Comments (5)
If there is no return and therefore return 0, 0 of 0 seems more logical to me.
Anyhow there is little to no chance we can provide this info, as it is dropped at a certain stage of the processing chain.
Thinking of mobile mapping datasets with 360° scanners a decent amount of pulses can't be assigned a return, when there is no overhead structure, thus resulting in a lot of null returns in the las file.
from las.
Agreed. I'm inclined to discourage this usage, but at the very least we can formalize its meaning. Not sure how those points would get digitized with a position anyway (0 XYZ?) since nothing ever got measured. It'd just be a timestamp record, basically.
from las.
We could allow a return number of zero to generally mean the xyz position that the pulse was fired from. Whenever we have a 0th return for a laser shot it means that we have stored one xyz sample of the optical origin's trajectory. It could possibly be stored at a lower frequency (i.e. not for every shot). When we have a 0th return for a shot that has produced zero returns (number of returns of given pulse = 0) then it would basically indicate a shot that did not produce returns. But when we have the 0th return of a shot producing 1, 2, 3, ... 15 returns then it would store a sample of the optical origin's trajectory.
from las.
I think i get the idea of what Martin is saying and i see the value, especially for mobile and terrestrial scan data, although i am a bit concerned this might blow up file size. If we include this in the specs, we should be careful with the term 'trajectory'. I think we don't want people to mix this up with trajectory related to IMU measurements. Also the magic word for me is 'allow'. One might use a 0 return this way, but it should NOT be mandatory to include 0 returns in a LAS file.
from las.
Certainly not mandatory!!! And if only stored at the resolution of the GPS track it won't blow up the file much. But if we add meaning to a return number of 0 and a number of pulses of 0 to mark missing pulses (with no obvious xyz on the ground) we may at least write it such that its xyz has a useful meaning and such that a return number of 0 (with the same meaning) can also be used when the number of pulses is not 0. I would probably recommend to keep these zero returns in a separate (trajectory + missing return file) because the LASzip compressor won't like frequent jumps up to the aircraft and back down.
from las.
Related Issues (20)
- Investigate ReadTheDocs to replace ad-hoc Sphinx implementation
- Standard System Identifiers downloads contain incorrect informat HOT 1
- List of all recognized LAS classes HOT 2
- Reserve LASF_Projection:4224 for 1.5 WKT2 HOT 2
- Detailed change documents for LAS 1.1 & 1.2 HOT 2
- Fully deprecate PDRFs 0-5 for LAS 1.5 HOT 10
- Epoch description support for 1.5 HOT 3
- Update spec links to permalinks
- Editorial updates for LAS 1.5
- Using LASZIP and requirement to set “Reserved” value to Zero(0) HOT 7
- Ambiguity of Creation Day of Year and Creation Year HOT 8
- Various Kinds of Point Clouds HOT 3
- Update OGC LAS community standard
- Add an optional MIME types VLR that describes other VLRs in the file HOT 1
- Clarify specification of X, Y, and Z Scale Factors HOT 4
- Clarification about backwards compatibility HOT 1
- Add External Standards VLR HOT 1
- Clarification about File Creation Day of Year HOT 2
- General question about undocumented extra bytes HOT 6
- Is it valid to have several extrabytes VLR? HOT 4
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from las.