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  lasreturn:

  Reports geometric return statistics for multi-return pulses and
  repairs the 'number of returns' field based on GPS times. Note 
  that input files need (currently) to be sorted based on their
  GPS time stamp.

  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:

>> lasreturn -i lidar.laz -repair_number_of_returns -odix _repaired -olaz

the 'number of returns' field of every point is set to the highest
return number that is found for each set of returns with the same
unique GPS time stamp. assumes sorted input (use lassort -gps_time).

>> lasreturn -i lidar.laz -histo return_distance 0.1

computes the distances between all subsequent returns from the same
pulse and prints a histogram with bin size 0.1 meter. assumes sorted
input (use lassort -gps_time).

>> lasreturn -i lidar.laz -histo return_distance 0.1 0.0 4.99

same as before but limits the histogram to the range rfrom 0 to 5

>> lasreturn -i in.laz -check_return_numbering

checks the file in.laz for whether all returns of each multi-return
pulse are in the file and prints a histogram of missing or duplicate
returns. assumes sorted input (use lassort -gps_time).

> lasreturn -i strips\*.laz -check_return_numbering -classify_as 8 -odix _marked -olaz
> lasreturn -i strips\*.laz -check_return_numbering -flag_as_synthetic -odix _flagged -olaz
> lasreturn -i strips\*.laz -check_return_numbering -flag_as_withheld -odir strips_flagged -olaz

same as above but also produces an output file where all remaining
returns (those that are from pulses that led to a multi-return but
whose returns are not all listed) are either marked by a requested
classification code or flagged as synthetic or withheld.

for more info:

E:\LAStools\bin>lasreturn -h
Filter points based on their coordinates.
  -keep_tile 631000 4834000 1000 (ll_x ll_y size)
  -keep_circle 630250.00 4834750.00 100 (x y radius)
  -keep_xy 630000 4834000 631000 4836000 (min_x min_y max_x max_y)
  -drop_xy 630000 4834000 631000 4836000 (min_x min_y max_x max_y)
  -keep_x 631500.50 631501.00 (min_x max_x)
  -drop_x 631500.50 631501.00 (min_x max_x)
  -drop_x_below 630000.50 (min_x)
  -drop_x_above 630500.50 (max_x)
  -keep_y 4834500.25 4834550.25 (min_y max_y)
  -drop_y 4834500.25 4834550.25 (min_y max_y)
  -drop_y_below 4834500.25 (min_y)
  -drop_y_above 4836000.75 (max_y)
  -keep_z 11.125 130.725 (min_z max_z)
  -drop_z 11.125 130.725 (min_z max_z)
  -drop_z_below 11.125 (min_z)
  -drop_z_above 130.725 (max_z)
  -keep_xyz 620000 4830000 100 621000 4831000 200 (min_x min_y min_z max_x max_y max_z)
  -drop_xyz 620000 4830000 100 621000 4831000 200 (min_x min_y min_z max_x max_y max_z)
Filter points based on their return number.
  -keep_first -first_only -drop_first
  -keep_last -last_only -drop_last
  -keep_first_of_many -keep_last_of_many
  -drop_first_of_many -drop_last_of_many
  -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
  -keep_scan_direction_change
  -keep_edge_of_flight_line
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 classifications or flags.
  -keep_class 1 3 7
  -drop_class 4 2
  -keep_extended_class 43
  -drop_extended_class 129 135
  -drop_synthetic -keep_synthetic
  -drop_keypoint -keep_keypoint
  -drop_withheld -keep_withheld
  -drop_overlap -keep_overlap
Filter points based on their user data.
  -keep_user_data 1
  -drop_user_data 255
  -keep_user_data_below 50
  -keep_user_data_above 150
  -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_abs_scan_angle_below 1
  -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 RGB/CIR/NIR channels.
  -keep_RGB_red 1 1
  -keep_RGB_green 30 100
  -keep_RGB_blue 0 0
  -keep_RGB_nir 64 127
  -keep_NDVI 0.2 0.7 -keep_NDVI_from_CIR -0.1 0.5
  -keep_NDVI_intensity_is_NIR 0.4 0.8 -keep_NDVI_green_is_NIR -0.2 0.2
Filter points based on their wavepacket.
  -keep_wavepacket 0
  -drop_wavepacket 3
Filter points based on extra attributes.
  -keep_attribute_above 0 5.0
  -drop_attribute_below 1 1.5
Filter points with simple thinning.
  -keep_every_nth 2
  -keep_random_fraction 0.1
  -thin_with_grid 1.0
  -thin_with_time 0.001
Boolean combination of filters.
  -filter_and
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
  -switch_x_y -switch_x_z -switch_y_z
  -clamp_z_below 70.5
  -clamp_z 70.5 72.5
  -copy_attribute_into_z 0
Transform raw xyz integers.
  -translate_raw_z 20
  -translate_raw_xyz 1 1 0
  -translate_raw_xy_at_random 2 2
  -clamp_raw_z 500 800
Transform intensity.
  -set_intensity 0
  -scale_intensity 2.5
  -translate_intensity 50
  -translate_then_scale_intensity 0.5 3.1
  -clamp_intensity 0 255
  -clamp_intensity_above 255
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
  -set_extended_return_number 10
  -change_return_number_from_to 2 1
  -set_number_of_returns 2
  -set_number_of_returns 15
  -change_number_of_returns_from_to 0 2
Modify the classification.
  -set_classification 2
  -set_extended_classification 0
  -change_classification_from_to 2 4
  -classify_z_below_as -5.0 7
  -classify_z_above_as 70.0 7
  -classify_z_between_as 2.0 5.0 4
  -classify_intensity_above_as 200 9
  -classify_intensity_below_as 30 11
  -change_extended_classification_from_to 6 46
  -move_ancient_to_extended_classification
Change the flags.
  -set_withheld_flag 0
  -set_synthetic_flag 1
  -set_keypoint_flag 0
  -set_extended_overlap_flag 1
Modify the extended scanner channel.
  -set_extended_scanner_channel 2
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
  -copy_user_data_into_point_source
  -bin_Z_into_point_source 200
  -bin_abs_scan_angle_into_point_source 2
Transform gps_time.
  -set_gps_time 113556962.005715
  -translate_gps_time 40.50
  -adjusted_to_week
  -week_to_adjusted 1671
Transform RGB/NIR colors.
  -set_RGB 255 0 127
  -set_RGB_of_class 9 0 0 255
  -scale_RGB 2 4 2
  -scale_RGB_down (by 256)
  -scale_RGB_up (by 256)
  -switch_R_G -switch_R_B -switch_B_G
  -copy_R_into_NIR -copy_G_into_NIR -copy_B_into_NIR
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
Fast AOI Queries for LAS/LAZ with spatial indexing LAX files
  -inside min_x min_y max_x max_y
  -inside_tile ll_x ll_y size
  -inside_circle center_x center_y radius
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)
usage:
lasreturn -i in.laz -repair_number_of_returns -o out.laz
lasreturn -i in.laz -check_return_numbering
lasreturn -i strips\*.laz -check_return_numbering -classify_as 8 -odix _marked -olaz
lasreturn -i strips\*.laz -check_return_numbering -flag_as_synthetic -odix _flagged -olaz
lasreturn -i strips\*.laz -check_return_numbering -flag_as_withheld -odir strips_flagged -olaz
lasreturn -i bad_files\*.laz -repair_number_of_returns -odir good_files -olaz
lasreturn -i in.laz -histo return_distance 0.05
lasreturn -i in.laz -histo return_distance 0.05 0.0 2.99
lasreturn -i in.laz -histo angle_bigger_error 0.1
lasreturn -h

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