filters.lof

Local Outlier Factor (LOF) 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 [Breunig2000].

The filter creates three new dimensions, all of which are doubles. The KDistance 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 KDistance. 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, where k is set with the minpts option.

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 filters.lof 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.

[Breunig2000]Breunig, M.M., Kriegel, H.-P., Ng, R.T., Sander, J., 2000. LOF: Identifying Density-Based Local Outliers. Proc. 2000 Acm Sigmod Int. Conf. Manag. Data 1–12.

Example

The sample pipeline below uses filters.lof to compute 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.

{
  "pipeline":[
    "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]