OpenTopography Blog

Information and discussion related to high-resolution LiDAR topography for the Earth sciences

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Category: Software

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Filtering vegetation from Terrestrial Laser Scanning data using the Point2Grid tool

Posted on Fri, February 26, 2010 by ccrosby in Software • TLS

Max Wilkinson, a PhD student in the Dept. of Earth Sciences at Durham University in the UK, recently sent us a link to a video of a seminar he gave on methods of vegetation filtration from Terrestiral Laser Scanning (TLS) data using the Points2Grid tool developed by the OpenTopography team (originally developed with GEON funding).  Points2Grid is a Windows application that implements the exact same DEM generation algorithm that is used in the OpenTopography point cloud processing system.  It was developed to allow users to get the same DEM generation capabilities as are available in OpenTopography for non-hosted data. 

Dealing with vegetation in TLS scanner data can be a significant problem, and Max shows in the video how he is able to use the MIN surface feature in P2G to remove much of the vegetation from the scan.  Points2Grid is not technically designed to filter vegetation, but the minimum surface function can act as a low budget filter.  P2G is also handy for reducing the complexity of the point cloud data down to something more manageable for analysis.  In this case, Max has much higher shot densities than he needs to construct a surface from the data, and P2G helps to reduce the amount of data he is working with:

From the TLS perspective, I’m finding P2G is tremendously useful for distilling large pointsets down to a size whereby the computing time required to test various interpolation methods becomes bearable! I had previously spent many hours waiting to see the output of an interpolation based on millions of points, only to find the parameters I used were not quite right. With the reduced computing time I’m now able to run more interpolations in various programs with a range of different parameters to see the results. I’m now using arcGIS for interpolations of TLS data, rather than just goCAD. For example, I’ve just finished a flow routing analysis of footwall drainage in arcGIS. A big thank you to the team for making P2G available!

Screen capture of the video:

You can download the video from Max’s page here: Vegetation Removal.avi (124 MB download). 

I had difficulties getting the file to play in Quicktime on a Mac, but the video played perfectly in open source (free) VLC player.  You may also need this ffdshow codec provided by Max.

The GEON Points2Grid tool can be downloaded via the lidar.asu.edu site.  I recommend reading the P2G Instructions / Help doc and also looking at the this page on how the search radius parameter in P2G works.

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LAS Conversion Tools:  LAStools and LASUtility

Posted on Wed, January 06, 2010 by ccrosby in Software

The ASPRS LAS format has become the standard for delivery of lidar point cloud data.  However, because LAS is a binary format, a reader of some kind is necessary to ingest the data, visualize it, or perform additional analysis.  Many of the major GIS software packages now support LAS (e.g. ArcGIS) and all of the commercial lidar specific software read and write LAS data.  Some users however lack access to software that will read the LAS format and thus would like to view lidar point cloud data in ascii format.  In these situations I typically recommend the excellent and free LAStools (or LAStools’ Open Source Geospatial foundation relative libLAS), a package of utilities for reading and writing LAS files.  The downside to LAStools is that the utilities are command line-based, which tends to scare away some users.  If you are interested in LAStools, take a look at the short LAStools tutorial that we’ve used in past OpenTopography short courses.

Today I learned about an alternative to LAStools, called LASUtility from an Indian company called GeoKno.  Like LAStools, LASUtility offers the ability to view LAS headers, convert LAS data to ascii and vice versa, merge or subset LAS files, etc.  However, LASUtility is a Java application that includes a graphical user interface (gui) to the commands unlike LAStools.  The fact that it is a Java app also means that in theory LASUtility will run on platforms other than windows (the readme says Windows, Linnux, Solaris), but the download only includes a Windows executable.  LASUtility is a free download, and a free 5 day license is available (read the “Readme.txt” file in the downloaded directory for info on obtaining a license).  It is unclear to me if longer term licenses are free or require payment.

A quick test drive of LASUtility using a sample LAS file (~220 MB .las) indicated that it generally works as promised but is quite a bit slower than the command line LAStools.  There also seems to to be less control over how file conversions are done - for example, the “LAS to XYZ” and “LAS to text” commands don’t allow you to define the delimiter in the ascii file (files are written as tab delimited).  On the plus side, LASUtility provides a simple (but slow) 3D viewer for the LAS file and claims to offer graphical sub-setting of files (allowing users to draw a bounding box), but I couldn’t get the interface to launch.

LASUtility:

If you are working with lidar point cloud data in LAS format, LAStools is a set of utilities that should definitely be part of your tool kit.  If you are command line averse, LASUtility may also be worth a look.

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