ASU LiDAR/ALSM Research

Comparing LiDAR shot counts: GeoEarthScope, ECSZ, and B4 surveys

Ramón Arrowsmith, May-June, 2010

Introduction

A question came up about the actual shot count per square meter or density for various LiDAR topographic surveys that have been done recently. I have written about this before (Analysis of LiDAR shot densities for NCALM Eastern California Shear Zone (ECSZ) survey and B4 and A few notes on the differences between data gathered by the 1233 and 5100 ASLM scanners), but I was not able at the time do do some direct comparisons.

Overlapping LiDAR data

OpenTopography now has LiDAR datasets on line with overlap which can be used to explore this question (see table below).
Data set nameSourceAcquisition dateScanner information (AGL, SR)*Metadata link
NSAFUSGS/NASAFebruary, 2003Terrapoint ALTMS 4036 (1000 m, 64kHz) NSAF metadata
Eastern California Shear Zone (ECSZ)Oskin/Perg (NSF)November, 2003NCALM Possibly Optech 1233 (600 m, 33kHz) None available
B4OSU/USGS/NCALM/UNAVCO/Optech**May 15-25, 2005NCALM Optech ATLM 3100 (600 m, 70kHz) See references noted**
Northern California EarthScope (NoCAL)EarthScope/NSFMarch 21 April 17, 2007NCALM Optech GEMINI (700 m, 125kHz^) NoCAL EarthScope metadata
Southern California EarthScope (SoCAL)EarthScope/NSFApril 2-24, 2008NCALM Optech GEMINI (700 m, 100kHz) SoCAL EarthScope metadata
*AGL=Above Ground Level, SR = Scan Rate

**B4 was supported by NSF and references include:
B4 project web pages at Ohio State University
Bevis, M. et.al., 2005. The B4 Project: Scanning the San Andreas and San Jacinto Fault Zones, Eos Trans. AGU, 86(52), Fall Meet. Suppl., H34B-01
Toth, et al., Extreme precision lidar mapping
Toth, et al., 2006, HIGH-RESOLUTION AIRBORNE LIDAR/CCD MAPPING OF SAN ANDREAS FAULT
Toth, et al., 2007, LIDAR MAPPING SUPPORTING EARTHQUAKE RESEARCH OF THE SAN ANDREAS FAULT

^Most of the NCAL EarthScope data were scanned at 100kHz, but the Gualala section compared here was done at 125kHz because of mostly heavy canopy.

Scan Comparisons

Overview

My effort to compare the scans was not exhaustive. Some more quantitative analyses might be warranted and looking at some of the differences between the scans as being due to actual ground displacement--rather than error--would be a good idea. I looked at 3 different overlaps in my comparison (Figure 1): 1) SoCAL/ECSZ overlap along the Calico Fault, 2) SoCAL/B4 northwest of Parkfield, and 3) NoCAL/NSAF near Point Arena.


Figure 1. Overview map of comparison areas (white rectangles) and active faults of western US.

Method

Overview

To compute the shot densities, I used the OpenTopography Point Cloud and Custom DEM tool to select the areas of interest, download the points, and then compute DEMs and point count maps. An important point is that we use a local binning approach which finds all points within a user specified search radius relative to the user specified grid node spacing for the DEM. This method is in production at OpenTopography and also explained as well as available as a Windows application at this link: http://lidar.asu.edu/points2grid.html. This link provides a simple overview of the idea: Exploration of search radius. The point counts are thus all the points in the search radius which was 1 and 4 m in this study (3.14 m2 and 50.26 m2 respectively). To convert to a shot density per square meter, I divided by the area of the search for each grid node. This tends to smear the results a little, but is representative and basically correct (see the results below). It also is appropriate in the sense that at least I think in terms of DEMs in many cases and so I need to know what resolution is supported by what density of points or points in the search radius for a given DEM resolution.

In addition to displaying the hillshades of the DEMs and image maps of the shot density maps, I computed histograms of the shot densities and also produced detailed maps of representative areas showing the actual points over the shot density map, and computed the average shot density in that rectangular area.

Sample MatLab scripts and functions used in the analysis

Once the OpenTopography computations were completed and the points and resulting grids downloaded, I analyzed the results and visualized the patterns using scripts and functions written in MatLab.

SoCAL/ECSZ overlap along the Calico Fault

I compared the overlap between SoCAL and ECSZ through the entire overlap between the two along the Calico fault just southeast of I-40 (Table and Figures 1 and 2).

ECSZ data

SoCAL data
Figure 2. Calico fault ECSZ and SoCAL data coverage (see Figure 1 for location).

ECSZ Calico Fault data

Figures 3-5 show the ECSZ Calico Fault data, starting with the entire data set and zooming to the fault zone. Single swaths have shot densities of about 0.7 shots/m2, whereas a double overlap shows 2-3 shots/m2 and a triple 4-5 shots/m2 (see figure 5).


Figure 3. Hillshade of entire ECSZ coverage of Calico Fault and zoom to fault zone.

Figure 4. Shot count per square meter of entire ECSZ coverage of Calico Fault and zoom to fault zone. Histograms at right show the shot count distributions. The mode for the entire coverage is 0.48 shots/m2 whereas the mode for the higher density zone along the fault zone is 2.23 shots/m2.

Figure 5. Shot count per square meter of ECSZ coverage of zoom to Calico Fault zone (upper left is the same area as is shown in figures 3 and 4). The three small boxes are 15x15 m on a side and show the shot count map overlain with the actual lidar returns and then the average shot per square meter is thus calculated independently and corresponds with the radially averaged shot count map.

EarthScope Calico Fault data

Figures 6-8 show the EarthScope Calico Fault data, starting with the overlap area and zooming to the fault zone. Single swaths have shot densities of about 2-3 shots/m2, whereas a double overlap shows 4-5 shots/m2 and a triple more than 6 shots/m2 (see figure 7).


Figure 6. Hillshade of EarthScope coverage of Calico Fault and zoom to fault zone.

Figure 7. Shot count per square meter of EarthScope coverage of Calico Fault and zoom to fault zone. Histograms at right show the shot count distributions. The mode for the entire coverage is 1.77 shots/m2 whereas the mode for the higher density zone along the fault zone is 3.5 shots/m2.

Figure 8. Shot count per square meter of EarthScope coverage of zoom to Calico Fault zone (upper left is the same area as is shown in figures 6 and 7). The three small boxes are 15x15 m on a side and show the shot count map overlain with the actual lidar returns and then the average shot per square meter is thus calculated independently and corresponds with the radially averaged shot count map.

NoCAL/B4 overlap along the San Andreas Fault northwest of Parkfield

I compared the overlap between NoCAL and B4 through the overlap between the two along the San Andreas Fault just northwest of Parkfield (Table and Figures 1 and 9).

B4 data

NoCAL data

B4 and NoCAL data
Figure 9. B4 and NoCAL data coverage (see Figure 1 for location).

B4 data

Figures 10-12 show the B4 data starting with the northwestern 6 km of the data set and zooming to the comparison zone. Single swaths have shot densities of about 2 shots/m2 whereas a double overlap shows 4 shots/m2 and a triple 6 shots/m2 (see figure 7).


Figure 10. Hillshade of B4 coverage of San Andreas Fault northwest of Parkfield and zoom to comparison zone (see figure 1 for location).

Figure 11. Shot count per square meter of B4 coverage of San Andreas Fault northwest of Parkfield and zoom to comparison zone. Histograms at right show the shot count distributions. Note the clear bimodal pattern with the lower mode at 2 shots/m2 contributed by the fringing single swath coverage and the second being double coverage at about 4 shots/m2.

Figure 12. Shot count per square meter of B4 coverage in comparison zone (upper left is the same area as is shown in figures 10 and 11). The three small boxes are 15x15 m on a side and show the shot count map overlain with the actual lidar returns and then the average shot per square meter is thus calculated independently and corresponds with the radially averaged shot count map.

NCAL EarthScope data

Figures 13-15 show the NoCAL data starting with the southeastern several km of the data set and zooming to the comparison zone. Single swaths have shot densities of about 2.2 shots/m2 whereas a double overlap shows 5 shots/m2 and a triple 7+ shots/m2 (see figure 7).


Figure 13. Hillshade of NoCAL coverage of San Andreas Fault northwest of Parkfield and zoom to comparison zone (see figure 1 for location).

Figure 14. Shot count per square meter of NoCAL coverage of San Andreas Fault northwest of Parkfield and zoom to comparison zone. Histograms at right show the shot count distributions. Both histograms are dominated by the double swath coverage through most of the area with 4-5 shots/m2. Shot densities as high as 15 shots/m2 are in areas of returns though vegetation in the southwest and maybe highre elevation (lower AGL(?)).

Figure 15. Shot count per square meter of NoCAL coverage in comparison zone (upper left is the same area as is shown in figures 13 and 14). The three small boxes are 15x15 m on a side and show the shot count map overlain with the actual lidar returns and then the average shot per square meter is thus calculated independently and corresponds with the radially averaged shot count map.

NSAF/NoCAL overlap along the San Andreas Fault east of Point Arena

Finally, I compared the overlap between NSAF and NoCAL through the overlap between the two along the San Andreas Fault east of Point Arena (Table and Figures 1 and 16).

NSAF data

NoCAL data

NSAF and NoCAL data
Figure 16. NSAF and NoCAL data coverage (see Figure 1 for location).

NSAF data

Figures 17-19 show the NSAF data along the San Andreas Fault directly east of the town of Point Arena. These older and lower pulse rate data (see Table) are in feet (California State Plane). I think I still got the conversions to metric correct. The flight pattern is evident in the shot density map (Figure 18) and I have chosen an area that spans the split in the single directed overlapping swaths and the second, higher shot density, zone with the additional N-S flight lines. The stated single swath goal for the survey was 1 shot/m2 (NSAF metadata). Figures 18 and 19 show that this was achieved. The double overlap zones have 1.8 shots/m2 and the multi-overlap (3 or 4x) shows 2.6 shots/m2 (Figure 19). Few laser shots are returned from the Garcia River.


Figure 17. Hillshade of NSAF coverage of San Andreas Fault east of Point Arena and zoom to comparison zone (see figure 1 for location).

Figure 18. Shot count per square meter of NSAF coverage of San Andreas Fault east of Point Arena and zoom to comparison zone. Histograms at right show the shot count distributions. Note the clear bimodal pattern coming from the single and double flight coverages (both have overlapping swaths).

Figure 19. Shot count per square meter of NSAF coverage in comparison zone (upper left is the same area as is shown in figures 17 and 18). The three small boxes are 15x15 m on a side and show the shot count map overlain with the actual lidar returns and then the average shot per square meter is thus calculated independently and corresponds with the radially averaged shot count map.

Acknowledgements

The idea for this study came from a question by Mike Oskin. Thanks to Chris Crosby for edits.
Dataset acknowledgements:
Data Access Acknowledgement: This material is based on processing services provided by the OpenTopography Facility with support from the National Science Foundation under NSF Award Numbers 0930731 & 0930643.

Last modified: June 19, 2010