I know this reads like a simple problem with a simple solution but I just can't wrap my head around it.
The documentation for laspy is a little sparse but I've managed well so far. I think that the issue here is now simply not being familiar enough with numpy.
I want to sort a numpy array based on the GPS Time.
Here is where I stand:
I am using the sample.las that comes with laspy for testing.
import laspy
import numpy as np
#open the file
lasFile = laspy.file.File("C:/Anaconda3/Lib/site-packages/laspytest/data/simple.las", mode = "rw")
#put points in numpy array
lasPoints = lasFile.points
What I am attempting to do is sort the array by the gps_time column.
print(lasPoints.dtype)
gives me
[('point', [('X', '<i4'), ('Y', '<i4'), ('Z', '<i4'), ('intensity', '<u2'), ('flag_byte', 'u1'), ('raw_classification', 'u1'), ('scan_angle_rank', 'i1'), ('user_data', 'u1'), ('pt_src_id', '<u2'), ('gps_time', '<f8'), ('red', '<u2'), ('green', '<u2'), ('blue', '<u2')])]
and
print(lasPoints)
gives me
[ ((63701224, 84902831, 43166, 143, 73, 1, -9, 132, 7326, 245380.78254963, 68, 77, 88),)
((63689633, 84908770, 44639, 18, 81, 1, -11, 128, 7326, 245381.45279924, 54, 66, 68),)
((63678474, 84910666, 42671, 118, 9, 1, -10, 122, 7326, 245382.13595007, 112, 97, 114),)
...,
((63750167, 85337575, 41752, 43, 9, 1, 11, 124, 7334, 249772.21013494, 100, 96, 120),)
((63743327, 85323084, 42408, 31, 9, 1, 11, 125, 7334, 249772.70733372, 176, 138, 164),)
((63734285, 85324032, 42392, 116, 73, 1, 9, 124, 7334, 249773.20172407, 138, 107, 136),)]
To access the gps_time I can run
lasPoints[0][0][9] ## first gps_time in array
lasPoints[1][0][9] ## second gps_time in array
Replacing "gps_time" for 9 gives the same result.
Now when I come to trying to sort my data it doesn't actually seem to sort the anything:
np.sort(lasPoints["point"]["gps_time"])
print(lasPoints)
The array is printed out unsorted and as is,
lasPoints=np.sort(lasPoints["point"]["gps_time"])
print(lasPoints)
Results in the gps_time being sorted as such:
[ 245370.41706456 245370.74331403 245371.06452222 ..., 249782.07498673
249782.64531958 249783.16215837]
Where am I going wrong here?
np.sort does not seem to support inplace sorting as far as i understand the documentation. np.ndarray.sort however does. So
np.sort(lasPoints["point"]["gps_time"])
print(lasPoints)
will always be unsorted.
But to your problem: You could slice the list of GPS-times out of your list and use argsort to get the indices for a sorted list. These can then be used to sort your laspoints. e.g.:
sorted_ind = np.argsort(list_of_gpstimes)
laspoints = laspoints[sorted_ind]