How to I control resolution and what is removed in a DTM?

[Nicholas Pilkington]

Hi there, 

I am trying to use LAStools to generate a DTM of this point cloud:

https://www.dropbox.com/s/af2jmz5aa7zaju6/points.las?dl=0

The commands I use are:

las2las.exe -i points.las -o points_utm.las -keep_every_nth 10 -meter -elevation_meter -target_utm auto

lassort -i points_utm.las -rescale 0.1 0.1 0.1 -o points.laz

lasground -i points.laz -city -ultra_fine -o points_ground.laz

las2dem -i points_ground.laz -false -keep_class 2 -thin_with_grid 0.125 -extra_pass -step 0.5  -otif -o dtm.tif

Here's the before and after. 

https://www.dropbox.com/s/vszxbzzr74ru0j0/Screenshot%202017-10-08%2021.00.16.png?dl=0

These settings some of the vehicles and buildings but also cut off the top of each stockpile which is very problematic? Because I need to measure volumes:

So my questions are:

1. Are there settings that can improve this? 

2. The rasterized image is very low resolution ~400x400, if there a setting to control the gronud sampling distance of the final image?

Regards, 

Nick

[Martin Isenburg]

Hello Nick,

could you upload your file (at centimeter resolution meaning scale factors of 0.01) in compressed LAZ format? That's why this format exists. To share data more easily over slow network links. I am on a surfbeach in India (i.e. in Mahabalipuram should you care (-:) and downloading large files is not feasible.

I cannot try myself now but try

lasground -i points.laz -town -extra_fine -o points_ground.laz

lasground -i points.laz -nature -fine  -o points_ground.laz

lasground -i points.laz -wilderness -o points_ground.laz

or switch to lasground_new.exe that is better with mountain tops

lasground_new -i points.laz -city -ultra_fine -o points_ground.laz

lasground_new -i points.laz -town -extra_fine -o points_ground.laz

lasground_new -i points.laz -nature -fine  -o points_ground.laz

The pixel size of the rastered image is controlled via '-step 0.5'. Try '-step 0.25' or '-step 0.1' instead.

Regards from India.

Martin @rapidlasso

--
Download LAStools at
http://lastools.org
http://rapidlasso.com
Be social with LAStools at
http://facebook.com/LAStools
http://twitter.com/LAStools
http://linkedin.com/groups/LAStools-4408378
Manage your settings at
http://groups.google.com/group/lastools/subscribe

[Nicholas Pilkington]

Here's the points.laz https://www.dropbox.com/s/23iy5zanzrwhheb/points.laz?dl=0

[Martin Isenburg]

Hello Nick,

I seem to get reasonable results with the following command line:

E:\LAStools\bin>lasground_new -i points.laz -olay -v -town -bulge 2

WARNING: do not expect LAY files to work across different versions

         of LAStools because the development of LASlayers is still

         not completed. this is just an awesome prototype as proof

         of concept. we continue to gather community input ... (-;

processing file 'points.laz'.

horizontal units are meter and vertical units are meter. town mode.

reading 1326816 points. step is 10 m, sub is 4, spike is 1+1 m, and offset is 0.05 m ...

took 3.12 sec. finding initial ground points ...

took 0.11 sec. generating initial ground estimate ...

took 0.062 sec. refining ground ...

took 0.344 sec. integrating points 0.05 above ground ...

took 4.112 sec. outputting ...

took 1.129 sec. 1145001 points classified as ground.

done with 'points.laz'. total time 8.877 sec.

E:\LAStools\bin> lasview -i points.laz  -ilay

E:\LAStools\bin> las2dem -i points.laz -ilay -step 0.25 -hillshade -odix _dsm -opng

WARNING: do not expect LAY files to work across different versions

         of LAStools because the development of LASlayers is still

         not completed. this is just an awesome prototype as proof

         of concept. we continue to gather community input ... (-;

E:\LAStools\bin> las2dem -i points.laz -ilay -keep_class 2 -step 0.25 -hillshade -odix _dtm -opng

WARNING: do not expect LAY files to work across different versions

         of LAStools because the development of LASlayers is still

         not completed. this is just an awesome prototype as proof

         of concept. we continue to gather community input ... (-;

See resulting hillshades that are attached.

Note that this little exercise uses LASlayers [1] (the '-olay' and '-ilay' switches) to reduce I/O by avoiding to write the complete new LAZ file despite only the classifications having changed.

Regards,

Martin @rapidlasso

[1] http://rapidlasso.com/LASlayers