GEOMORPHIC CHANGE DETECTION

April 30th to May 1st , 2014

Sponsored by:

I. THRESHOLDING ALTERNATIVES IN GCD

WORKSHOP PLAN – DAY 1… (AFTERNOON)

E. Essential Best Practices to Support GCD

F. Traditional Approaches to Change Detection

G. Change Detection in Raster Calculator vs. GCD

H. Getting to know GCD Software

I. Thresholding Alternatives

J. Recap & Preview of Day 2

1:00 to 1:45

A Plan.. Not a Contract

1:45 to 2:15

2:15 to 3:00

3:15 to 4:00

4:00 to 4:45

4:45 to 5:00

3:00 to 3:15 BREAK

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 6

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

GCD 6 THRESHOLDING

• Simple defined minLoD

• Propagated Errors

• Probabilistic Confidence Interval

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

APPLICATION OF A MINLoD

• You take original DoD, and remove all changes <= minLoD

• For example +/- 20 cm

VARYING minLoD THRESHOLDS

22zzz

newold DEMDEM

© Wheaton 2008

newoldDEMDEMf z,zz

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

MINLoD USING ERROR PROPAGATION

• Distinguish those changes that are real from noise

• Use standard Error Propagation

• DEM Errors can vary temporally and spatially

z z DEM old

2

z DEM new

2

Elevation (Time 1)

Elevation (Time 2)

See •Brasington et al (2000): ESPL

•Lane et al (2003): ESPL

•Brasington et al (2003): Geomorphology

z 10 2 20

2 22.36

22.36 cm ≈ 8.8 in

e.g.

z DEM old

10cm

z DEM new

20cm

ERROR PROPAGATION GETS APPLIED SAME WAY AS MINLoD

• Does not matter whether the minLoD is specified, or calculated from error propagation

• Just on a cell-by-cell basis!

• In background a perror grid is produced

WHAT ARE TYPICAL ERRORS?

• LiDaR : +/- 12 to 25 cm• Aerial Photogrammetry : +/- 10 to 15 cm

• Total Station Surveys : +/- 2 to 10 cm

• GPS: : +/- 3 to 12 cm• Terrestrial Laser Scanning: +/-

0.5 to 4 cm

Ground-Based Surveys

Remotely Sensed or Aerial Surveys

SO WHAT WOULD PROPAGATED ERRORS BE?

• LiDaR : +/- 12 to 25 cm (17 to 36 cm

minLoD)• Aerial Photogrammetry : +/- 10 to 15

cm(14 to 22 cm minLoD)

• Total Station Surveys : +/- 2 to 10 cm (3 to 14 cm minLoD)

• GPS: : +/- 3 to 12 cm (4 to 17 cm minLoD)

• Terrestrial Laser Scanning: +/-0.5 to 4 cm (0.7 to 6 cm minLoD)

Ground-Based Surveys

Remotely Sensed or Aerial Surveys

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

HOW COULD I REPRESENT AS PROBABILITY?

• Using inferential statistics, we’ll calculate a t-score

• σDoD is the characteristic uncertainty

– In this case σDoD =

minLoD

• Just the ratio of actual change to minLoD change

• Assuming two-tailed test, t is significant at:

– 68% confidence limit when t= 1

– 95% confidence limit when t=1.96

t zDEM new

zDEM old

DoD

PROBABILITY THAT CHANGE IS REAL

Even when minLoD is spatially constant, probability varies in space… why?

© Wheaton (2008)

SENSITVITY OF THRESHOLD?

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

WHERE DOES THIS FIT?

• Choosing the Threshold Method

GCD 6 THRESHOLDING - TUTORIAL

• Simple defined minLoD

• Propagated Errors

• Probabilistic Confidence Interval

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting

CONCEPT: EXCLUSION OR SUBTRACTION?

• LoD raster must be applied independently to cut raster and fill raster to preserve signage.

• Is it better to exclude points whose LoD > DoD or subtract LoD from DoD?

Slide from Greg Pasternack(2012) – USBoR Lecture

COMPARISON OF EXCLUSION VS. SUBTRACTION

• Blue is deposition; grey is within 1’; others are erosion

• Subtraction is always a more conservative estimate…

Slide from Greg Pasternack(2012) – USBoR Lecture

Exclusion raster

Subtraction raster

Net Change33,456 yds3

Net Change17,555 yds3

Raw DoD

WEIGHTING BELOW THE THESHOLD

• Instead of subtraction or exclusion, values below the threshold can be weighted based on their probabilities…

• For example, if the probability that change is real is 80%, for a vertical change of + 10 cm, the value used could be 8 cm.

See Lane SN, Westaway RM and Hicks DM (2003). DOI: 10.1002/esp.483.

DETAIL PLAN – H.

H. THRESHOLDING ALTERNATIVES FOR GCD

1. Three Types in GCD 5

2. Recall minLoD

3. Error Propagation

4. Probabilistic Thresholding

5. Tutorial

6. Thresholding by Subtraction vs. Exclusion vs. Weighting