Using the SINK Hydrologic Using the SINK Hydrologic Function to Identify Treetops in Function to Identify Treetops in Small-Footprint Multiple-Return Small-Footprint Multiple-Return

Lidar DataLidar Data

Jason StokerJason StokerLidar Remote Sensing ScientistLidar Remote Sensing Scientist

USGS EROS Data CenterUSGS EROS Data CenterSioux Falls, SDSioux Falls, SD

Symposium on Terrain Analysis for Water Resources Symposium on Terrain Analysis for Water Resources ApplicationsApplications

December 16-18, 2002December 16-18, 2002University of Texas - AustinUniversity of Texas - Austin

What……….????What……….????

Do I care about?Do I care about?

• Trees?Trees?• Lidar– OKLidar– OK• SINKs– OKSINKs– OK• Results?Results?

Site Description

• Location of Cheesman Lake

The 15 ha PlotThe 15 ha Plot

Description of the Lidar Description of the Lidar SensorSensor

• 3Di 3Di Technologies Technologies of Boulder, COof Boulder, CO• Small-footprint Small-footprint

multiple-multiple-return systemreturn system•Up to five Up to five

returns per returns per pulsepulse

• Intensity Intensity recorded for recorded for each returneach return

Courtesy of 3Di Technologies

Description of the Lidar Description of the Lidar SensorSensor

• 3Di 3Di Technologies of Technologies of Boulder, COBoulder, CO• Pulse rate of Pulse rate of

36.5 kHz36.5 kHz• Fast retrace Fast retrace

linear scan linear scan patternpattern

• Posting density Posting density from 1m x 1m from 1m x 1m to 2.8m x 2.8mto 2.8m x 2.8m

MethodsMethods• Freestanding, individual treesFreestanding, individual trees

• Separate freestanding trees from clumps Separate freestanding trees from clumps of treesof trees

• Qualitative analysis of point cloudsQualitative analysis of point clouds• Height, Length of Live Crown, DBH, and Height, Length of Live Crown, DBH, and

VolumeVolume

• Identify the number of overstory Identify the number of overstory treestrees

• All overstory trees in 15 ha plotAll overstory trees in 15 ha plot• Delineate tree crownsDelineate tree crowns• Height, DBH, and VolumeHeight, DBH, and Volume

Veg Return - Bare Ground Return = Veg Returns on GRID GRID Flat Surface

Who Needs Topography?

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Freestanding Trees versus ClumpsFreestanding Trees versus Clumps

FreestandingFreestanding

FreestandingFreestanding

ClumpsClumps

Clumps

Identifying Identifying freestanding treesfreestanding trees

• Vegetation-Vegetation-only contoursonly contours

• 3 meter 3 meter contourcontour

• Only one Only one surveyed treesurveyed tree

• Clipped 1Clipped 1stst-5-5thth and BG returnsand BG returns

Qualitative Analysis of Qualitative Analysis of Point CloudPoint Cloud

•Overlaid Overlaid return points return points on TINson TINs

•Photograph Photograph treetree•GPS photo GPS photo

locationlocation

Qualitative Analysis of Point CloudQualitative Analysis of Point Cloud

Identifying the number of Identifying the number of trees in a clumptrees in a clump

• Identify Identify “treetops”“treetops”

• Compare # of Compare # of treetops per treetops per clump to clump to surveyed trees surveyed trees per clumpper clump

• Use threshold Use threshold method to raise method to raise barbar

Treetops

The SINK Hydrologic FunctionThe SINK Hydrologic Function

• Creates a grid identifying all sinks, or Creates a grid identifying all sinks, or areas of internal drainage areas of internal drainage (ESRI, Redlands, CA)(ESRI, Redlands, CA)

• Sinks are considered to have undefined Sinks are considered to have undefined flow directionsflow directions

• Usually caused by errors in the dataUsually caused by errors in the data

Sink

Identifying the number of Identifying the number of trees in a clumptrees in a clump

• Identify Identify “treetops”“treetops”

• Inverted Inverted vegetation only vegetation only gridgrid

• Used Hydrological Used Hydrological GIS method to GIS method to identify “sinks”identify “sinks” Sinks

Height Thresholds and the Height Thresholds and the Wilcoxon Signed Rank TestWilcoxon Signed Rank Test

•Compared surveyed number to Lidar number in clumpCompared surveyed number to Lidar number in clump

•Many trees in understory- not detectableMany trees in understory- not detectable

•Wilcoxon- HWilcoxon- H00: Median of Differences = 0: Median of Differences = 0

•Different Height thresholds to get into the overstoryDifferent Height thresholds to get into the overstory

Identifying the number of Identifying the number of trees in a clumptrees in a clump

All surveyed trees

Identifying the number of Identifying the number of trees in a clumptrees in a clump

Surveyed trees > 4 m

Identifying the number of Identifying the number of trees in a clumptrees in a clump

Surveyed trees > 6 m

Identifying the number of Identifying the number of trees in a clumptrees in a clump

Surveyed trees > 8 m

Identifying the number of Identifying the number of trees in a clumptrees in a clump

Surveyed trees > 10 m

Identifying the number of Identifying the number of trees in a clumptrees in a clump

Surveyed trees > 12 m

Number of trees in a Number of trees in a ClumpClump

p < 0.0001

p =0.11p = 0.007p < 0.0001

p = 0.37p = 0.83p = 0.04p = 0.0001

p < 0.0001

p < 0.0001p < 0.0001

H0: Median of Differences = 0

• Surveyed stem locations were not accurate enough to Surveyed stem locations were not accurate enough to compare locations of treetopscompare locations of treetops

• Can resolve only overstory treesCan resolve only overstory trees

• Next step- delineating individual overstory crownsNext step- delineating individual overstory crowns

• Useful for even aged / managed stands?Useful for even aged / managed stands?

Further thoughts:Further thoughts:

Acknowledgements Acknowledgements

• David Box- 3Di Technologies, Inc.David Box- 3Di Technologies, Inc.

• Dr. Roger Hoffer- Colorado State UniversityDr. Roger Hoffer- Colorado State University

• Dr. Merrill Kaufmann- USFS Rocky Mountain Dr. Merrill Kaufmann- USFS Rocky Mountain Research StationResearch Station

• Dr. Luis Garcia- Colorado State UniversityDr. Luis Garcia- Colorado State University