Filtering LiDAR Data

[RashidiIsa]

Hi..I'm a student, nedd help for my thesis project 😃
Does anyone know how to filter the LiDAR data? I had created the raster file from LiDAR ascii format. I need to produce an EDM. The problem is, I do not know how to filter the non ground data from the raster, so that there are only ground data left. Any help is much appreciated 😃

[JeffreySwain]

Since you said Lidar from ascii points, it may be rather difficult since you did not mention LAS format.  If you had the LAS format and if it was classified, you would just need to select the proper class.  Since it may not be LAS and it may not be classified, you may have to look to other means of selecting points. In ArcGIS with LAS data, you can use the information on the LAS to Multipoint tool to select the proper class.  Otherwise, you face the daunting prospect of having to manually classify the data.  There are additional tools at 10.1 to help you classify them, but they require the data to be in the LAS format.

[RashidiIsa]

ooo...thanks for the info..=) then, how to manually classify those data..? I really do not have any idea..
Mr Jeff, you said that there is a tool that may help in classify,but it needs the data to be in LAS format..I just want to ask, is there any method that the ascii format can be converted into LAS format...?

[JeffreySwain]

There is a tool called LAStools that should be able to help with the conversion to LAS.  There is an enhancement request mentioned here for the same behavior in ArcMap. 

After you have the points in LAS, then you will need to utilize 10.1 to take advantage of the functionality to set the codes for the individual points with the Set LAS Code Class with a Feature Tool.

[JeffreyEvans]

Selecting an appropriate filtering approach is somewhat dependent on what type of features dominate the data. Zhang's (2003) morphological filter works quite well if you need to filter buildings in an urban landscape but does not work well in forested environments. In this case the Multiscale Curvature Classification (Evans & Hudak 2007) model is recommended. The USFS-RMRS software FUSION has an implementation of Vosselmans's (2000) slope based filter but it produces inconsistent results and is not very flexible. Martin Isenburg's LASTools has a good general filter implemented as well and I believe that it is now distributed with an ArcGIS toolbox written in Python.

All of these models operate on the lidar point cloud, not an interpolated raster, and mostly support the use or import of ASCII data. The De facto lidar format is LAS, so if you exported your data from LAS to ASCII I would stick with the original format during the filtering process. Since this is for your thesis work I would highly recommend reading the provided references as a starting point.   

References:
Evans, J.S., Hudak, A.T. (2007) A multiscale curvature algorithm for classifying discrete return LiDAR in forested environments. IEEE Transactions on Geoscience and Remote Sensing. 45(4): 1029-1038. Available online at: http://www.treesearch.fs.fed.us/pubs/29032

G. Vosselman (2000) Slope based filtering of laser altimetry data. Int. Arch. Photogramm. Remote Sens., vol. XXXIII, pp. 935�??942
Available online at: http://www.isprs.org/proceedings/XXXIII/congress/part3/935_XXXIII-part3.pdf

Zhang, K., S.-C. Chen, D.Whitman,M.-L. Shyu, J. Yan, and C. Zhang (2003) A progressive morphological filter for removing nonground measurements from airborne LiDAR data. IEEE Transactions on Geoscience and Remote Sensing. 41(4):872�??882.

Links:

Multiscale Curvature Classification
http://sourceforge.net/apps/trac/mcclidar/

FUSION (Slope filter)
http://www.fs.fed.us/eng/rsac/fusion

I believe that BCAL has Zhang's morphological filter implemented
http://code.google.com/p/bcal-lidar-tools/

LASTools
http://www.cs.unc.edu/~isenburg/lastools/

[MartinIsenburg]

Hello from @lastools,

please use the URL http://lastools.org/ to refer to LAStools from now on. To turn ASCII to LAS or SHP you can use txt2las.exe or las2shp.exe. But all LAStools can directly read TXT or SHP files via on-the-fly conversion to LAS.

In terms of point classification I do not think that ArcGIS 10.1 will have the full functionality you need. The most crucial task is that of ground / building / vegetation classification. Here you need to look at lasground.exe or lasheight.exe or lasclassify.exe. Or have a look at the other tools that Andrew mentioned.

Cheers,

Martin @lastools