Scale factor x,y,z - las-file

[cloudmap]

Hello.

I have a las file with a big point cloud. I have used the subsample method to decimate the cloud. After export to las, the scale factors of x,y and z has changed from 0.001 to something like 0.0000004635473823. Also, the offset has changed..

Is this a bug in the export? Or is this correct? Can I thrust the point cloud for volume measurements and so on? Can I/should I change the scale factors back to 0.001??

Thanks in advance.

CM

[daniel]

Hi I tried to reply on lasgroup directly, but I'm not sure it worked ;)

Here is the message:

First, let me explain how CloudCompare works:

- CloudCompare doesn't handle units. It reads the data as they are stored in the file (whatever its format) and the user is responsible for any conversion of the coordinates that would be necessary for its own process

- for efficiency reasons values are stored internally as 'standard' floating point values (as most formats do by the way - LAS and E57 files are the only formats that use this offset/scale coding scheme).

- therefore, when CC saves a cloud (that can come from ANY file format) to LAS format, it doesn't have any particular information and it saves the cloud with a very high relative accuracy (= scaling) of 10^-9 multiplied by the cloud bounding-box dimension (10^-9 because the integer saved in LAS files is coded on 32 bits which roughly corresponds to +/-2.10^9).

This is why you can see very small values for the scale. But there's no real loss of accuracy with this way of working and it works whatever the cloud source is.

But I understand that it's more the value of the 'scale' itself that bugs you?

What would you expect exactly?

• that CloudCompare remembers the original offset/scale information when loading the cloud from a LAS file? (the issue is that if you modify the values inside CC, this information might not be adapted to the new coordinates of the cloud)
• or that CloudCompare asks the user for the right offset/scale values when saving the file? (in a kind of 'advanced mode' of course)

I hope you'll understand that CloudCompare is not a LAS-only tool and therefore those kind of 'particularities' are quite painful to manage (especially when it's not for the sake of accuracy ;).