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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Tools for Lidar Point Cloud Filtering / Classification

Posted on Tue, March 08, 2011 by Chris Crosby in point cloud • Software

A relatively common question from OpenTopography users is how they can filter or classify a lidar dataset that was delivered by the data provider without ground (bare earth) returns differentiated from vegetation returns. The B4 lidar dataset, which covers the southern San Andreas and San Jacinto faults is a good example of a dataset where the lack of classification can be problematic for users, especially those working at higher elevations where vegetation can be dense.  In other cases, the classification was done hastily or not as well as a user would like, and thus re-classification is needed.  A good example here are the 2010 post-Haiti earthquake data which were processed rapidly in order to quickly distribute them to users.

Most lidar vendors use expensive black box commercial software to classify data - TerraScan is the industry standard - not readily available to your average Earth scientist.  This of course raises the question: For users who don’t have access to a commercial product like TerraScan, what other tools are available for performing lidar point cloud classification?

This week, someone on the libLAS developers list asked essentially that very question.  The question triggered a series of posts by other list members on software currently available to classify lidar data and develop bare earth terrain models.  Some of the suggestions were tools that I was already aware of (including some already registered to our new OpenTopography Tool Registry), while one or two were new to me.  Below is a compilation of suggestions provided by the libLAS-devel listserv members.

Tools for Lidar Classification / Filtering:

• MCC-LIDAR - An open source command-line tool for processing discrete-return LIDAR data in forested environments. It classifies data points as ground or non-ground using the Multiscale Curvature Classification algorithm developed by Evans and Hudak, 2007


• GRASS GIS - Open source GIS software.  Includes a suite of tools related to lidar data processing as discussed in this GRASS lidar wiki entry.  Currently doesn’t offer direct support for point cloud data in LAS format.


• BCAL Lidar Tools - Open source tools developed by the Idaho State University Boise Center Aerospace Lab in IDL as a plugin for the ENVI software package.  Includes a Height Filtering tool optimized for open rangeland (sagebrush) vegetation developed by Streutker and Glenn, 2006.


• SAGA GIS - Open source GIS package.  “SAGA includes several tools to manipulate the point cloud, e.g. an attribute calculator, reclassifier, subset extractor and many other methods like gridding and interpolation. There is also (a grid based) bare earth filter, adapted from Vosselman (2001).”

What other options are out there?  The libLAS list discussion emphasized open source tools, but I know there are other commercial options (e.g., Merrick’s MARS), that also offer point cloud classification capability.  If you are aware of tools not listed above, please leave a comment.  I have varied experience with these tools, so your thoughts and recommendations are also welcome in the comments. TerraScan is presumably the industry standard because when applied by a skilled operator the classification results are generally good.  Another issue inherent in all lidar data processing tasks is software that has the scalability to handle the massive data volumes typically encountered.  I assume that TerraScan does relatively well on this front, but it is an issue to also keep in mind when looking at the alternatives listed above.

User controlled point cloud classification (or re-classification) is a feature that we’d like to offer through OpenTopography, and one or more of these open source tools may be a viable option for integration into OT in the future.

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Filtering vegetation from Terrestrial Laser Scanning data using the Point2Grid tool

Posted on Fri, February 26, 2010 by Chris Crosby in Points2Grid • Software • TLS • Video

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 Chris Crosby 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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