Change Detection Goal: Use remote sensing to detect change on a landscape over time.
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Transcript of Change Detection Goal: Use remote sensing to detect change on a landscape over time.
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Change Detection
• Goal: Use remote sensing to detect change on a landscape over time
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Change Detection
• Plan for today– What is change?– Avoiding “uninteresting” change – Methodologies– Dr. Ripple: Examples
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Change Detection
• To use remote sensing, the change must be detectable with our instruments– Spectrally
• Distinguish use from cover• Allow sufficient time between images for changes to be
noticeable
– Spatially• Generally, grain size of change event >> pixel size
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Change Detection
• Write a list of potential “changes” that you think might be interesting to observe with remote sensing– Any type of remote sensing– Any period of time over which change occurs
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Change Detection
• Describing change– abrupt vs. subtle– human vs. natural – “real” vs. detected
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Change Detection
• We need to separate interesting change from uninteresting change
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Change Detection: Uninteresting Change?
• Phenological changes– Use anniversary date image acquisition
• Sun angle effects– Radiometrically calibrate– Use anniversary date image acquisition
• Atmospheric effects– Radiometrically calibrate
• Geometric– Ensure highly accurate registration
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Change Detection: Radiometric Calibration
• Minimize atmospheric, view and sun angle effects• Radiometric normalization
– Histogram equalization or match– Noise reduction– Haze reduction
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Change Detection: Radiometric Calibration
• Histogram matching
Band 4
Pixe
l Cou
nt
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Change Detection: Radiometic Calibration
• Regression Approach
Band 4, 1999
Ban
d 4,
198
8
Dark Objects
Light Objects
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Change Detection
• Atmospheric correction– Model atmospheric effects using radiative transfer
models• Aerosols, water vapor, absorptive gases
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Change Detection Methods
Complexity groupings Techniques
Linear procedures Difference imagesRatioed images
Classification routines Post-classification change detectionSpectral change pattern analysisLogical pattern change detectionRadiance vector shift
Transformed data sets Albedo difference imagesPrincipal components analysisVegetation indices
Others Regression analysisKnowledge-based expert systems
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Change Detection: Methods
• Basic model:– Inputs:
• Landsat TM image from Date 1• Landsat TM image from Date 2
– Potential output:• Map of change vs. no-change• Map describing the types of change
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Change Detection: Methods
• Display bands from Dates 1 and 2 in different color guns of display– No-change is greyish– Change appears as non-grey
• Limited use– On-screen delineation– Masking
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Change Detection: Methods
• Image differencing– Date 1 - Date 2– No-change = 0– Positive and negative values interpretable– Pick a threshold for change– Often uses vegetation index as start point, but not
necessary
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Image Differencing
8 10 8 11
240 11 10 22
205 210 205 54
220 98 88 46
5 9 7 10
97 9 8 22
98 100 205 222
103 98 254 210
3 1 1 1
143 2 2 0
107 110 0 -168
117 0 -166 -164
Image Date 1
Image Date 2 Difference Image = Image 1 - Image 2
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Change Detection: Methods
• Image differencing: Pros– Simple (some say it’s the most commonly used method)– Easy to interpret– Robust
• Cons:– Difference value is absolute, so same value may have
different meaning depending on the starting class– Requires atmospheric calibration for expectation of “no-
change = zero”
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Change Detection: Methods
• Image Ratioing– Date 1 / Date 2– No-change = 1– Values less than and greater than 1 are interpretable– Pick a threshold for change
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Change Detection: Methods
• Image Ratioing: Pros– Simple– May mitigate problems with viewing conditions, esp.
sun angle• Cons
– Scales change according to a single date, so same change on the ground may have different score depending on direction of change; I.e. 50/100 = .5, 100/50 = 2.0
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Change DetectionImage Difference (TM99 – TM88) Image Ratio (TM99 / TM88)
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Change Detection: Methods
• Change vector analysis– In n-dimensional
spectral space, determine length and direction of vector between Date 1 and Date 2
Band 3
Ban
d 4 Date 1
Date 2
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Change Detection: Methods
• No-change = 0 length• Change direction may be
interpretable• Pick a threshold for change
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Change Detection: Methods
• Change detection: Pros– Conceptually appealing– Allows designation of the type of change occurring
• Cons– Requires very accurate radiometric calibration– Change value is not referenced to a baseline, so
different types of change may have same change vector
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Change Detection: Methods
• Post-classification (delta classification)– Classify Date 1 and Date 2 separately, compare class
values on pixel by pixel basis between dates
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Change Detection: Methods
• Post-classification: Pros– Avoids need for strict radiometric calibration– Favors classification scheme of user– Designates type of change occurring
• Cons– Error is multiplicative from two parent maps– Changes within classes may be interesting
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Change Detection: Methods
• Composite Analysis– Stack Date 1 and Date 2 and run unsupervised
classification on the whole stack
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Change Detection: Methods
• Composite Analysis: Pros– May extract maximum change variation– Includes reference for change, so change is anchored at
starting value, unlike change vector analysis and image differencing
• Cons– May be extremely difficult to interpret classes
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Change Detection: Summary
• Radiometric, geometric calibration critical• Minimize unwanted sources of change
(phenology, sun angle, etc.)• Differencing is simple and often effective• Post-classification may have multiplicative error• Better to have a reference image than not