shp2las - need help

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Chris Geary

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Nov 7, 2021, 1:25:18 PM11/7/21
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Hi all,

I'm currently trying to convert a shapefile with a collection of points I surveyed into las format so I can generate a DTM.

I keep getting stuck, shp2las imports the file and converts it into a laz file, but when I open the points up in something like cloudcompare they are total nonsense. I'm assuming this has something to do with the projection but I can't figure it out.

Attached is a copy of the shapefile I am working with. Any ideas?
DTM shapefile.rar

Chris Geary

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Nov 8, 2021, 2:23:10 AM11/8/21
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Update: I managed to convert my points into a laz file by stripping all the coordinates out and using the txt2las tool.

I now have my manually staked points, now I'm trying to use las2dem to densify my DTM. Unfortunately when I use this tool my output is always a laz file with no points!

This is what the output looks like from the GUI:

las2dem -cpu64 -i "F:\WebODM Outputs\Masters\shp2las\attempt4.laz" -step 0.5 -elevation -odir "F:\WebODM Outputs\Masters\shp2dem2" -o "densified_DTM.laz"

Here is a link to my manually staked points converted to laz format


Thanks again,

Chris

Alex Zscheile

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Nov 8, 2021, 3:19:38 AM11/8/21
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Your output is specifying a .laz file. What type of file format do you desire? For example, if you want a .tif, make your output file densified_DTM.tif.

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Jorge Delgado García

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Nov 8, 2021, 3:55:54 AM11/8/21
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Your LAS file (obtained from your points) is in geographic coordinates (EPSG:4326) and your coordinates in degrees. In my opinion, it is better to convert to a projected system (in meters). You can make it using txt2las ... using the projection option and define your EPSG code.

Your points dataset has important differences in spacing in both axes. The differences between lines are around 22m ... do you want a 0.5m spacing DTM?

Of course, you need to define what is your desired output format ... could be a GeoTIFF o LAZ file ... or another format.

Jorge



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Chris Geary

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Nov 9, 2021, 1:48:59 AM11/9/21
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Hey, thanks for the tip! You were correct, the issue was that I hadn't reprojected the laz file, once I did that using the las2las tool I managed to get the DTM working.

Jorge Delgado García

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Nov 10, 2021, 3:26:34 PM11/10/21
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Dear Chris, some additional instructions ... sorry, but yesterday I had really a very busy day (a typical Monday).

1) You can transform the original data (I suppose that are in geographical coordinates and WGS84 datum) into a UTM-WGS84 CRS using this command:
las2las -cpu64 -i "D:\lastools_google\attempt4.laz" -target_epsg 32755 -odir "D:\lastools_google" -o "data_UTMWGS84_32755.laz" -epsg 4326

2) Always is very important to check out the LAZ file and analyse what is the real average point density and spacing in our study area. You can use the lasinfo command.
lasinfo -cpu64 -i "D:\lastools_google\data_UTMWGS84_32755.laz" -odix _info -otxt -odir "D:\lastools_google" -cd

In this case, the results are... 
number of point records:    405
covered area in square meters/kilometers: 1592/0.00
point density: all returns 0.25 last only 0.25 (per square meter)
spacing: all returns 1.98 last only 1.98 (in meters)

In my opinion, the number of points available is scarce to be able to propose a model with a spacing of 0.5m, although the area is apparently flat 
(with differences between 78.78 and 87.78m, in a distance of about 240m). Nevertheless, I will generate the model with 0.5m spacing. 
In this respect it is important to consider the maximum allowable distance for the triangles, which with this data configuration, and doing several tests, 
should be at least 25m. The command is the following, and you will obtain a DEM in GeoTIFF format in the defined CRS and with a spacing of 0.5m.

las2dem -cpu64 -i "D:\lastools_google\data_UTMWGS84_32755.laz" -step 0.5 -kill 25 -elevation -odir "D:\lastools_google" -otif


I hope it will be useful for you. 

Jorge Delgado

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