Workflow for generating a Las File which can be used as a "imaginary" ground ? (ANGKOR WAT Mapping Project)

[ninah...@gmail.com]

Hi!

For 3D Visualizations from Temple Sites in Cambodia (CALI Angkor Lidar Project), I colored a big file of LAS Points with the RGB from an orthophoto using LAScolor - I've split the file into different Z value areas and colored them step by step and then merged - so I got a very nice output without uncolored point.

The problem I am trying to solve is, for water areas there are no Lidar Points, so the 3D Model of the pointcloud I has some not so good looking gaps (small rivers, reservoirs...)... 

I would like to create a point cloud with a simple consistent Z value a little below the lowest Z Value from the file and merge it with the las file, so that I can fill the "Water gaps" with some colored points and make the visualization look a little more pretty.

What i tried: 

- create a polygon at the area of interest

- create a point feature class with random points inside the polygon area

- used shp2las and now have a las file including those randomly created points.

BUT i cannot apply the LAScolor function with this file, because its missing tags describing tiepoints and pixscale. 

So is there A. a solution, so i can use the self created file, or B. another completely different way of creating a LAS pointcloud with by my own as a "dummy" las layer?

Thx for any suggestions.

Cheers,

Nina H.

Lidar Analytics/GIS technician

CALI Angkor Lidar Initiave

[Martin Isenburg]

Hello Nina,

to generate "fake" points that have a certain elevation you can use the '-clamp_z' transform as shown here:

lasview -i ..\data\fusa.laz -clamp_z 22 22

if you first have to close the water bodies to create points there you could simply first rasterize the point cloud. let's assume the buildings are your lakes and drop them.

lasview -i ..\data\fusa.laz -drop_class 6 -clamp_z 22 22 

In order to "close" the lakes with "fake points" it might be easiest just to create a DTM in BIL format and then read those as if they were points:

las2dem -i ..\data\fusa.laz -drop_class 6 -clamp_z 22 22 -step 0.5 -o fake.bil 

Now we simply clamp those fake points via on-the-fly conversion to LAS points against your lake polygons that we'll quickly generate like this:

lasboundary -i ..\data\fusa.laz ^

                    -keep_class 6 ^

                    -disjoint -concavity 5 ^

                    -o fake_lakes.shp

And now we clip only the points that have the fake elevation of 22 meters and fall into one of the fake lakes. In your case you can also color them blue at the same time (assuming you convert them to point type 2 or 3 first).

lasclip -i  fake.bil  -poly fake_lakes.shp -o fake_lakes.laz

Attached is a typical result of these "fake" points with an elevation of your choosing.

Regards,

Martin @rapidlasso

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