We will use CloudCompare, a free GNU 3D point cloud processing software, please install so we can continue with this workflow.
To begin, please go to File -> Open the LAS file you would like to clean:
Click on Apply in the next dialog:
On the next dialog, click on Yes to All (this won’t change nor modify the real coordinates of the point cloud).
Click on the pointcloud file name on the items section on the top-left:
Click on Plugins-> CSF Filter:
Depending on the characteristics of your point cloud/site, choose accordingly between "steep slope", "relief" or "flat" as well as "slope processing".
The CSF Filter algorithm will use scalar field to classify your point cloud and give you two new sets of groups, one for "ground points" (surface) and one for "off-ground points" (vertical structures, trees, bushes, poles, cars, etc).
Click to highlight the "ground points" group and then click on "Save" to export your file:
Choose between high, original or custom resolutions; we recommend you keep to the Higher Resolution:
You will now have your point cloud .las file cleaned of "vertical points" ready for format conversion or import to CAD software.