limits and potentials of high resolution terrestrial laser...

Limits and Potentials of High Resolution Terrestrial Laser Scanning in Monitoring Estuarine Geomorphologic Variability

Charlie Endris

January 20, 2010

From Van Dyke and Wasson, 2005.

Salt Marsh Vegetation Cover

Mean X-section width of tidal creeks

Evolution of salt marsh to mudflat

1980

2001

Elkhorn Slough: Erosion and Habitat Loss

~10 – 25 cm/yr

Wetland monitoring

How do we identify change on a shorter time-scale and at high resolutions??

• Direct observations and measurements– SET’s and Marker Horizons– PVC pipe markers along bank edges (Malzone, 1999)– Tidal creek flora and fauna indicate morphologic and hydrologic change (Lindquist, 1998;

Lowe, 1999)

• Observations using Remote sensing/survey techniques

Wetland monitoring

*Attempted in Elkhorn Slough

Current methods of monitoring geomorphologic change in a wetland environment?

GPS

Theodolite

Aerial Photos

Airborne LiDAR

PhotogrammetryTerrestrial LiDAR (TLS)

Spatial Scale

Modified from Heritage and Heatherington, 2007

Spatial and Temporal Limits

TSC2 Controller

Reflectiveprism

Trimble VX Spatial Station

Time Scale1 day 1 month 1 year 1000 years

1mm

1m

1km

1000km

Survey Techniques: Remote Sensing

“Time of fl ight” principle

2

Time of beam travel to a target and backX Speed of Light = Distance

TLS Operation: Calculating Distance

“Time of fl ight” principle

TLS Operation: Calculating Distance

2

Time of beam travel to a target and backX 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

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

Case Studies

Spatial and temporal scales…

a) “Long-term” monitoring of marsh environments including:

- pickleweed edge

- mudbanks

- Mudflats

- mudflat creeks

Elkhorn Slough

Site #4: North Azavedo Pond

Site #3: Bird Observatory

Site #2: Sandholdt Bridge

Site #1: Potrero Rd

Case StudiesElkhorn Slough




Site #1: Potrero Rd



- pickleweed edge

- mudbanks

- Mudflats

- mudflat creeks

Case Studies



- pickleweed edge

- mudbanks

- Mudflats

- mudflat creeks


Elkhorn Slough




Site #1: Potrero Rd

1993 2004 2005 2008

Potrero Rd. (Site 1): Historic change

1160 m2 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

Feb 10, 2009

Potrero Rd. (Site 1): Mudflat change- “Long-term”

Potrero Rd. (Site 1): Mudflat DEM’s

Mar-6-2009

Elevation

-1.0

-2.3

Measured 118 x-sectionand thalweg widths and depths

Potrero Rd. (Site 1): Measuring mudflat creeks

Mudflat creek: X-section width / Thalweg width ratioDec. 2007 - Mar. 2009

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Rat

io

Dec 2007 Oct 2008 Mar 2009

Mudflat creek comparisonsDec. 2007 - Mar. 2009

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Dis

tanc

e (m

)

X-section widthThalweg widthThalweg depth

Dec 2007 Oct 2008 Mar 2009

Potrero Rd. (Site 1): Mudflat Creek Comparison Results- “Long-term”

Thalweg width

Depth

X-section width

‘V’-shape ‘U’-shape

‘V’-shape to ‘U’-shape

35 cm increase in mean creek width

Bird Observatory (Site 3)

Looking north

Looking east

Bird Observatory (Site 3)

Pickleweed MarshPickleweed Edge

Mudflat

Typical Edge Profile: Marsh to Mudflat

Dec. 9-11, 2008

Bird Observatory (Site 3): Comparison results of mudflat and mudbank, 48 hrs.

N

N

Raster resolution: 3 cm

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

Pickleweed MarshPickleweed Edge

Mudflat

Typical Edge Profile: Marsh to Mudflat

Bird Observatory (Site 3): Pickleweed edge, VERTICAL PLANE

Dec. 11- Jan. 9

Bird Observatory (Site 3): Pickleweed edge variability over 1 month (LONG-TERM)

BEFOREDec. 11, 2008

AFTERJan. 9, 2009

Vertical Plane

Sloped Plane

0.30

-0.30

0

(m)

0.10

0.20

-0.10

-0.20

BEFOREDec. 11, 2008

AFTERJan. 9, 2009

Dec. 11- Jan. 9

Pickleweed edge

Mudbank

Comparison Results

Vertical Plane

Sloped Plane

Bird Observatory (Site 3): Pickleweed edge variability over 1 month (LONG-TERM)

Change

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


2. Limitations: Investigate sources of measurement error with respect to laser beam characteristics

Objectives/Goals

Question: How do distance, angle of incidence, and grain size affect the accuracy of TLS measurements?

Distance = 100 m

48

Footprint (cm) created by laser at angle 0o

114

Footprint elongated by 45o target angle

Beam Divergence and Angle of Incidence

Trimble VX beam divergence: 8 cm vertical and 4 cm horizontal at 100 m

Distance = 100 m

0o Angle

45o Angle

Beam Diameter

Trimble VX Spatial Station: IR Laser Footprint Vertical Diameter

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120

Distance from station (m)

Footprint Diameter (cm)

858070605045403020100

Angle of Incidence (deg O)

* Footprint diameter calculations based on factory technical specifications of beam divergence of 8 cm vertical and 4 cm horizontal at 100 m

Beam Footprint

Surface Properties

SAND45o

70o

80o

MUD

45o

70o

80o

**Experiment reveals less accurate results when measuring mud at high angles of incidence**

• 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

Accuracy Experiment: Summary

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 fine-scale variability of wetland environments

Take-home message




- Applications: salt panne development in salt marsh habitats

Take-home message




- Applications: large-scale engineering projects

Parsons Slough proposed sill structure

www.elkhornslough.org

Recently constructed dike at N. Azavedo Pond

Funding Support• California Sea Grant• David and Lucile Packard Foundation

(Friends of MLML)

Committee Members• Ivano Aiello, MLML• John Oliver, MLML• Ellen Hines, SFSU

Acknowledgements

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