decode the GPS time of a lidar cloud

[Alessandro Montaghi]

Dear All, 

I have a las file format of laser points. When i convert in text file using las2txt in order to obtain x, y, z, and GPS(t) time i have the following results (just the first 3 lines)

X, Z, GPS_time

285304.40 4615498.07 57.38 525502.384000

285304.88 4615498.82 69.23 525502.384000

285305.11 4615498.78 61.77 525502.384000

Is it 525502.384000 value with sense? there is a way do understand the data when the laser pulses was shhoted?

Thanks in advance

Alessandro

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Alessandro Montaghi

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[Kirk Waters - NOAA Federal]

Alessandro,

Your data is almost certainly stored with the time in GPS seconds of the week. In theory you could get back to a real clock time, but only if you know which week the data was collected. Unfortunately, a lot of data has been stored that way. It was the only LAS spec available until LAS 1.2 and you still see a lot of data as seconds of the week since that spec. I typically assume any times that are between zero and 7 * 86400 (7 days times number of seconds in a day) are not in modified GPS time, but are in seconds of the week. There is a week that could be either way.

Regards,

Kirk

--

Kirk Waters, PhD
Coastal Remote Sensing Program Manager
NOAA Coastal Services Center
843-740-1227

www.csc.noaa.gov/digitalcoast

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[Konstantin Lisitsyn]

Hello Alessandro,

that value is "Seconds of Week" - number of seconds since midnight between Saturday and Sunday.

Here is a part of Excel worksheet to calculate DD:HH:MM:SS. The logic should be quite clear.

residual	divider	value	floor of value	field
525502.4	86400	6.082204	6	<=day
7102.384	1440	4.932211	4	hour
1342.384	60	22.37307	22	minute
22.384	1	22.384	22	second

With formulas:

residual	divider	value	floor of value	field
525502.384	=60*60*24	=A2/B2	=FLOOR(C2,1)	<=day
=A2-B2*D2	=60*24	=A3/B3	=FLOOR(C3,1)	hour
=A3-B3*D3	60	=A4/B4	=FLOOR(C4,1)	minute
=A4-B4*D4	1	=A5/B5	=FLOOR(C5,1)	second

Note that it only gives you day and time of the week. The week has to be identified somehow else.

--

Konstantin

[Ruben Valbuena]

Thanks all. Still another question:

Is that time recorded in local time or following a certain zone (GMT?). Seasonal changes apply?

[Konstantin Lisitsyn]

Hello Ruben,

Usually LAS's timestamps are referred to GPS time. Check out this site showing the relation of GPS time frame to UTC and others:

http://leapsecond.com/java/gpsclock.htm

--

Konstantin

[martin...@live.ca]

Hello,

I think that the formula isn't correct for hour. The divisor should be "=60*60" (seconds per hour)... not "=60*24".

The right answer is 1:58:22.

Best regards,

Martin Riopel

Université Laval