatter terrains.
Maximum Distance
The max allowed distance between a candidate point (c) and the closest 3D point between c and the triangle facet. Higher values will insert points that are further from the tin facet into the surface. Larger distances generally produce more slope in the resulting surface. Larger distance values may need to be used in rolling terrain as opposed to flatter terrains.
Iteration Control
Each iteration of the Adaptive TIN Ground filter adds new data to the surface. The first iteration can cause the surface to be above or below the "actual" surface. Each iteration adds more points to the TIN causing the surface to flow more smoothly.
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