Lasground problem from a photogrammetrically-derived point cloud

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Jung-kuan Liu

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Mar 14, 2018, 3:19:21 PM3/14/18
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Hello,

I have a photogrammetrically-derived point cloud and was saved as a LAZ file. I tried to classify it as category of ground- and nonground- points by using Lasground. However, it came out there is a block area cannot be processed as shown in the attached image. Looks like this block area came from different flight strip (or recorded as different point_source_id). Since the original LAZ file is huge, I attached a clipped patch here. Can any one tell me how to resolve it? Thanks!

The argument of Lasground I applied is,

>lasground -i test2.laz -o test2_cls.laz -step 7.5



--- 
Ernie
test2.laz

Jean-François Bourdon

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Mar 14, 2018, 4:20:25 PM3/14/18
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lasground correctly classify your ground points but didn't include any points from PointSourceID 32949 because those seem to be shifted a bit more than 1 meter above those from PointSourceID 28236. The software used to produce the point cloud seems to have failed to match together pixels from the the two datasets (photos taken on different days with completely different lighting conditions ?).

You may try to find the altitude shift over a road or any stable flat surface and then use las2las with -translate_xyz 0 0 -1 for exemple to bring down the point cloud (supposing that the shift is constant which I can't tell)

Martin Isenburg

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Mar 15, 2018, 12:33:15 PM3/15/18
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Hello,

the vertical shift between "flightlines" that Jean-François found is visualized and "roughly measured" with lasview in the attached image. 

lasview -i test2.laz -set_min_max 1503 1510 -color_by_flightline

So are those two point clouds with different point source IDs from two different surveys? I was under the impression the concept of a "swath" or a "flightline" does not really exist in photogrammetry. Was the photogrammetry solution run on different overlapping "batches of images" but the resulting surfaces were not aligned with each other (or to global control points) in the end?

The LAS file uses pretty "nasty" offsets in the header. You can fix that with 

las2las -i in.laz -auto_reoffset -o out.laz

And you should also set the global encoding to 1 as the points have "Adjusted Standard GPS time". That is also a bit unusual for  photogrammetry. Where do the time stamps come from? The moment the pictures where taken? The granularity of times is rather high as you can see at the end. Are you sure that this is photogrammetry?

:\LAStools\bin>lasinfo -i test2.laz -histo point_source 1 -histo gps_time 0.25
asinfo (180303) report for test2.laz
eporting all LAS header entries:
 file signature:             'LASF'
 file source ID:             0
 global_encoding:            0
 project ID GUID data 1-4:   00000000-0000-0000-0000-000000000000
 version major.minor:        1.2
 system identifier:          'libLAS'
 generating software:        'libLAS 1.8.0'
 file creation day/year:     70/2018
 header size:                227
 offset to point data:       321
 number var. length records: 1
 point data format:          3
 point data record length:   34
 number of point records:    510019
 number of points by return: 0 0 0 0 0
 scale factor x y z:         0.01 0.01 0.01
 offset x y z:               514731.6698339844 4637916.7800097652 1499.8799989509582
 min x y z:                  514731.67 4637916.78 1499.88
 max x y z:                  515447.77 4638464.11 1522.03
ARNING: full resolution of max_x not compatible with x_offset and x_scale_factor: 515447.76980957034
ARNING: full resolution of max_y not compatible with y_offset and y_scale_factor: 4638464.1100878902
ariable length header record 1 of 1:
 reserved             43707
 user ID              'LASF_Projection'
 record ID            34735
 length after header  40
 description          'by LAStools of rapidlasso GmbH'
   GeoKeyDirectoryTag version 1.1.0 number of keys 4
     key 1024 tiff_tag_location 0 count 1 value_offset 1 - GTModelTypeGeoKey: ModelTypeProjected
     key 3072 tiff_tag_location 0 count 1 value_offset 6341 - ProjectedCSTypeGeoKey: NAD83(2011) / UTM zone 12N
     key 3076 tiff_tag_location 0 count 1 value_offset 9001 - ProjLinearUnitsGeoKey: Linear_Meter
     key 4099 tiff_tag_location 0 count 1 value_offset 9001 - VerticalUnitsGeoKey: Linear_Meter
ASzip compression (version 2.2r0 c2 50000): POINT10 2 GPSTIME11 2 RGB12 2
eporting minimum and maximum for all LAS point record entries ...
 X                   0      71610
 Y                   0      54733
 Z                   0       2215
 intensity        1219      17941
 return_number       0          0
 number_of_returns   0          0
 edge_of_flight_line 0          0
 scan_direction_flag 1          1
 classification      2          2
 scan_angle_rank   -15         -5
 user_data           0          0
 point_source_ID 28236      32949
 gps_time 117911164.104240 118850621.604240
ARNING: range violates GPS week time specified by global encoding bit 0
 Color R 0 1536
       G 0 2816
       B 0 2560
umber of first returns:        510019
umber of intermediate returns: 0
umber of last returns:         510019
umber of single returns:       510019
ARNING: there are 510019 points with return number 0
ARNING: there are 510019 points with a number of returns of given pulse of 0
istogram of classification of points:
         510019  ground (2)
oint source id histogram with bin size 1
 bin 28236 has 337869
 bin 32949 has 172150
 average point source id 29826.809263968596 for 510019 element(s)
ps_time histogram with bin size 0.25
 bin [117911164,117911164.25) has 11256
 bin [117911164.25,117911164.5) has 19235
 bin [117911164.5,117911164.75) has 18727
 bin [117911164.75,117911165) has 19292
 bin [117911165,117911165.25) has 18604
 bin [117911165.25,117911165.5) has 19284
 bin [117911165.5,117911165.75) has 19255
 bin [117911165.75,117911166) has 18564
 bin [117911166,117911166.25) has 19254
 bin [117911166.25,117911166.5) has 19189
 bin [117911166.5,117911166.75) has 18835
 bin [117911166.75,117911167) has 18946
 bin [117911167,117911167.25) has 19236
 bin [117911167.25,117911167.5) has 18614
 bin [117911167.5,117911167.75) has 19256
 bin [117911167.75,117911168) has 19276
 bin [117911168,117911168.25) has 18567
 bin [117911168.25,117911168.5) has 19195
 bin [117911168.5,117911168.75) has 3284
 bin [118850619.25,118850619.5) has 14502
 bin [118850619.5,118850619.75) has 18929
 bin [118850619.75,118850620) has 18740
 bin [118850620,118850620.25) has 18548
 bin [118850620.25,118850620.5) has 18930
 bin [118850620.5,118850620.75) has 18922
 bin [118850620.75,118850621) has 18930
 bin [118850621,118850621.25) has 18299
 bin [118850621.25,118850621.5) has 18924
 bin [118850621.5,118850621.75) has 7426
 average gps_time 118228266.33177079 for 510019 element(s)


E:\LAStools\bin>las2txt -i test2.laz -parse xyzt -stdout | more
515447.16 4638463.64 1517.58 117911164.104240
515447.16 4638462.56 1517.57 117911164.111381
515447.18 4638461.49 1517.47 117911164.120994
515447.18 4638460.41 1517.48 117911164.129844
515447.21 4638459.33 1517.37 117911164.139518
515447.24 4638458.26 1517.23 117911164.149101
515447.25 4638457.18 1517.18 117911164.158286
515447.23 4638456.10 1517.29 117911164.167167
515447.22 4638455.03 1517.32 117911164.175346
515447.24 4638453.95 1517.26 117911164.183647
515447.23 4638452.87 1517.29 117911164.192039
515447.22 4638451.80 1517.33 117911164.200279
515447.24 4638450.72 1517.25 117911164.211021
515447.24 4638449.65 1517.23 117911164.219749
515447.25 4638448.57 1517.22 117911164.228050
515447.27 4638447.49 1517.11 117911164.236991
515447.28 4638446.42 1517.10 117911164.245506
515447.27 4638445.34 1517.13 117911164.254630
515447.28 4638444.27 1517.09 117911164.261802
515447.28 4638443.19 1517.07 117911164.271537
515447.30 4638442.11 1516.99 117911164.280998
515447.31 4638441.04 1516.94 117911164.289634
515447.32 4638439.96 1516.90 117911164.298759
515447.34 4638438.88 1516.81 117911164.306877
515447.35 4638437.81 1516.77 117911164.316001
515447.35 4638436.73 1516.81 117911164.324668
515447.34 4638435.65 1516.84 117911164.333427
515447.34 4638434.58 1516.82 117911164.342307
515447.34 4638433.50 1516.83 117911164.350669
515447.37 4638432.43 1516.72 117911164.360557
515447.39 4638431.35 1516.61 117911164.369132
515447.39 4638430.28 1516.62 117911164.377708
515447.39 4638429.20 1516.65 117911164.386039
515447.38 4638428.12 1516.68 117911164.394126
515447.38 4638427.05 1516.69 117911164.402915
515447.39 4638425.97 1516.65 117911164.411430
515447.40 4638424.89 1516.59 117911164.421867
515447.41 4638423.82 1516.55 117911164.429771
515447.41 4638422.74 1516.55 117911164.438957
515447.43 4638421.66 1516.46 117911164.448295
515447.47 4638420.59 1516.30 117911164.456108
515447.47 4638419.51 1516.29 117911164.464500
515447.47 4638418.43 1516.29 117911164.472129
515447.46 4638417.36 1516.33 117911164.481925
515447.47 4638416.28 1516.31 117911164.490806
515447.47 4638415.20 1516.32 117911164.499595
515447.47 4638414.13 1516.33 117911164.509178
515447.46 4638413.05 1516.36 117911164.517570
515447.46 4638411.97 1516.39 117911164.526420
515447.45 4638410.90 1516.40 117911164.535331
515447.46 4638409.82 1516.39 117911164.543785
515447.48 4638408.75 1516.30 117911164.552390
515447.48 4638407.67 1516.31 117911164.560691
515447.48 4638406.59 1516.30 117911164.570030
515447.50 4638405.52 1516.21 117911164.578819
515447.51 4638404.44 1516.17 117911164.587577
515447.51 4638403.36 1516.18 117911164.597404
515447.52 4638402.29 1516.12 117911164.605979
515447.53 4638401.21 1516.08 117911164.614616
515447.54 4638400.14 1516.04 117911164.623008
515447.54 4638399.06 1516.04 117911164.632347
[...]



lasview_visualize_missalignments.jpg
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