point density calculation from raw las files

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Luka Šolar

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Mar 7, 2015, 2:59:17 AM3/7/15
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Dear all

Can someone help me around how i can calculate point density based on 10x10m grid for each flightline straight from surveying. My aim is to find immideately information if i have enough point density in each flightline in 90 percent certain point density

Thanks, Luka

Martin Isenburg

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Mar 7, 2015, 4:29:02 AM3/7/15
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Hello Luka,

I assume you mean the pulse density (just counting one return per shot) that is usually computed as the last-return point density? There is an (undocumented ... shame on my head) option to directly compute the density per cell in lasgrid:

lasgrid -i mapua\strips_raw\strip000001.laz ^
           -last_only ^
           -step 10 -point_density_32bit ^
           -odix _dns -obil

You can quickly get a histogram by using the ASC (or better BIL) rasters as input for lasinfo and by creating a histogram over the z value (which stores the area-normalized point density).

lasinfo -i mapua\strips_raw\strips_000001.asc ^
           -histo z 0.5 ^
           -nh -nv -nmm
lasinfo for mapua\strips_raw\strips_000001.asc
WARNING: there is coordinate resolution fluff (x10) in XY
WARNING: there is serious coordinate resolution fluff (x100) in XY
z coordinate histogram with bin size 0.5
  bin [0,0.5) has 430
  bin [0.5,1) has 425
  bin [1,1.5) has 1670
  bin [1.5,2) has 29560
  bin [2,2.5) has 82947
  bin [2.5,3) has 5049
  bin [3,3.5) has 1677
  bin [3.5,4) has 1003
  bin [4,4.5) has 649
  bin [4.5,5) has 537
  bin [5,5.5) has 491
  bin [5.5,6) has 557
  bin [6,6.5) has 527
  bin [6.5,7) has 284
  bin [7,7.5) has 91
  bin [7.5,8) has 37
  bin [8,8.5) has 6
  bin [8.5,9) has 3
  bin [9,9.5) has 2
  bin [9.5,10) has 2
  average z coordinate 2.21584

And of course you can run this in parallel over a whole folder of strilps:

lasgrid -i mapua\strips_raw\strip*.laz ^
            -last_only ^
            -step 10 -point_density_32bit ^
            -odix _dns -oasc ^
            -cores 4

and visualize it in QGIS (see attached picture) and notice how the densities increase towards the edges of the scan because an oscillating mirror was used. More on LiDAR scan patterns here:


Regards,

Martin @rapidlasso

lasgrid_point_density.png

Luka Šolar

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Mar 10, 2015, 4:14:18 PM3/10/15
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Hello Martin

What i need from point density calculation is ti get procentage of required point density in each stripe, not average...i must see if i have enough point density in each stripe (if i have more than 5 pulses/m2 by 90%)

Regards, Luka


Dne sobota, 07. marec 2015 10.29.02 UTC+1 je oseba Martin Isenburg napisala:

David Herries

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Mar 10, 2015, 9:40:57 PM3/10/15
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Luka

 

This is what Martin did by generating the histogram.   (I enjoyed martin exposing this feature in lasgrid as have used this already  “–point_density_32bit”)

 

Taking his example using a 10x10 grid: 90% of the swath has a pulse density of 2 or greater.

 

 

Cheers

 

 

David Herries     Interpine Group Ltd

Mobile:      021 43 5623   DDI:  +64 7 350 3209 or Australia 0280113645 ext 721

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