new lasnoise.exe removes or reclassifies isolated points in LAS/LAZ files

[Martin Isenburg]

Hello,

as mentioned in a few footnotes but not yet properly announced. There is a new addition to LAStools that can help you get rid of isolated noise points in your LAS / LAZ data as they occasionally happen when the laser beam hits a few cloud flurries, some birds, or a swarm of bees (-: or when a belated reflection produces a return well below the ground.

Here a typical use case and attached is the result. The points that are now pink/lilac were reclassified into classification code 7 which is the default mode. There is also an option to remove the points from the file.

lasnoise -i tiles_raw\tile_654000_1614000.laz ^
             -step 2 -isolated 5 ^

             -odix _denoised -olaz

The lasnoise tool flags or removes noise points in LAS/LAZ/BIN/ASCII files. The tool looks for isolated points according to certain criteria that can be modified via '-step 3' and '-isolated 3' as needed. The default for step is 4 and for isolated is 5.

The tool tries to find points that have only few other points in their surrounding 3 by 3 by 3 grid of cells with the cell the respective point falls into being in the center. The size of each of the 27 cells is set with the '-step 5' parameter. The maximal number of few other points in the 3 by 3 by 3 grid still designating a point as isolated is set with '-isolated 6'.

By default the noise points are given the classification code 7 (low or high noise). Using the '-remove_noise' flag will instead remove them from the output file. Alternatively with the '-classify_as 31' switch a different classification code can be selected.

It is also important to tell the tool whether the horizontal and vertical units are meters (which is assumed by default) or '-feet' or '-elevation_feet'. Should the LAS file contain proper projection VLRs then there is no need to specify this explicitly. Internally the step size in xy or in z direction will be multiplied with 3 if the coordinates are found or are specified to be in feet. The size of the grid cells needs to be changed with '-step 0.0001' or similar if the input is in geographic longlat representation.

example usage:

>> lasnoise -i lidar.las ^

                  -remove_noise ^

                  -o lidar_without_noise.laz

removes all points that have only 5 or fewer other points in their surrounding 3 by 3 by 3 grid (with the respective point in the center cell) where each cell is 4 by 4 by 4 meters in size.

>> lasnoise -i tiles_raw\*.laz ^

                  -classify_as 31 ^

                  -step 2 - isolated 2 ^

                  -odir tiles_denoised\ -olaz ^

                  -cores 3

running on whole folder of files on 3 cores where each cell being only 2 by 2 by 2 meters in size and where points that have only two or fewer other points in the 27 cell neighborhood of their surrounding 3 by 3 by 3 grid are considered noise. here points are classified to class 31 instead of being removed.

[Ilves Risto]

Hi,

The problem is the lampposts, power lines,.. where we get only few points (when
we use low point density), but we don't want to classify them as a noise.

Could you use some additional information about locations, where not to
classify noise points, if we have these features already mapped (ie. shape
files, gml, xy(z) points,...)?


Regards,

Risto




Martin Isenburg (5.8.2013 14:37):

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***************************************************************

Risto Ilves - risto...@nls.fi
http://www.maanmittauslaitos.fi/

Maanmittauslaitos National Land Survey of Finland
Ilmakuvakeskus Aerial Image Centre
Oikokatu 11 Oikokatu 11
40700 Jyväskylä FIN-40700 Jyväskylä


Puhelin 040 514 6085 Phone +358 40 514 6085

[Wes]

Hi martin, good work!  Is there a point file size ceiling, such as with lasground, or no?

Wes

[Martin Isenburg]

Risto & Wes,

in the lastest release of LAStools (130817) there is a new option for lasnoise.exe that allows you to exclude points from being classified as noise.

lasnoise -i lidar.laz -ignore_class 2 -o denoised.laz

there is also a (still undocumented) option that allows you to specify the '-step_xy' and the '-step_z' separately. Not entirely sure what the largest LAS/LAZ file is that you can process but it should be filry large. I designed lasnoise.exe to be extremely sclable for large point clouds. Let me know if you manage to break it ... (-:

Martin @rapidlasso

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