Limits and Potentials of High Resolution Terrestrial Laser Scanning in Monitoring Estuarine Geomorphologic Variability Charlie Endris January 20, 2010
Elkhorn Slough: Erosion and Habitat Loss Evolution of salt marsh to mudflat Salt Marsh Vegetation Cover 1980 Mean X-section width of tidal creeks ~10 25 cm/yr 2001 From Van Dyke and Wasson, 2005.
Wetland monitoring How do we identify change on a shorter time-scale and at high resolutions??
Wetland monitoring Current methods of monitoring geomorphologic change in a wetland environment? Direct observations and measurements SET s and Marker Horizons PVC pipe markers along bank edges (Malzone, 1999) *Attempted in Elkhorn Slough Tidal creek flora and fauna indicate morphologic and hydrologic change (Lindquist, 1998; Lowe, 1999) Observations using Remote sensing/survey techniques
Survey Techniques: Remote Sensing Spatial and Temporal Limits Trimble VX Spatial Station 1000km Aerial Photos 1km GPS Airborne LiDAR Spatial Scale Theodolite TSC2 Controller Reflective prism 1m 1mm Terrestrial LiDAR (TLS) Photogrammetry 1 day 1 month 1 year 1000 years Time Scale Modified from Heritage and Heatherington, 2007
TLS Operation: Calculating Distance Time of flight principle Time of beam travel to a target and back 2 X Speed of Light = Distance
TLS Operation: Calculating Distance Time of flight principle Time of beam travel to a target and back 2 X Speed of Light = Distance
TLS Operation: Calculating Angles
TLS Operation Trimble VX Technical Specifications Single point measurements Direct surface scanning (point cloud) Robotic magnetic drive Range: 2500 m (prism), 150 m (direct) Speed: up to 15 pts/sec Measures reflectivity (intensity) 3 mm point precision
Data collection Top-down view
Post-processing Point Cloud Digital Elevation Model (DEM) Inverse Distance Weighted (IDW) interpolation Sun-illuminated topography
Objectives/Goals 1. Potential: Can we use TLS to monitor geomorphologic change in wetlands at high resolution and at multiple spatial and temporal scales? a) Long-term monitoring of different marsh environments b) Short-term effects of single tidal cycles on tidal creek margin stability 2. Limitations: Investigate sources of measurement error with respect to laser beam characteristics
Objectives/Goals 1. Potential: Can we use TLS to monitor geomorphologic change in wetlands at high resolution and at multiple spatial and temporal scales? a) Long-term monitoring of different marsh environments b) Short-term effects of single tidal cycles on tidal creek margin stability 2. Limitations: Investigate sources of measurement error with respect to laser beam characteristics
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Case Studies Elkhorn Slough Spatial and temporal scales a) Long-term monitoring of marsh environments including: - pickleweed edge - mudbanks - Mudflats - mudflat creeks b) Short-term effects of single tidal cycles on tidal creek margin stability Site #4: North Azavedo Pond Site #3: Bird Observatory Site #2: Sandholdt Bridge Site #1: Potrero Rd
Potrero Rd. (Site 1): Historic change 1993 2004 2005 2008 1160 m 2 increase in mudflat area ~ 21 cm/yr of marsh loss (horizontal) Aerial imagery courtesy of GoogleEarth
Potrero Rd. (Site 1): Mudflat change- Long-term Dec 19, 2007
Potrero Rd. (Site 1): Mudflat change- Long-term Feb 10, 2009
Potrero Rd. (Site 1): Mudflat DEM s
Potrero Rd. (Site 1): Mudflat DEM s
Potrero Rd. (Site 1): Mudflat DEM s
Potrero Rd. (Site 1): Measuring mudflat creeks Mar-6-2009 Elevation -1.0-2.3 Measured 118 x-section and thalweg widths and depths
Potrero Rd. (Site 1): Mudflat Creek Comparison Results- Long-term Mudflat creek comparisons Dec. 2007 - Mar. 2009 2.0 1.5 1.0 Distance (m) 0.5 0.0-0.5 X-section width Thalweg width Thalweg depth X-section width -1.0-1.5 V -shape U -shape -2.0 Dec 2007 Oct 2008 Mar 2009 Depth Mudflat creek: X-section width / Thalweg width ratio Dec. 2007 - Mar. 2009 5.5 5.0 Thalweg width 4.5 4.0 V -shape to U -shape Ratio 3.5 3.0 35 cm increase in mean creek width 2.5 2.0 1.5 1.0 Dec 2007 Oct 2008 Mar 2009
Bird Observatory (Site 3) Looking north Looking east
Bird Observatory (Site 3) Typical Edge Profile: Marsh to Mudflat Pickleweed Edge Pickleweed Marsh Mudflat
Bird Observatory (Site 3): Comparison results of mudflat and mudbank, 48 hrs. Dec. 9-11, 2008 N N 2 cm of elevation change! < -4.0-4.0 - -2.0-2.0 - -0.5 No change 0.5 2.0 2.0 4.0 4.0 10 >10 Raster resolution: 3 cm
Bird Observatory (Site 3): Pickleweed edge, VERTICAL PLANE Typical Edge Profile: Marsh to Mudflat Pickleweed Edge Pickleweed Marsh Mudflat
Bird Observatory (Site 3): Pickleweed edge variability over 1 month (LONG-TERM) Dec. 11- Jan. 9 BEFORE Dec. 11, 2008 AFTER Jan. 9, 2009 Vertical Plane Sloped Plane
Bird Observatory (Site 3): Pickleweed edge variability over 1 month (LONG-TERM) Dec. 11- Jan. 9 BEFORE Dec. 11, 2008 AFTER Jan. 9, 2009 (m) Change 0.30 0.20 Comparison Results 0.10 Pickleweed edge Vertical Plane 0-0.10 Mudbank Sloped Plane -0.20-0.30
Objectives/Goals 1. Potential: Can we use TLS to monitor geomorphologic change in wetlands at high resolution and at multiple spatial and temporal scales? a) Long-term monitoring of different marsh environments b) Short-term effects of single tidal cycles on tidal creek margin stability 2. Limitations: Investigate sources of measurement error with respect to laser beam characteristics Question: How do distance, angle of incidence, and grain size affect the accuracy of TLS measurements?
Beam Divergence and Angle of Incidence 0 o Angle Distance = 100 m Beam Diameter 8 4 Footprint (cm) created by laser at angle 0 o 45 o Angle Distance = 100 m 11 4 Trimble VX beam divergence: 8 cm vertical and 4 cm horizontal at 100 m Footprint elongated by 45 o target angle
Beam Footprint Footprint Diameter (cm) 100 90 80 70 60 50 40 30 20 10 0 Trimble VX Spatial Station: IR Laser Footprint Vertical Diameter 0 20 40 60 80 100 120 Distance from station (m) Angle of Incidence (deg O ) 85 80 70 60 50 45 40 30 20 10 0 * Footprint diameter calculations based on factory technical specifications of beam divergence of 8 cm vertical and 4 cm horizontal at 100 m
Surface Properties SAND 45 o 70 o 80 o 45 o 70 o MUD 80 o **Experiment reveals less accurate results when measuring mud at high angles of incidence**
Accuracy Experiment: Summary Measurements with high angle of incidence and distance produce large footprint diameters- reducing resolution and accuracy At high angles and distance, fine-grained (mud) surfaces produce less accurate results than coarse-grained (sand) surfaces
Take-home message Important to be aware of the limitations of TLS surveys TLS is a rapid and efficient method used to monitor geomorphologic change in a wetland at high resolution and at multiple spatial scales Ability to identify and interpret change is critical to understanding the finescale variability of wetland environments
Take-home message Important to be aware of the limitations of TLS surveys TLS is a rapid and efficient method used to monitor geomorphologic change in a wetland at high resolution and at multiple spatial scales Ability to identify and interpret change is critical to understanding the finescale variability of wetland environments - Applications: salt panne development in salt marsh habitats
Take-home message Important to be aware of the limitations of TLS surveys TLS is a rapid and efficient method used to monitor geomorphologic change in a wetland at high resolution and at multiple spatial scales Ability to identify and interpret change is critical to understanding the finescale variability of wetland environments - Applications: large-scale engineering projects Parsons Slough proposed sill structure Recently constructed dike at N. Azavedo Pond www.elkhornslough.org
Acknowledgements Funding Support California Sea Grant David and Lucile Packard Foundation (Friends of MLML) Committee Members Ivano Aiello, MLML John Oliver, MLML Ellen Hines, SFSU