Problem interpreting GPS Time

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d weasson

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Oct 18, 2017, 12:38:43 PM10/18/17
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OK, I'm an amateur here, and my searching for an answer has been unsuccessful.  I'm hoping someone can help me interpret the lasinfo's GPS time.

A sample record is at the bottom of this post, and here is how I interpreted it so far:

version major minor 1.2 means LAS specification version 1.2.
according to those standards 
global_encoding 0 means GPS Week Time
file creation day/year 174/2010 means June 23, 2010, which was a Wednesday.
gps_time 334368.065945 336746.871043 means...

I found this conversion for GPS Week time to time:

GPS Time of Week to Day of Week with Time of Day

The value given for GPS Time of Week represents the number of seconds into the week. Therefore, to determine the day and time from that value, calculations are performed to break down the number of seconds into day, hour, minute, and second values.

( https://www.novatel.com/support/knowledge-and-learning/published-papers-and-documents/unit-conversions/ )

Example:  511200                               511200           334368.065945       336746.871043
Day 511200 / 86400 seconds per day              5.916667 days    3.870000763 days    3.89753323 days
Hour 0.916667 x 86400 / 3600 seconds per hour  22.000000 hours  20.880018320 hours  21.54079751 hours
Minute 0.000 x 3600 / 60 seconds per minute     0.000000 mins   52.801099080 mins   32.44785072 mins
Second 0.000 x 60 seconds per minute            0.000000 secs   48.065945000 secs   26.87104300 secs
                                               Th 10:00:00pm    Tu 8:52:48pm        Tu 9:32:27pm


So, right off the bat there is a Tuesday (Day 3) vs Wednesday (Day 174/2010) issue.
So... maybe the GPS time is the actual seconds within a day? 

                                                                334368.065945       336746.871043
GPS time / 86400 seconds per day                                 3.870000763 hours   3.89753323 hours
Minutes remainder * 24 hours                                    20.880018320 mins   21.54079751 mins
Seconds remainder * 60 seconds                                  52.801099080 secs   32.44785072 secs
                                             Wed June 23, 2010     3:20:53am          3:21:32am


So, are either of those computations correct?  And if not, what is the correct computation?

At a more basic level, is the time stamp the time of the FLIGHT, or the time of the PROCESSING of the file.  Basically, I need the former, not the latter!

Any help would be appreciated.  Thanks, Darlene

Here is the output from the lasinfo:

lasinfo (170828) report for D:\...\a1.las
reporting all LAS header entries:
  file signature:             'LASF'
  file source ID:             0
  global_encoding:            0
  project ID GUID data 1-4:   2364A7A9-A882-4FFD-4193-A44E48D7280E
  version major.minor:        1.2
  system identifier:          'NIIRS10'
  generating software:        'GeoCue GeoCoder'
  file creation day/year:     174/2010
  header size:                227
  offset to point data:       830
  number var. length records: 4
  point data format:          1
  point data record length:   28
  number of point records:    7238701
  number of points by return: 5681927 1378521 173893 4360 0
  scale factor x y z:         0.01 0.01 0.01
  offset x y z:               0 0 0
  min x y z:                  2420000.00 1737500.00 213.04
  max x y z:                  2424999.99 1742499.99 1704.09
variable length header record 1 of 4:
  reserved             43707
  user ID              'LASF_Projection'
  record ID            34735
  length after header  192
  description          'GeoTiff Projection Keys'
    GeoKeyDirectoryTag version 1.1.0 number of keys 23
      key 1024 tiff_tag_location 0 count 1 value_offset 1 - GTModelTypeGeoKey: ModelTypeProjected
      key 2048 tiff_tag_location 0 count 1 value_offset 4269 - GeographicTypeGeoKey: GCS_NAD83
      key 2049 tiff_tag_location 34737 count 24 value_offset 76 - GeogCitationGeoKey: GCS_North_American_1983
      key 2050 tiff_tag_location 0 count 1 value_offset 6269 - GeogGeodeticDatumGeoKey: Datum_North_American_Datum_1983
      key 2051 tiff_tag_location 0 count 1 value_offset 8901 - GeogPrimeMeridianGeoKey: PM_Greenwich
      key 2054 tiff_tag_location 0 count 1 value_offset 9102 - GeogAngularUnitsGeoKey: Angular_Degree
      key 2055 tiff_tag_location 34736 count 1 value_offset 9 - GeogAngularUnitSizeGeoKey: 0.01745329252
      key 2056 tiff_tag_location 0 count 1 value_offset 7019 - GeogEllipsoidGeoKey: Ellipse_GRS_1980
      key 2057 tiff_tag_location 34736 count 1 value_offset 6 - GeogSemiMajorAxisGeoKey: 6378137
      key 2059 tiff_tag_location 34736 count 1 value_offset 7 - GeogInvFlatteningGeoKey: 298.2572221
      key 2061 tiff_tag_location 34736 count 1 value_offset 8 - GeogPrimeMeridianLongGeoKey: 0
      key 3072 tiff_tag_location 0 count 1 value_offset 26995 - ProjectedCSTypeGeoKey: NAD83 / Mississippi West
      key 3073 tiff_tag_location 34737 count 52 value_offset 0 - PCSCitationGeoKey: NAD_1983_StatePlane_Mississippi_West_FIPS_2302_Feet
      key 3075 tiff_tag_location 0 count 1 value_offset 1 - ProjCoordTransGeoKey: CT_TransverseMercator
      key 3076 tiff_tag_location 0 count 1 value_offset 9003 - ProjLinearUnitsGeoKey: Linear_Foot_US_Survey
      key 3077 tiff_tag_location 34736 count 1 value_offset 5 - ProjLinearUnitSizeGeoKey: 0.3048006096
      key 3081 tiff_tag_location 34736 count 1 value_offset 4 - ProjNatOriginLatGeoKey: 29.5
      key 3082 tiff_tag_location 34736 count 1 value_offset 0 - ProjFalseEastingGeoKey: 2296583.333
      key 3083 tiff_tag_location 34736 count 1 value_offset 1 - ProjFalseNorthingGeoKey: 0
      key 3088 tiff_tag_location 34736 count 1 value_offset 2 - ProjCenterLongGeoKey: -90.33333333
      key 3092 tiff_tag_location 34736 count 1 value_offset 3 - ProjScaleAtNatOriginGeoKey: 0.99995
      key 4097 tiff_tag_location 34737 count 24 value_offset 52 - VerticalCitationGeoKey: NAVD88 - Geoid03 (Feet)
      key 4099 tiff_tag_location 0 count 1 value_offset 9003 - VerticalUnitsGeoKey: Linear_Foot_US_Survey
variable length header record 2 of 4:
  reserved             43707
  user ID              'LASF_Projection'
  record ID            34736
  length after header  80
  description          'GeoTiff double parameters'
    GeoDoubleParamsTag (number of doubles 10)
      2.29658e+006 0 -90.3333 0.99995 29.5 0.304801 6.37814e+006 298.257 0 0.0174533
variable length header record 3 of 4:
  reserved             43707
  user ID              'LASF_Projection'
  record ID            34737
  length after header  101
  description          'GeoTiff ASCII parameters'
    GeoAsciiParamsTag (number of characters 101)
      NAD_1983_StatePlane_Mississippi_West_FIPS_2302_Feet|NAVD88 - Geoid03 (Feet)|GCS_North_American_1983|
variable length header record 4 of 4:
  reserved             43707
  user ID              'NIIRS10'
  record ID            4
  length after header  10
  description          'NIIRS10 Timestamp'
the header is followed by 4 user-defined bytes
reporting minimum and maximum for all LAS point record entries ...
  X           242000000  242499999
  Y           173750000  174249999
  Z               21304     170409
  intensity           0        255
  return_number       1          4
  number_of_returns   1          4
  edge_of_flight_line 0          1
  scan_direction_flag 0          1
  classification      1         12
  scan_angle_rank   -28         28
  user_data         170        205
  point_source_ID 13013      13015
  gps_time 334368.065945 336746.871043
number of first returns:        5681927
number of intermediate returns: 182131
number of last returns:         5674804
number of single returns:       4300161
overview over number of returns of given pulse: 4300161 2405698 513380 19462 0 0 0
histogram of classification of points:
          167863  unclassified (1)
         1747599  ground (2)
          194347  medium vegetation (4)
          826544  high vegetation (5)
             236  noise (7)
            2382  water (9)
         4299730  overlap (12)

Martin Isenburg

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Oct 19, 2017, 11:16:23 AM10/19/17
to The LAS room - a friendly place to discuss the LAS and LAZ formats
Hello,

elaborate answers were posted in the LAStools user forum:

https://groups.google.com/d/topic/lastools/x0fgpTfvOD8/discussion

Regards from India,

Martin @rapidlasso
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