Number of waveforms in las 1.3 FWF file

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gen...@ualberta.ca

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Apr 5, 2019, 2:43:07 PM4/5/19
to PulseWaves - no pulse left behind
Hi there,

I have a question relevant to the number of waveforms in las 1.3 FWF file. From  the Byte offset to waveform data in the data record, it seems every point data has a corresponding  waveform packet. However the truth is that some point data relating to same waveform packet. So,does anyone know how to determine the total number of wavefoms in the .WDP file and how to index the data points from same waveform?


Regards,
Genping

Martin Isenburg

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Apr 5, 2019, 3:13:03 PM4/5/19
to PulseWaves - no pulse left behind
Hello,

when the waveform descriptor index is zero then the return has no waveform and the offset value can be disregarded. when the waveform descriptor index is non-zero then the return is pointing to *some* waveform that can in theory be located anywhere in the file and whose size and sampling is described by the referenced waveform descriptor and whose location is described by the offset. In theory (unlikely to happen except for LAS files with just 1 - 5 returns) there could be just one waveform in the WDP file that is shared by all returns. More likely is that for each shot there is one complete waveforms and all returns point to this same waveform. But each waveform can also be replicated once for every return. Either in its entirety or only the 32 digitization samples before and after it. So there is not one definite answer. 

For asking a question about how to use the LAS format properly we actually now have an official forum where you can go on record with the ASPRS LAS Working group itself:

https://github.com/ASPRSorg/LAS/ 

And because the FWF extension in LAS is so "tricky" to get right, there is a whole section devoted to clarifying it:

https://github.com/ASPRSorg/LAS/wiki/Waveform-Data-Packet-Descriptors-Explained 

Regards,

Martin  

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