1.. _filters.elm: 2 3filters.elm 4=============================================================================== 5 6The Extended Local Minimum (ELM) filter marks low points as noise. This filter 7is an implementation of the method described in [Chen2012]_. 8 9ELM begins by rasterizing the input point cloud data at the given cell_ size. 10Within each cell, the lowest point is considered noise if the next lowest point 11is a given threshold above the current point. If it is marked as noise, the 12difference between the next two points is also considered, marking points as 13noise if needed, and continuing until another neighbor is found to be within the 14threshold. At this point, iteration for the current cell stops, and the next 15cell is considered. 16 17.. embed:: 18 19Example #1 20---------- 21 22The following PDAL pipeline applies the ELM filter, using a cell_ size of 20 23and 24applying the :ref:`classification <class>` code of 18 to those points 25determined to be noise. 26 27.. code-block:: json 28 29 { 30 "pipeline":[ 31 "input.las", 32 { 33 "type":"filters.elm", 34 "cell":20.0, 35 "class":18 36 }, 37 "output.las" 38 ] 39 } 40 41Example #2 42---------- 43 44This variation of the pipeline begins by assigning a value of 0 to all 45classifications, thus resetting any existing classifications. It then proceeds 46to compute ELM with a threshold_ value of 2.0, and finishes by extracting all 47returns that are not marked as noise. 48 49.. code-block:: json 50 51 [ 52 "input.las", 53 { 54 "type":"filters.assign", 55 "assignment":"Classification[:]=0" 56 }, 57 { 58 "type":"filters.elm", 59 "threshold":2.0 60 }, 61 { 62 "type":"filters.range", 63 "limits":"Classification![7:7]" 64 }, 65 "output.las" 66 ] 67 68Options 69------------------------------------------------------------------------------- 70 71_`cell` 72 Cell size. [Default: 10.0] 73 74_`class` 75 Classification value to apply to noise points. [Default: 7] 76 77_`threshold` 78 Threshold value to identify low noise points. [Default: 1.0] 79 80.. include:: filter_opts.rst 81 82