Select: the 'select' argument specifies the data that will actually be loaded. For example,
'xyzia' means that the x, y, and z coordinates, the intensity and the scan angle will be loaded.
The supported entries are t - gpstime, a - scan angle, i - intensity, n - number of returns,
r - return number, c - classification, s - synthetic flag, k - keypoint flag, w - withheld flag,
o - overlap flag (format 6+), u - user data, p - point source ID, e - edge of flight line flag,
d - direction of scan flag, R - red channel of RGB color, G - green channel of RGB color,
B - blue channel of RGB color, N - near-infrared channel. C - scanner channel (format 6+).
Also numbers from 1 to 9 for the extra bytes data numbers 1 to 9. 0 enables all extra bytes to be
loaded and '*' is the wildcard that enables everything to be loaded from the LAS file.
Note that x, y, z are implicit and always loaded. 'xyzia' is equivalent to 'ia'.
Filter: the 'filter' argument allows filtering of the point cloud while reading files.
This is much more efficient than lasfilter in many ways. If the desired filters are known
before reading the file, the internal filters should always be preferred. The available filters are
those from LASlib
and can be found by running the following command: rlas:::lasfilterusage().
(see also rlas::read.las)