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Distance and average distance between point cloud
Posted: Mon Apr 25, 2022 8:45 am
by GabrielMulero
Good day, I am new cloud compare and LiDAR generally.
I would like to know how to get average point density, like I saw in a research paper, the author reports getting an average point density of 8 points/m2. I am currently working on my LiDAR data and would like to generate such report for presentation from my data.
Also, in the attached image from my result I a get Z-values that are negative (scalar bar to the right), I am wondering how it could possibly be that the elevation is negative.
Kindly help.
Re: Distance and average distance between point cloud
Posted: Wed Apr 27, 2022 6:29 am
by daniel
For the negative altitudes, it all depends on the coordinate system in which the points are expressed. It can be a nice geolocalized global coordinate system, in which you can expect zero to be the sea level, but it can also be a local coordinate system, centered on the sensor, in which case you cannot expect anything!
And for the point density, you will have to use the 'Tools > Others > Compute Geometric features' tool:
https://www.cloudcompare.org/doc/wiki/i ... c_features
For instance, you can choose to compute the 'Number of neighbors' and input a radius that correspond to 1 square-meter (r = 0,5641896). Or if the density is too low, you can also let CC do the maths and choose the 'Surface density' with a larger radius.
Re: Distance and average distance between point cloud
Posted: Tue May 31, 2022 9:28 am
by GabrielMulero
daniel wrote: ↑Wed Apr 27, 2022 6:29 am
For the negative altitudes, it all depends on the coordinate system in which the points are expressed. It can be a nice geolocalized global coordinate system, in which you can expect zero to be the sea level, but it can also be a local coordinate system, centered on the sensor, in which case you cannot expect anything!
And for the point density, you will have to use the 'Tools > Others > Compute Geometric features' tool:
https://www.cloudcompare.org/doc/wiki/i ... c_features
For instance, you can choose to compute the 'Number of neighbors' and input a radius that correspond to 1 square-meter (r = 0,5641896). Or if the density is too low, you can also let CC do the maths and choose the 'Surface density' with a larger radius.
Thank you Daniel.
In the end I realize that the elevation for this location is usually below sea-level, hence the negative Z-value.
The point density is well understood.
I have a further question:
Is it possible to retrieve laser return values (1st, 2nd, and last return values), and how can that be done in cloud compare?
Thank you.
Re: Distance and average distance between point cloud
Posted: Tue May 31, 2022 4:43 pm
by daniel
So it depends on the version of LAS files you have:
- with older versions, you may have the 'return number' scalar field that indicates whether it's the first, second, or more pulse. You can then split the cloud based on this information (especially with the new 'Edit > Scalar fields > Split cloud (integer values)' tool).
- if you have a FullWaveform (FWF) LAS file (= version 1.4 with an associated wdp file), then CC is able to load them (at least the Windows version, with the 'LAS 1.3 or 1.4' filter). But then you can only display the waves for now. You would need to add some code if you want to process these waves and extract new points out of it.
Re: Distance and average distance between point cloud
Posted: Mon Jun 06, 2022 12:36 pm
by GabrielMulero
Thank you Daniel.
I do not have waveform data, but I tried the to do the first suggestion you gave but just may be I am getting something wrong.
Can I share a simple .las file with you (in the zip file) and show you exactly what I am trying to do?
Can I possibly do this with what I have (the attached)?
Re: Distance and average distance between point cloud
Posted: Thu Jun 09, 2022 10:19 am
by daniel
Well, in your example file, I can see a 'ReturnNumber' scalar field. But it has only 2 values:
- return_number.JPG (77 KiB) Viewed 3481 times