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  lasground:

  This is a tool for bare-earth extraction: it classifies LIDAR
  points into ground points (class = 2) and non-ground points
  (class = 1). This tools works very well in natural environments
  such as mountains, forests, fields, hills, or other terrain
  with few man-made objects.

  The tool also produces excellent results for town or cities
  but buildings larger than the step size can be problematic.
  The default step size is 5 meters, which is good for forest
  or mountains. For towns or flat terrains '-town' the step
  size is increased to 10 meters. For cities or warehouses
  '-city' the step size is increased to 25 meters. For very
  large cities use '-metro' and the step size is increased
  to 50 meters You can also set it directly with '-step 35'.

  It is 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
  projection information then there is no need to specify this
  explicitly. If the input coordinates are in an earth-centered
  or a longlat representation, the file needs converted to, for
  example, a UTM projection first. That said, some folks have
  successfully ground-classified longlat represtations using a
  very small '-step 0.000005' or so.

  By default the tool only considers the last return. Earlier
  returns are considered non-ground. You can turn this off by
  requesting '-all_returns'. If you want to leave out certain
  classifications from the bare-earth calculation you can do
  so with '-ignore_class 6'.

  For very steep hills you can intensify the search for initial
  ground points with '-fine' or '-extra_fine' and similarly for
  flat terrains you can simplify the search with '-coarse' or
  '-extra_coarse' but try the default setting first. 

  The experienced user can fine-tune the algorithm by specifing
  the threshold in meters at which spikes get removed. setting
  '-spike 0.5' will remove up-spikes above 50 centimeter and
  down-spikes below 5 meters in the coarsest TIN.

  The maximal standard deviation for planar patches in centimeter
  can be set with '-stddev 10' and the maximal offset in meters up
  to which points above the current ground estimate get included
  can be set with '-offset 0.1'.

  Finally you can ask lasground to compute the height above the
  ground for each point (so you can use lasclassify next without
  needing to run lasheight first) with '-compute_height' or even
  ask to have the computed height replace the elevation value with
  option '-replace_z'. Then you directly get a height normalized
  LAS/LAZ file that can be used, for example, with lascanopy or
  lasgrid for canopy height model (CHM) computations.

  Please license from martin@rapidlasso.com to use lasground
  commercially. Please note that the unlicensed version will set
  intensity, gps_time, user data, and point source ID to zero,
  slightly change the LAS point order, and randomly add a tiny
  bit of white noise to the points coordinates.

  For updates check the website or join the LAStools mailing list.

  http://lastools.org/
  http://groups.google.com/group/lastools/
  http://twitter.com/lastools/
  http://facebook.com/lastools/
  http://linkedin.com/groups?gid=4408378

  Martin @lastools

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example usage:

>> lasground -i terrain.las -o classified_terrain.las

classifies a terrain with the default settings.

>> lasground -i terrain.las -o classified_terrain.las -feet -elevation_feet

classifies a terrain where both horizontal and vertical units are
in feet instead of in meters (which is assumed by default unless
there is projection information in the LAS file saying otherwise).

>> lasground -i terrain.las -o classified_terrain.las -all_returns

classifies a terrain considering all points - not just the last
returns (as is the default behavior).

>> lasground -i *.las

classifies all LAS files with the default settings.

>> lasground -i *.las -town

the same as above but uses wider spacing to allow for small
buildings and other man-made structures.

>> lasground -i *.las -city

the same as above but uses even wider spacing to allow for
very large buildings.

for more info:

C:\lastools\bin>lasground -h

Please license from 'martin.isenburg@gmail.com' to use LAStools commercially.
Filter points based on their coordinates.
  -clip_tile 631000 4834000 1000 (ll_x ll_y size)
  -clip_circle 630250.00 4834750.00 100 (x y radius)
  -clip_box 620000 4830000 100 621000 4831000 200 (min_x min_y min_z max_x max_y max_z)
  -clip 630000 4834000 631000 4836000 (min_x min_y max_x max_y)
  -clip_x_below 630000.50 (min_x)
  -clip_y_below 4834500.25 (min_y)
  -clip_x_above 630500.50 (max_x)
  -clip_y_above 4836000.75 (max_y)
  -clip_z 11.125 130.725 (min_z max_z)
  -clip_z_below 11.125 (min_z)
  -clip_z_above 130.725 (max_z)
  -clip_z_between 11.125 130.725 (min_z max_z)
Filter points based on their return number.
  -first_only -keep_first -drop_first
  -last_only -keep_last -drop_last
  -keep_middle -drop_middle
  -keep_return 1 2 3
  -drop_return 3 4
  -keep_single -drop_single
  -keep_double -drop_double
  -keep_triple -drop_triple
  -keep_quadruple -drop_quadruple
  -keep_quintuple -drop_quintuple
Filter points based on the scanline flags.
  -drop_scan_direction 0
  -scan_direction_change_only
  -edge_of_flight_line_only
Filter points based on their intensity.
  -keep_intensity 20 380
  -drop_intensity_below 20
  -drop_intensity_above 380
  -drop_intensity_between 4000 5000
Filter points based on their classification.
  -keep_class 1 3 7
  -drop_class 4 2
  -drop_synthetic -keep_synthetic
  -drop_keypoint -keep_keypoint
  -drop_withheld -keep_withheld
Filter points based on their user data.
  -keep_user_data 1
  -drop_user_data 255
  -keep_user_data_between 10 20
  -drop_user_data_below 1
  -drop_user_data_above 100
  -drop_user_data_between 10 40
Filter points based on their point source ID.
  -keep_point_source 3
  -keep_point_source_between 2 6
  -drop_point_source 27
  -drop_point_source_below 6
  -drop_point_source_above 15
  -drop_point_source_between 17 21
Filter points based on their scan angle.
  -keep_scan_angle -15 15
  -drop_abs_scan_angle_above 15
  -drop_scan_angle_below -15
  -drop_scan_angle_above 15
  -drop_scan_angle_between -25 -23
Filter points based on their gps time.
  -keep_gps_time 11.125 130.725
  -drop_gps_time_below 11.125
  -drop_gps_time_above 130.725
  -drop_gps_time_between 22.0 48.0
Filter points based on their wavepacket.
  -keep_wavepacket 1 2
  -drop_wavepacket 0
Filter points with simple thinning.
  -keep_every_nth 2
  -keep_random_fraction 0.1
  -thin_with_grid 1.0
Transform coordinates.
  -translate_x -2.5
  -scale_z 0.3048
  -rotate_xy 15.0 620000 4100000 (angle + origin)
  -translate_xyz 0.5 0.5 0
  -translate_then_scale_y -0.5 1.001
  -clamp_z_below 70.5
  -clamp_z 70.5 72.5
Transform raw xyz integers.
  -translate_raw_z 20
  -translate_raw_xyz 1 1 0
  -clamp_raw_z 500 800
Transform intensity.
  -scale_intensity 2.5
  -translate_intensity 50
  -translate_then_scale_intensity 0.5 3.1
Transform scan_angle.
  -scale_scan_angle 1.944445
  -translate_scan_angle -5
  -translate_then_scale_scan_angle -0.5 2.1
Change the return number or return count of points.
  -repair_zero_returns
  -set_return_number 1
  -change_return_number_from_to 2 1
  -set_number_of_returns 2
  -change_number_of_returns_from_to 0 2
Modify the classification.
  -set_classification 2
  -change_classification_from_to 2 4
Modify the user data.
  -set_user_data 0
  -change_user_data_from_to 23 26
Modify the point source ID.
  -set_point_source 500
  -change_point_source_from_to 1023 1024
Transform gps_time.
  -translate_gps_time 40.50
  -adjusted_to_week
  -week_to_adjusted 1671
Transform RGB colors.
  -scale_rgb_down (by 256)
  -scale_rgb_up (by 256)
Supported LAS Inputs
  -i lidar.las
  -i lidar.laz
  -i lidar1.las lidar2.las lidar3.las -merged
  -i *.las - merged
  -i flight0??.laz flight1??.laz
  -i terrasolid.bin
  -i esri.shp
  -i nasa.qi
  -i lidar.txt -iparse xyzti -iskip 2 (on-the-fly from ASCII)
  -i lidar.txt -iparse xyzi -itranslate_intensity 1024
  -lof file_list.txt
  -stdin (pipe from stdin)
  -rescale 0.01 0.01 0.001
  -rescale_xy 0.01 0.01
  -rescale_z 0.01
  -reoffset 600000 4000000 0
Supported LAS Outputs
  -o lidar.las
  -o lidar.laz
  -o xyzta.txt -oparse xyzta (on-the-fly to ASCII)
  -o terrasolid.bin
  -o nasa.qi
  -odir C:\data\ground (specify output directory)
  -odix _classified (specify file name appendix)
  -ocut 2 (cut the last two characters from name)
  -olas -olaz -otxt -obin -oqfit (specify format)
  -stdout (pipe to stdout)
  -nil    (pipe to NULL)
LAStools (by martin@rapidlasso.com) version 130120 (commercial)
usage:
lasground -i in.las -o out.las
lasground -i in.las -o out.las -foot -elevation_foot
lasground -i in.las -o out.las -town
lasground -i in.las -o out.las -city
lasground -i in.las -o out.las -metro
lasground -i in.las -o out.las -verbose -step 10 -spike 2 -offset 0.1
lasground -i *.las -verbose -odix _g -olaz
lasground -i *.laz -verbose -odir ground_classified\ -olaz
lasground -h

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if you find bugs let me (martin.isenburg@rapidlasso.com) know.