filters.lof

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filters.lof#

The Local Outlier Factor (LOF) filter was introduced as a method of determining the degree to which an object is an outlier. This filter is an implementation of the method described in [Breunig et al., 2000].

The filter creates three new dimensions, NNDistance, LocalReachabilityDistance and LocalOutlierFactor, all of which are double-precision floating values. The NNDistance dimension records the Euclidean distance between a point and it’s k-th nearest neighbor (the number of k neighbors is set with the [minpts] option). The LocalReachabilityDistance is the inverse of the mean of all reachability distances for a neighborhood of points. This reachability distance is defined as the max of the Euclidean distance to a neighboring point and that neighbor’s own previously computed NNDistance. Finally, each point has a LocalOutlierFactor which is the mean of all LocalReachabilityDistance values for the neighborhood. In each case, the neighborhood is the set of k nearest neighbors.

In practice, setting the [minpts] parameter appropriately and subsequently filtering outliers based on the computed LocalOutlierFactor can be difficult. The authors present some work on establishing upper and lower bounds on LOF values, and provide some guidelines on selecting [minpts] values, which users of this filter should find instructive.

Note

To inspect the newly created, non-standard dimensions, be sure to write to an output format that can support arbitrary dimensions, such as BPF.

Note

In earlier PDAL releases (up to v2.2.0), NNDistance was stored in the KDistance Dimemsion.

Default Embedded Stage

This stage is enabled by default

Example#

The sample pipeline below computes the LOF with a neighborhood of 20 neighbors, followed by a range filter to crop out points whose LocalOutlierFactor exceeds 1.2 before writing the output.

[
    "input.las",
    {
        "type":"filters.lof",
        "minpts":20
    },
    {
        "type":"filters.range",
        "limits":"LocalOutlierFactor[:1.2]"
    },
    "output.laz"
]

Options#

minpts

The number of k nearest neighbors. [Default: 10]

where

An expression that limits points passed to a filter. Points that don’t pass the expression skip the stage but are available to subsequent stages in a pipeline. [Default: no filtering]

where_merge

A strategy for merging points skipped by a where option when running in standard mode. If true, the skipped points are added to the first point view returned by the skipped filter. If false, skipped points are placed in their own point view. If auto, skipped points are merged into the returned point view provided that only one point view is returned and it has the same point count as it did when the filter was run. [Default: auto]

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