Device Reference

Filter Device - PPA

Available in WM Pro Beta 5 (2007-07-15)

PPA stands for "Profile-recognition and Polygon-breaking Algorithm". It is an algorithm developed by Dr. Yet-Chung Chang for automatic extraction of ridge and valley axes from digital terrain elevation data.

Device Parameters
Terrain type (TT) Ridge | Valley. When "Ridge" is selected, the "Edge Threshold" is set to 0.0; it is automatically set to 1.0 when "Valley" is selected.
Edge Threshold (ET) The height threshold of axis edges. For example, when TT = "Ridge", setting ET = 0.3 removes any ridge axis with a height less than 30% of the maximum height of the current height field.
Height Difference (HD) The height difference threshold of neighboring sites. When the height difference between two neighboring sites is less than HD, they are considered by PPA as "equal" in height. The default is 0.05.
Downsampling Rate (DR) The downsampling rate for the resulting axis graph. The default is 5.0.
End Point Reduction (EPR) How many end segments to remove. For example, setting EPR = 3 will remove 3 segments from each end of all axes. The The default is 3.
Short Branch Threshold (SBT) The segment length threshold of a branch, below which the branch is considered as "short" and is removed from the final axis graph. For example, setting SBT = 7 removes all branch with length less than 7 segments from the result. The default is 7.
Small Group Threshold (SGT) The segment length threshold of a group of isolated branches, below which the group is considered as "small" and is  removed from the final axis graph. The larger the SGT is, the less isolated islands of axes remain in the result. The default is 10.
Half Profile Length (HPL) The number of profile points on either side. The larger the HPL is, the more small height variations are lost. The default is 3.