Highest LiDAR point return per sq m?

Question

I currently have at my disposal QGIS and Trimble Business Center.

I am looking for semi-automated solution for extracting the single highest lidar point return per square meter. My area of interest is roughly 290 000 sq m.

I have LAS and a SHP which I have created for the grid. The output needed is an ASCII file containing x, y and z of each point extracted.

For QGIS I have read that SAGA might have a similar capability. u/firefly-orange once wrote:

"You can use saga - raster values to points. Then use 'join attributes by location' to join the points to the polygon layer. You can then select the max value from the attribute table of the joined points/polygon layer using 'statistics by categories'"

is this the right track?

For TBC, there is tool called "Create High/Low Point". However there is little customization with this command and their macro community doesn't appear to have any further information.

Does anyone have any input on this? Has anyone produced a similar product?

Answer 1

The GRASS GIS module r.in.lidar (see manual page) creates a raster from a LiDAR point cloud using one of a list of univariate statistics. Among them is max. So you can create a raster map in GRASS where each pixel value will be the maximum LiDAR value in that cell. When calling the module in GRASS, the resolution= option sets the output raster resolution, so setting this to 1 would result in a raster of 1 meter resolution (providing your original LiDAR data are projected in a meter based CRS).

Then using r.out.xyz you can export the GRASS raster to an ascii text file of x,y,z as needed.

So:

r.in.lidar -e input=<your_lidar.las> output=lidar_max method=max resolution=1
r.out.xyz input=lidar_max output=lidar_max.txt

The -e flag tells r.in.lidar to set the computational region to the extent of the lidar point could.

Answer 2

LASTools has a macro called "lasgrid" which does exactly this.

https://rapidlasso.com/lastools/