[GIS] Shifting an entire .las file in Z

inpho-match-atlaslidarvectorz-value

I have a set of .las tiles. I needed to edit one of the tiles in Trimble's Inpho DTMaster to remove some noise. The coordinates are correct in the program, showing a my desired elevation. On export, however, the program shifts the entire file up in Z by 90 feet. It is not a spatial reference/coordinate problem, I've ruled that out already (unless the software is doing something unbelievably dumb). This is all done in a local spatial rectangular system.

enter image description here

Is there an easy way to subtract 90 from the elevation field of every point in the .las? I'd like to try to offset the Z. I've tried to convert it to an ascii file to do it semi-manually, but it is too large and fails after it grows to about 2.5 GB.

I have access to various software, including ArcGIS, QGIS, Inpho Photogrammetry, Global Mapper.

Best Answer

There are some options. With Fusion there is the command line ClipData and its switch biaselev:#:

biaselev:# Add an elevation offset to every LIDAR point: # can be + or -.

The command line syntax for shifting the z values down 90 ft:

\ClipData /biaselev:-90 InputSpecifier SampleFile [MinX MinY MaxX MaxY]

Another option is the las2las tool wit the argument --offset from liblas (as pointed out by Michael Miles-Stimson).

Header modification options:
--offset arg
A comma-separated or quoted, space-separated list of offsets to set on the output file: --offset 0,0,0 or --offset "1234 5678 91011"

An example would be:

las2las -i in.las -o out.las --offset:0,0,-90

Related Solutions

[GIS] Problem with Y offset shift of LAS file (las2las)

Fusion LTK has a tool specifically for what you are trying to do. Its name is ClipData.

You'll need two switches (variations of the command line) to accomplish the task. The first switch is the biaselev:# (bias elevation of number equal to #). From Fusion's manual:

biaselev:# Add an elevation offset to every LIDAR point: # can be + or -.

So, let's say the minimum z value is 702, then, you would have /biaselev:702 to subtract all z coordinates with 702.

The second switch, which offsets x and y, is the nooffset. See its description from the manual:

nooffset: Produce an output point file that no longer has the correct georeferencing. This is used when you need to work with the point cloud but cannot reveal the actual location of the features represented in the point cloud. This option modifies the header values in the LAS header for the output files.

The complete command line for your case would be written like the following:

/ClipData /biaselev:# /nooffset InputSpecifier SampleFile [MinX MinY MaxX MaxY]

where InputSpecifier is the gross lidar cloud (.las file), the SampleFile is the new .las file with the offsets and [MinX MinY MaxX MaxY] are the minimums and maximums x and y from the gross lidar cloud, respectively.

Here I wrote a detailed answer which teaches how to use ClipData that you can adapt for your case using information from this post.

Just to exemplify that this works, I ran a test with one file of mine. Here is a screenshot of the new .las file's header.

enter image description here

Using Laspy to Create LAS Files from Scratch

Rather than setting the entire points array in one go, try setting each dimension in turn:

import laspy

header = laspy.header.Header()
outfile = laspy.file.File("output.las", mode="w", header=header)
outfile.X = [1, 2, 3]
outfile.Y = [0, 0, 0]
outfile.Z = [10, 10, 11]
outfile.close()

Best Answer

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[GIS] Problem with Y offset shift of LAS file (las2las)

Using Laspy to Create LAS Files from Scratch

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