Correctly reading GPS_time from LiDAR .las file using laspy (python)

Question

I am trying to read in some .las LiDAR data into python and to correctly access the GPS_time attribute.

path = r"\\xxxx\las_data.las"
las_data = laspy.read(path)

To access the gps time I use:

timestamp = las_data.gps_time

The return array is numbers like:

array([3.43864063e+08, 3.43864063e+08, 3.43864063e+08, ...,
       3.41800751e+08, 3.41800751e+08, 3.41800751e+08])

However the actual GPS_time (inspected in GIS) should be:

array([1343864063, 1343864063, 1343864063, ...,
       1341800751, 1341800751, 1341800751])

Basically it has dropped the first digit when reading the data.

I have manually added the missing order of magnitude, converting 3.43864063e+08 to 1343864063 using code such as:

tss = [int(_)  + int(1000000000) for _ in timestamps_scientific]

tss looks like:

[1343864063,
 1343864063,
 1343864063,
 1343864063,
 1343864063,
...
]

Which is correct.

However I am looking for a more efficient process which correctly reads the GPS_time from the start which will avoid possible manipulation errors.

Answer 1

You need to read the standard. That's exactly how GPS time is encoded in these files. They call it "adjusted standard GPS time", and it is "satellite GPS time" minus 1 times 10^9.

Check page 6 of https://www.asprs.org/wp-content/uploads/2010/12/LAS_1_4_r13.pdf .

So, just do the addition you're doing.