Hi,
Is it possible to have different point densities within a point cloud file of a large area? If so, which parameters play role in it?
Also, does it matter if the point cloud is created from image photogrammetry or airborne laser scanning (ALS)?
Thanks,
Mohammad
for TLS (Terrestrial Laser Scanning) or other scanning techniques with a laser like ALS the point density is depending on:
- angel of incidence
- distance sensor to object
- angular scanning increment of scanner
But not only the density, also the quality of points, which is influenced by the aforementioned and additional radiometric and material proberties is of vital importance. See further information here:
https://www.researchgate.net/publication/313249939_An_intensity-based_stochastic_model_for_terrestrial_laser_scanners
Concerning point clouds from images, i think you mean something like the "structure from motion" principle (SFM), the point desnity higly depends on the automatic feature detection and dense matching procedure aftrewards, in regions with good contrast and varying image information (edges etc.) you will have more dense points than lets say on a white wall.
So for both techniques you will have differnt point density
And if you want to use point clouds generated by a laser scanner and from imaging photogrammetry in common, you should be careful to have the same scale, often point clouds from SFM do have an arbitrary scale!
Regards,
Mathias
Article An intensity-based stochastic model for terrestrial laser scanners
Speaking about ALS:
About exactly point cloud, it has different point densities:
- firstly, depending on the flight trajectory, scanner type & scanning parameters, relief and land cover.
- secondly, point density value depends on measurement system, because it is statistic value, it can be calculated differently.
If question is related with data saving procedure. For example, LAS 1.4v has attribute "Project ID". Therefore it is possible divide point cloud into many files and get different point densities as sum of point clouds from different files.
In ALS the point cloud density will differ because your typical flight trajectory is at a near constant elevation whilst the terrain elevation below changes. The point density is a function of distance from measurement system to terrain feature and angle from vertical line to feature position. The closer the terrain is to the measurement system the higher the point density and vice versa. The further away the feature point is from the vertical line the greater the distance interval between successive positions assuming constant angular deviation of your measurement system.
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