Working with full waveform LiDAR data in SPDLib (part 2)

The first post of this series was written quite a while ago now. Apologies it has taken so long for a follow up. Since the first post has been written there have been two exciting developments:

1. The methods described for generating full waveform metrics have been used to perform the LiDAR analysis for a paper led by Chloe Brown, University of Nottingham.Brown, C.; Boyd, D.S.; Sjögersten, S.; Clewley, D.; Evers, S.L.; Aplin, P. Tropical Peatland Vegetation Structure and Biomass: Optimal Exploitation of Airborne Laser Scanning. Remote Sens. 2018, 10, 671. https://doi.org/10.3390/rs10050671
2. SPDLib is now available on Windows, macOS and Linux through conda-forge (as is RSGISLib). See part one for updated install instructions.

At the end of part one we had imported the LAS 1.3 file into SPDLib and decomposed the waveforms. This next section will cover ground classification and metrics generation.

1. Spatially index data

   In part one we had been working with an SPD file without a spatial index (UPD file). However, for subsequent processing steps a spatial index is needed so a spatially indexed file is generated using the spdtranslate command.

   As the gridding can use a lot of RAM we are going to process in tiles and then stitch them together. We create a temporary directory to store the tiles using:

   mkdir spd_tmp
   

   Then run the translate command:

   spdtranslate --if SPD --of SPD \
                -x LAST_RETURN \
                -b 1 \
                --temppath spd_tmp \
                -i LDR-FW-RG13_06-2014-303-05_subset_decomp.spd \
                -o LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded.spd
   

   The temporary directory can be removed after processing has completed:

   rm -fr spd_tmp
   

2. Classify ground returns and populate height

   Many metrics use the height about ground rather than absolute elevation so this must be defined. To derive heights from LiDAR data it is first necessary to determine the ground elevation so heights can be calculated above this. Within SPDLib the ground classification results are achieved using a combination of two classification algorithms: a Progressive Morphology Filter (PMF; [1]) followed by the Multi-Scale Curvature algorithm (MCC; [2]). Both these algorithms use only the discrete points rather than the waveform information.

   Apply a Progressive Morphology Filter using the following command:

   spdpmfgrd --grd 1 -i LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded.spd \
             -o LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_grd.spd
   

   Then apply the Multi-Scale Curvature algorithm to the output file using:

   spdmccgrd --class 3 --initcurvetol 1 \
             -i LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_grd.spd \
             -o LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_mcc_grd.spd
   

3. Attribute with height

   The final step of the SPD processing is to attribute each pulse with heights above ground level. An interpolation is used for ground points, similar to generating a Digital Terrain Model (DTM), but rather than using a regular grid the ground height is calculated for the position of each point.

   spddefheight --interp --in NATURAL_NEIGHBOR_CGAL \
                -i LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_mcc_grd.spd \
                -o LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_mcc_grd_defheight.spd
   

4. Calculate metrics

   After all the pre-processing steps to convert the LAS 1.3 file into a gridded SPD format file with a defined height it is possible to generate a number of metrics from the waveform data. The command to calculate metrics within SPDLib (`spdmetrics`) takes an XML file in which the metrics are defined. There are a large number of metrics available and operators (addition, subtraction etc.,) allowing existing metrics to be combined to implement new metrics. The full list of metrics is available in the The full list of metrics is available in the SPDMetrics.xml file, distributed with the source of SPDLib. Most metrics have an option to specify the minimum number of returns (`minNumReturns`), setting this to 0 will use the waveform information to calculate the metric, setting to 1 (default) or above will use the discrete data. In this way full waveform and discrete metrics can be created at the same time.For this exercise we will be calculating Height of Medium Energy (HOME) and waveform distance (WD), a detailed description of these metrics is given in [3].

   First, create a file containing these metrics. Create a text file called ‘spd_metrics.xml’ and paste the text below into it:

   
   <!-- SPDLib Metrics file -->
   
   <spdlib:metrics
   xmlns:spdlib="http://www.spdlib.org/xml/">
   
   <!-- HOME -->
   
   <spdlib:metric metric="home" field="HOME"/>
   
   <!-- WD -->
   
   <spdlib:metric metric="maxheight" field="WD" minNumReturns="0"/>
   
   </spdlib:metrics>
   

   If this doesn’t display, try copying from https://gist.github.com/danclewley/4eefda2200e7593f1e5e2aaa6bae2c03

   To calculate the metrics and produce an image as an output run.

   spdmetrics --image -o LDR-FW-RG13_06-2014-303-05_subset_metrics.bsq \
              -f ENVI \
              -i LDR-FW-RG13_06-2014-303-05_subset_decomp_gridded_pmf_mcc_grd_defheight.spd \
              -m spd_metrics.xml
   

   Once the command has finished, open the metrics image using:

   tuiview LDR-FW-RG13_06-2014-303-05_subset_metrics.bsq
   

More metrics can be added to the ‘spd_metrics.xml’ file as needed, it is also possible to define new metrics using the operator tags.

This post was derived from the LiDAR practical given as part of the NERC-ARF workshop held at BAS, Cambridge in March 2018. If you have any questions about working with NERC-ARF data contact the NERC-ARF Data Analysis Node (NERC-ARF-DAN) see https://nerc-arf-dan.pml.ac.uk/ or follow us on twitter: @NERC_ARF_DAN.

[1] Keqi Zhang, Shu-Ching Chen, Whitman, D., Mei-Ling Shyu, Jianhua Yan, & Zhang, C. (2003). A progressive morphological filter for removing nonground measurements from airborne LIDAR data. IEEE Transactions on Geoscience and Remote Sensing, 41(4), 872–882. http://doi.org/10.1109/TGRS.2003.810682
[2]: 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.
[3] Cao, L., Coops, N., Hermosilla, T., Innes, J., Dai, J., & She, G. (2014). Using Small-Footprint Discrete and Full-Waveform Airborne LiDAR Metrics to Estimate Total Biomass and Biomass Components in Subtropical Forests. Remote Sensing, 6(8), 7110–7135. http://doi.org/10.3390/rs6087110

Working with full waveform LiDAR data in SPDLib (part 1)

The SPD file format was designed around storing LiDAR pulses with digitised waveforms and associated points. The most recent version (3.3) has the ability to import waveform data from LAS files using LASlib, which is part of LAStools. Binaries are available for Linux, macOS and Windows, they can be installed through conda.

conda create -n spdlib -c conda-forge spdlib tuiview
. activate spdlib
conda update -c conda-forge --all

For this example LAS 1.3 files acquired by the NERC Airborne Research Facility (NERC-ARF, previously ARSF) over Borneo using a Leica ALS50-II instrument with full waveform digitiser will be used.

Once you have registered with CEDA and applied for access to the ARSF archive these data can be downloaded from: http://browse.ceda.ac.uk/browse/neodc/arsf/2014/RG13_06/RG13_06-2014_303_Maliau_Basin/LiDAR/flightlines/fw_laser/las1.3

You can also follow through with any of the other NERC-ARF datasets or other waveform LAS files you have.

1. Convert LAS to SPD format
   First you need a WKT file to define the projection. This step is optional but is more reliable than reading from a LAS file. For the example the projection is UTM50N, you can download a WKT file using:

   wget https://bitbucket.org/petebunting/rsgis_scripts/raw/c8cf94528cdb58b753029df3bc631a2509740ad1/WKT/UTM_WGS84/UTM_WGS84_Z50_N.wkt
   

   Then convert to an unsorted SPD file (UPD).

   spdtranslate --if LAS --of UPD \
                -x LAST_RETURN \
                --input_proj UTM_WGS84_Z50_N.wkt \
                -i LDR-FW-RG13_06-2014-303-05.LAS \
                -o LDR-FW-RG13_06-2014-303-05.spd
   

2. Subset SPD file (optional)

   As full waveform processing is quite intensive it is recommended to subset the data for the purpose of running though this tutorial, you can do this using the spdsubset command.

   spdsubset --xmin 494400 --ymin 524800 \
             --xmax 494800 --ymax 525000 \
             -i LDR-FW-RG13_06-2014-303-05.spd \
             -o LDR-FW-RG13_06-2014-303-05_subset.spd
   

3. Decompose waveform

   One of the limitations of discrete systems is there is are only a given number of ‘points’ recorded (normally 2 – 4) and the rest of the information is lost. As full waveform data records the entire waveform it is possible to extract more returns after data are acquired. A common approach to this ‘Gaussian Decomposition’ which involves fitting Gaussian distributions to the peaks, within SPDLib this is available as the ‘spddecomp’ command.

   spddecomp --all --noise --threshold 25 \
             -i  LDR-FW-RG13_06-2014-303-05_subset.spd \
             -o  LDR-FW-RG13_06-2014-303-05_subset_decomp.spd
   

   This will still take around 5 minutes to run. If you decide to decompose the full dataset after, expect it to take an hour or so.

4. Export returns to LAS file
   The final step for this part of the tutorial is to export the returns to a LAS file, using the spdtranslate command.

   spdtranslate --if SPD --of LAS \
                -i LDR-FW-RG13_06-2014-303-05_subset_decomp.spd \
                -o LDR-FW-RG13_06-2014-303-05_subset_decomp.las
   

Classifying ground returns and calculating LiDAR metrics using SPDLib are covered in part two

If you have further questions about using SPDLib please contact the mailing list (details available from https://lists.sourceforge.net/lists/listinfo/spdlib-develop). If you have any questions about working with NERC-ARF data contact the NERC-ARF Data Analysis Node (NERC-ARF-DAN) see https://nerc-arf-dan.pml.ac.uk/ or follow us on twitter: @NERC_ARF_DAN).