General question about undocumented extra bytes

[Rainer Wiesenfarth]

Hi,

I was coming along an unclear part in the LAS specification concerning extra bytes, and think this group might be a good place to ask for clarification.

The LAS specification states that the number of extra bytes is the Point Data Record Length minus the point size implied by the point format type. As the Point Data Record Length is an unsigned short value, there - theoretically - may be close to 64k extra bytes.

If the extra bytes should be described as "undocumented" using an Extra Bytes VLR, the specification states that the EXTRA_BYTES data_type must be 0 (= "undocumented extra bytes") with the options bit field storing the number of undocumented bytes.  However, the options field is an unsigned char and thus has a maximum value of 255.

So, my question is: How to provide a correct Extra Bytes VLR for LAS files where points have more than 255 undocumented extra bytes? Just add multiple descriptors? Or did I miss something?

Cheers, Rainer

[Kirk Waters - NOAA Federal]

That's probably a good question/issue for the LAS format github at https://github.com/ASPRSorg/las

Kirk

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[Rainer Wiesenfarth]

Thanks, Kirk, for pointing me to the ASPRS GitHub page. I created an issue with the question there.

Cheers, Rainer

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[Jochen Rapidlasso]

Hi Rainer,

each extra byte definition will get an table entry in the extra byte vlr.

If you add a extra byte with data_type=0 you give the size in the options field (1..255).

This size will extend your point data record size by this value.

The total size of a point data record is limited by 64k.

The size of the extra byte vlr will be large enough to hold around 340 extra_byte entries (64k / 192byte per table entry).

You can't add more extra_bytes than this.

If your extra bytes has a larger size you hit the 64k point data record size limit maybe earlier.

Best regards,

Jochen @rapidlasso

[Rainer Wiesenfarth]

Hi Jochen,

thanks for the clarification. I was not sure about this, but your reply implies that having multiple EXTRA_BYTES entries with data_type == undocumented in an Extra Bytes VLR is perfectly normal and the preferred (if not only) way to achieve this.

Cheers, Rainer

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[Evon Silvia]

Link to the LWG discussion thread: https://github.com/ASPRSorg/LAS/issues/149

I agree with Jochen's assessment here.