Re: [LAStools] Step Size Basis

[Ty Kennedy-Bowdoin]

Aaron,

In this context, step size would be the output resolution (pixel size) of your raster data (DEM).  LAStools will interpolate a value for every output pixel based on the grid that you designate using the step parameter.

Usually, you would want a step size close to the spot spacing of your lidar data to give you the best ability to resolve small objects. So, if the lidar flight plan was to collect data at 1 m spot spacing, you might use something like 1 m step size. Some applications might require a smoother surface, in which case you might want to make your step size larger than the spot spacing like maybe 5 m.

Cheers,

Ty

 

On Mon, Apr 1, 2013 at 7:50 AM, <aaron.dougl...@gmail.com> wrote:

Hi All,

I'm a beginner user of LASTOOLS, and I was wondering if someone could please explain what step size does.  For example in this command:

Las2dem –I CLIP_37.las –o CLIP_37_dsm.tif –v –step 1 –nad83 –utm 17T –meter –elevation_meter –kill 200

I see from the README file that it causes the raster to have a stepsize of the specified value, but I am unclear what this means.  When would it be beneficial to have a larger/smaller step size, and why?

Thanks in advance for your help!!

Aaron

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Ty Kennedy-Bowdoin
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[Terje Mathisen]

aaron.dougl...@gmail.com wrote:
> Hi All,
>
> I'm a beginner user of LASTOOLS, and I was wondering if someone could
> please explain what step size does. For example in this command:
>

> Las2dem �I CLIP_37.las �o CLIP_37_dsm.tif �v�step 1 �nad83 �utm 17T
> �meter �elevation_meter �kill 200

>
> I see from the README file that it causes the raster to have a stepsize
> of the specified value, but I am unclear what this means. When would it
> be beneficial to have a larger/smaller step size, and why?

The step size is the resolution of the final grid, in your case measured
in -meter (which is also the canonical value).

You would use something like -step 2 or -step 5 if you don't need to
know the elevation for every single square meter, instead getting a
lower resolution grid.

Terje
--
- <Terje.M...@tmsw.no>
"almost all programming can be viewed as an exercise in caching"

[How Yien Chang]

Hi,

What about if i use step size of 0.05 m ? and i don't know the spot spacing of my lidar data, how can i know that?

Regards,

HY

[Martin Isenburg]

Hello,

Use lasinfo to compute the average last return density and last return spacing as follows

E:\LAStools\bin>lasinfo -i ..\data\fusa.las -nh -nv -nmm -nr -cd

lasinfo (171011) report for ..\data\fusa.las

covered area in square units/kilounits: 62492/0.06

point density: all returns 4.44 last only 4.21 (per square units)

      spacing: all returns 0.47 last only 0.49 (in units)

By counting only the last return from each laser pulse fired we essentially count each pulse that was fired only once (in contrast to the all return density that is more of a measure of "how much fluffy stuff / vegetation / structures / wires / building edges / etc" is there on the terrain. In this case each pulse generated an average of 4.44 / 4.21 = 1.05 returns. An empty soccer field generates an average of 1.0 return per pulse. A tropical forest or a power station may generate up to average of 5 returns per pulse.

The image resolution for generating a DTM does not have to be much finer than the pulse spacing (i.e. the last return spacing) as the best we can hope for is that each pulse hits the ground and that therefore for the "fusa.laz" file that the ground was sampled with returns that are on average 0.49 units apart. So for "fusa.laz" a DTM with a resolution of 0.5 meters would probably capture all the elevation information that the laser pulses could have possibly captured ... assuming the spacing of pulses hitting the ground is more or less uniform.

However, some scanning systems / flight patterns / ... produce highly non-uniform pulse distributions on the ground so that the "average spacing" may be misleading. Have a look at this blog post:

http://rapidlasso.com/2014/03/20/density-and-spacing-of-lidar/

To more exactly compute the spacing between your last return LiDAR samples run this:

E:\LAStools\bin>las2tin -i ..\data\fusa.laz ^

                                      -last_only ^

                                      -histo_only edge_length 0.1

edge length histogram histogram with bin size 0.1

  bin [0,0.1) has 663

  bin [0.1,0.2) has 3634

  bin [0.2,0.3) has 10288

  bin [0.3,0.4) has 247678

  bin [0.4,0.5) has 46179

  bin [0.5,0.6) has 108588

  bin [0.6,0.7) has 106505

  bin [0.7,0.8) has 122776

  bin [0.8,0.9) has 94903

  bin [0.9,1) has 31486

  bin [1,1.1) has 5721

  bin [1.1,1.2) has 2441

  bin [1.2,1.3) has 2755

  bin [1.3,1.4) has 2176

  bin [1.4,1.5) has 807

  bin [1.5,1.6) has 409

  bin [1.6,1.7) has 361

  bin [1.7,1.8) has 1175

  bin [1.8,1.9) has 239

[...]

So maybe a DTM pixel spacing of 0.3 units will allow you to also capture the terrain variations in open areas whenever the pulses happened to be closer to another than 0.5 units.

Regards,

Martin @rapidlasso