Holes in DEM?

[Nicholas Pilkington]

I am trying to create a DTM and get most of the removed and big missing pieces in the DEM.

I'm using this point cloud: https://www.dropbox.com/s/04sxwuphdy3v1d0/holes.las?dl=0

and running:

las2las -i holes.las -epsg 4326  -meter -elevation_meter -target_utm auto -odix _utm -olaz

lasthin -i holes_utm.laz -step 1 -percentile 10 30 -classify_as 8 -odix _p10 -olaz

lasnoise -i holes_utm_p10.laz -ignore_class 0 -step_xy 5 -step_z 1 -isolated 10 -classify_as 7 -odix _denoised -olaz

lasground -i holes_utm_p10_denoised.laz -ignore_class 0 7 -city -extra_fine -odix _g -olaz

las2dem -i holes_utm_p10_denoised_g.laz -false -extra_pass -keep_class 2 -thin_with_grid 0.125 -step 0.1  -opng -o result.png

and the result is:

https://www.dropbox.com/s/h2p8nbbsdjay94t/preview_dtm_2.png?dl=0

What is happening?

[Martin Isenburg]

Hello Nick,

what you see is the effect of '-kill 50' which is the default for las2dem. This kills of any triangles with edge lengths larger than 50 prior to rasterizing the DEM. You have rather few ground points left:

lasview -i holes_utm_p10_denoised_g.laz -keep_class 2

so when I triangulate those then the large distances between the ground points do not get rendered, just like las2dem does it by increasing it to 1000 with '-concavity 1000' in lasview

lasview -i holes_utm_p10_denoised_g.laz -concavity 1000

the (near-identical) concept of concavity is called '-kill 1000' in las2dem

las2dem -i holes_utm_p10_denoised_g.laz -false -extra_pass -keep_class 2 -kill 1000 -thin_with_grid 0.125 -step 0.1  -opng -o result.png

Regards.

Martin

PS: Would you mind compressing the LAS file to a LAZ file next time? I am often in the tropics on 3G or 4G networks.

[Nicholas Pilkington]

The result doesn't look great with kill. How come the script is removing so much? How do I stop it removing so much geometry. 

[Martin Isenburg]

Hello,

you remove too much geometry in this step here as the first two attached pictures show.

lasthin -i holes_utm.laz -step 1 -percentile 10 30 -classify_as 8 -odix _p10 -olaz

the '-percentile 10 30' will only the 10th percentile elevation point as 8 when there are a minimum of 30 points per cell. In your case the cell size was set to 1 by 1 meter with '-step 1' . You need to make sure that you have this density. How? Use lasgrid:

lasgrid -i holes_utm.laz -step 1 -counter -false -set_min_max 29 30 -odix _1m_29_30 -opng

with this settings the false color image will be blue where we have only 29 or fewer points per square meter and red where we have the required 30 or more points. The result image makes clear why you loose so much geometry. Either lower the 30 or increase the step or do it adaptive like shown here:

lasthin -i holes_utm.laz -step 1 -percentile 10 30 -classify_as 8 -odix _p10_100cm -olaz

lasthin -i holes_utm_p10_100cm.laz -step 1.5 -percentile 10 30 -classify_as 8 -odix _150cm -olaz

lasthin -i holes_utm_p10_100cm_150cm.laz -step 2.0 -percentile 10 30 -classify_as 8 -odix _200cm -olaz

lasthin -i holes_utm_p10_100cm_150cm_200cm.laz -step 2.5 -percentile 10 30 -classify_as 8 -odix _250cm -olaz

This gives you 10th percentile geometry more evenly everywhere as you can see in the last attached picture. Then use this holes_utm_p10_100cm_150cm_200cm_250cm.laz file as input to your next processing step

lasview -i holes_utm_p10_100cm_150cm_200cm_250cm.laz

Regards,

Martin

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