2

I am having difficulty properly visualizing aerial LIDAR data from the csc.noaa.gov source.

Taking any of the .laz files, the point cloud displays as an extremely long thin line of points. Zoomed in, there are cyclic perturbations in the points along that line, which leads me to believe that some sort of transform is necessary to properly visualize the data. For comparison sake, the aerial LIDAR data set found in opentopo.sdsc.edu visualizes as one would expect.

Both claim to use NAD83 as the horizontal coordinate system. From what I can tell, the only major difference is one is las 1.0 and one is las 1.2. That being said, I don't really know what I'm doing.

A little bit of background to help guide an answer. I do not come from a GIS background, but rather am interested in comparing my LIDAR point clouds against a geographically accurate model, say a quality aerial LIDAR collect. For now, while learning, I have been using the free applications LAStools and CloudCompare.

1

3 Answers 3

2

This could be because csc.noaa.gov lidar files are stored with the .laz file format extension. It is a compressed version for .las extension.

Read this* article to know more about compressed las files (.laz).

You can try to retransform it from .laz to .las. For this, use the open-source LiDAR compressor LASzip to accomplish the task.

Here is one example:

Suppose that LASZip is installed under the drive C:, and the .laz file is stored under a fold named project (C:\project). Do the following:

c:\LASZip\laszip -i c:\project\aaa.laz -o c:\project\aaa.las

aaa is just a name for the file which I have made up.

*Reference

ISENBURG, M. LASzip: lossless compression of LiDAR data, 2012.


Examples

See below some examples about how .laz files become disfigured in comparison with its uncompressed version .las.:

This is the Fugro Viewer. While the dots in the .las file are concentrated in the upper and lower layers in the z direction (left panel), the dots in the .laz version are more homogeneously spread (right panel).

enter image description here

This is the Quick Terrain Reader. The dots in the .laz file are flattened in the z direction (right panel).

enter image description here

1

It is always a good idea to check any new LiDAR in LAS or LAZ format with the LAS validator tool as this will report any warnings or - more severe - failures in the file's adherence to the ASRPS specification. Also and read or decompression errors (e.g. due to rare bitflips and more common file truncations) will become obvious. You also find the free and open source lasvalidate.exe tool included as part of the LAStools distribution ...

1

I experienced the same problem with .laz files distributed on NOAA's S3 server for example https://noaa-nos-coastal-lidar-pds.s3.amazonaws.com/laz/geoid18/6325/Delivery_1/44123D8224.laz, which I cannot use in QGIS, displaz, or lidR. I was able to solve this problem by finding the original files on the FTP server at lidar.engr.oregonstate.edu. I'm guessing something NOAA did when they re-hosted the files messed them up, because the originals (44123D8224.laz for example) open fine when downloaded from FTP.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.