# filters.pmf¶

The Progressive Morphological Filter (PMF) is a method of segmenting ground and non-ground returns. This filter is an implementation of the method described in [Zhang2003].

## Example¶

{
"pipeline":[
"input.las",
{
"type":"filters.pmf"
},
{
"type":"writers.las",
"filename":"output.las"
}
]
}


## Notes¶

• slope controls the height threshold at each iteration. A slope of 1.0 represents a 1:1 or 45º.
• initial_distance is _intended_ to be set to account for z noise, so for a flat surface if you have an uncertainty of around 15 cm, you set initial_distance large enough to not exclude these points from the ground.
• For a given iteration, the height threshold is determined by multiplying slope by cell_size by the difference in window size between the current and last iteration, plus the initial_distance. This height threshold is constant across all cells and is maxed out at the max_distance value. If the difference in elevation between a point and its “opened” value (from the morphological operator) exceeds the height threshold, it is treated as non-ground. So, bigger slope leads to bigger height thresholds, and these grow with each iteration (not to exceed the max). With flat terrain, keep this low, the thresholds are small, and stuff is more aggressively dumped into non-ground class. In rugged terrain, open things up a little, but then you can start missing buildings, veg, etc.
• Very large max_window_size values will result in a lot of potentially extra iteration. This parameter can have a strongly negative impact on computation performance.

Note

[Zhang2003] describes the consequences and relationships of the parameters in more detail and is the canonnical resource on the topic.

## Options¶

max_window_size
Maximum window size. [Default: 33]
slope
Slope. [Default: 1.0]
max_distance
Maximum distance. [Default: 2.5]
initial_distance
Initial distance. [Default: 0.15]
cell_size
Cell Size. [Default: 1]
classify
Apply classification labels? [Default: true]
extract
Extract ground returns? [Default: false]
approximate
Use approximate algorithm? [Default:: false]