Question about "-slope" output of las2dem

[Ranjith Gopalakrishnan]

Hi all,

I am trying to get slope rasters for my las files (they are quite small; square plots of 120 meters on the side). I did something like this:

C:> las2dem -slope -meter -elevation_meter -step 20 -i lasfile.las -o slope_xyz.xyz

Note that lasfile.las had been ground classified and cleaned up.

Now, in slope_xyz.xyz, in "z" values, I get values like 77, 88, 86, 81, etc. Are these the slope of the normal to the terrain, in degrees? That is, the slope of the ground would be (90 - slope_normal), right? That is, the corresponding terrain slopes would be 13, 2, 4, 9 etc. This is not clear in the documentation.

Thanks in advance for your responses!

Ranjith.

[Martin Isenburg]

Hello Ranjith,

easiest - as always - a little experiment with the provided sample data. First let's view heavily thinned ground-points from 'fusa.laz' with LASview and triangulate them. Then we know what to expect:

D:\LAStools\bin>lasview -i ..\data\fusa.laz ^

                                       -keep_class 2 ^

                                       -thin_with_grid 5

A picture after triangulation <t> is attached. Pretty flat! Now we create a very coarse slope raster in ASC format and look at its values. Should all be clear now ... close to zero means flat. Close to 90 means steep.

D:\LAStools\bin>las2dem -i ..\data\fusa.laz ^

                                         -keep_class 2 ^

                                         -thin_with_grid 5 ^

                                         -step 10 ^

                                         -slope ^

                                         -o slope.asc

D:\LAStools\bin>more slope.asc

ncols 25

nrows 25

xllcorner 277750.000000

yllcorner 6122250.000000

cellsize 10.000000

NODATA_value -9999

-9999 5 3 4 8 12 14 1 3 11 8 6 7 6 15 15 13 9 8 8 7 5 3 3 -9999

15 2 2 2 1 4 9 4 4 3 3 2 4 2 3 3 4 3 4 5 3 1 3 2 5

10 1 1 1 2 1 5 3 3 2 2 2 2 1 3 5 2 2 3 6 8 2 3 2 6

9 1 2 2 2 2 2 2 2 2 2 2 1 2 3 3 2 1 1 1 6 3 2 2 6

1 1 1 2 2 2 2 2 2 1 2 1 2 3 2 3 2 2 4 4 4 2 2 4 8

7 2 1 2 2 2 2 2 2 2 2 1 4 1 2 2 2 2 3 3 3 2 2 2 19

1 2 1 1 2 2 2 1 2 2 2 2 3 2 2 2 2 3 2 2 2 1 2 3 13

5 1 2 3 2 1 2 2 3 2 1 2 3 2 2 2 2 3 2 2 1 1 2 2 6

7 2 2 3 2 1 1 2 3 2 1 3 2 1 2 2 3 3 2 2 2 1 2 2 28

7 2 2 1 1 1 1 2 2 1 2 2 2 1 2 2 2 2 2 2 2 2 2 2 3

4 2 1 1 1 2 1 2 2 2 2 2 3 2 2 2 2 2 2 2 2 2 2 2 3

9 1 6 1 1 1 3 2 2 2 2 3 3 2 2 2 2 2 2 3 2 2 2 2 3

7 1 1 1 1 1 3 3 2 2 1 2 2 2 2 2 2 2 2 2 2 2 3 1 2

10 2 3 4 2 3 3 3 2 1 2 2 2 2 2 2 2 2 2 2 2 3 2 2 3

9 4 4 2 2 2 2 1 1 1 1 3 4 2 2 2 2 2 2 2 2 2 2 1 5

10 4 2 2 2 2 1 1 0 0 1 4 4 1 2 2 2 2 2 2 2 2 2 0 3

3 2 2 2 1 1 1 1 0 1 1 5 4 2 2 1 2 2 1 2 2 3 2 1 3

3 3 2 1 1 1 1 1 0 0 1 6 2 1 2 2 2 2 1 2 2 2 1 0 1

3 2 2 1 1 1 1 1 1 0 1 6 3 1 2 2 2 2 2 2 3 1 1 1 2

1 2 1 2 1 1 1 1 0 0 1 7 1 1 2 2 2 2 2 2 2 2 1 0 1

9 2 2 1 2 1 1 1 1 1 2 4 1 1 2 2 2 2 2 2 2 2 1 0 3

13 3 2 2 1 1 1 1 1 1 2 4 2 2 2 1 2 2 2 2 2 2 0 1 5

5 3 2 2 2 1 1 1 2 2 2 3 3 2 1 2 2 2 2 2 2 2 0 2 8

3 3 1 1 2 2 1 2 2 2 2 3 2 2 1 2 2 2 2 2 3 1 1 4 4

-9999 3 1 2 2 2 2 2 2 2 2 5 1 2 2 2 2 2 2 3 2 1 3 2 -9999

Cheers,

Martin @rapidlasso

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[Ranjith Gopalakrishnan]

Hi Martin,

Thanks! It all looks ok now. It seems that " -keep_class 2" is crucial when one is trying to get the DEM, right? If so, I think it is better to point this out in the documentation. I had assumed that as the tool is called "las2dem", it would look only at ground points.

Ranjith.

[Martin Isenburg]

Hello Ranjith,

I would agree with you if the tool would be called las2dtm. But it is called las2dem because it can generate any kind of DEM ... not just a DTM. Here is how i call them:

DEM: any of the elevation models below
DTM: bare-earth terrain (ground points only)
DSM: highest surface during survey (first or highest returns)
CHM: normalized  height of highest surface above terrain

There are many variations on that:

archaeological DTMs: include man-made objects but no vegetation

lowest- or last-return DSMs: includes buildings and thick vegetation but few wires or thin poles

wireless DSMs: important for solar radiation studies where wires in the standard DSM produce shadowing walls
pit-free CHMs: provide less spiky canopy structure for better single tree detection

etc ...

las2dem - if used with different parameters - can produce all of the DEMs listed above ... not just standard DTMs.

Regards,

Martin

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