Still confusing "number of returns" and not the "return number"

[Phisan Santitamnont]

Hi Group,

l am still confusing with number of returns from lasinfo.

Testing with 'DOGAMI' sample  20090429_42122c8225_ld_p23.laz

i got 

        overview over number of returns of given pulse: 9520359 1642560 246960 8756 0 0

but using lasinfo -gui , it shows:- 

#of 1st returns: 10486547

#of 2nd returns: 861825

#of 3rd returns: 70263 

#of 4th returns: 0

Totally different! , any clue ?

Br,

Phisan

[Kirk Waters - NOAA Federal]

Phisan,

I just took a look at that file. lasinfo is actually reporting both. The first time is what it gets from the header (which matches what lasinfo -gui gave you). The second time is based on what it counted up from all points. The header isn't matching the data for some reason. That shouldn't be the case, and since that file is on our system, I'll have to figure out why.

Regards,

Kirk

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[Kirk Waters - NOAA Federal]

Phisan,

I did a few experiments and now I'm as perplexed as you are. I took another file from that dataset that appeared to have similar issues (ftp://csc.noaa.gov/pub/DigitalCoast/lidar1_z/geoid12a/data/1171/20090429_42123d2415_ld_p23.laz).

In that case, not only were the numbers different, but additional return numbers are reported. According to lasinfo, the header says:

number of points by return: 10983036 1532893 197689 0 0

while counting up the points returns:

overview over number of returns of given pulse: 9317011 2715479 648942 32186 0 0 0

Notice that there are only returns 1, 2, and 3 according to the header, but after looking through the points, lasinfo says there are also return 4 points. The total number of points is the same though. If I then try to just grab the return 4 points with:

las2las -keep_return 4 

I get: WARNING: written 0 points but expected 12713618 points

So, there really are no return 4 points. At this point, I'm not trusting the report on the return numbers after scanning the file, but I can't imagine what sort of bug would give these results.

Regards,

Kirk

[Phisan Santitamnont]

Many thanks Kirk!

Martin! please , we ,probably , found one 'real' bug in your lasinfo

Br,

เมื่อ วันพฤหัสบดีที่ 6 มีนาคม ค.ศ. 2014, 17 นาฬิกา 39 นาที 23 วินาที UTC+7, Phisan Santitamnont เขียนว่า:

[Evon Silvia]

I found the data file you mentioned and ran the LAZ through my own software. I can confirm that the return number counts are correctly encoded in the header, so it must be a lasinfo issue.

Evon

[Antonio Ruiz]

Phisan,

You get a 1st return from almost any lidar shot you throw if the target is reflective enough and it is not too far. For the 2nd and following returns you need more than one target simultaneously illuminated by the lidar footprint (of a single shot) and the separation between the targets must be large enough to be distinguished as different. The minimum distance required depends on the lidar sensor but they range between 60 cm to 3.5 m. 

The numbers you get are typical. To get some 4th returns you have to flight over a forest of high trees. 

I hope this helps

[Martin Isenburg]

Hi,

I am on travel in remote tropical islands. Will do so in a few days (11th to 15th) when i get to better internet.

Kirk wrote:
"According to lasinfo, the header says:
number of points by return: 10983036 1532893 197689 0 0
while counting up the points returns:
overview over number of returns of given pulse: 9317011 2715479 648942 32186 0 0 0

Notice that there are only returns 1, 2, and 3 according to the header, but after looking through the points, lasinfo says there are also return 4 points."

Wrong. There are points that are specified to be from a shot that resulted in 4 returns. Whether all 4 returns are stored to the file is a different issue. Maybe someone ran '-drop_return 4' on the file.

"The total number of points is the same though. If I then try to just grab the return 4 points with:

las2las -keep_return 4 

I get: WARNING: written 0 points but expected 12713618 points
So, there really are no return 4 points. At this point, I'm not trusting the report on the return numbers after scanning the file, but I can't imagine what sort of bug would give these results."

That is to be expected. From all the reports that i have read in this thread it seems to me there is a fundamental confusion about "number of returns of given pulse". Anyone remember the Terrasolid .bin files that are converted to .las ...? Look what they did to those two numbers to code the 4 options: single, first of many, last of many, intermediate into the two numbers: "return count" and "number of returns of given pulse" ...

Please ... no more wrong lasinfo bug reports while i am exploring the blue lagoons of a Philippino archipelago ...

(-;

Martin @rapidlasso

[Kirk Waters - NOAA Federal]

Martin,

That make sense. I was a bit worried that I wasn't interpreting "overview over number of returns of given pulse" correctly and apparently I wasn't. Now that I understand it, the wording seems clear, but it wasn't clear to me before, so I suspect it could use rewording. Have fun in the lagoons.

Kirk

--

[Martin Isenburg]

Hi,

Evon wrote:
"I found the data file you mentioned and ran the LAZ through my own software. I can confirm that the return number counts are correctly encoded in the header, so it must be a lasinfo issue."

This is not problem. The "return number counts" in the LAS header are a histogram of the "return number" field of all points in the file. The reported (non-)issue is with the  "number of returns of given pulse" that is not reported in the header.

These "number of returns of given pulse" numbers are not as rigorously defined as the "return number" in my experience (because they do not really have to be "correct" to encode the desired information ...

Martin

[Heidemann, Hans]

Martin, regarding your last statement,

"

These "number of returns of given pulse" numbers are not as rigorously defined as the "return number" in my experience (because they do not really have to be "correct" to encode the desired information ...

"

Perhaps I am not understanding what you mean, but ...

• I would disagree that they are not defined
  
• I would disagree that the do not have to be correct.
  

I would question if there is any real reason to histogram or summarize them in the first place.

Karl

H. Karl Heidemann, GISP

Physical Scientist, Lidar Science

U.S. Geological Survey

Mundt Federal Building

47914 252nd Street

Sioux Falls, SD  57110

605-594-2861

kheid...@usgs.gov

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