1 Information Extraction Principles for Hyperspectral Data David Landgrebe Professor of Electrical &...

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1

Information Extraction Principles for Hyperspectral Data

David LandgrebeProfessor of Electrical & Computer Engineering

Purdue Universitylandgreb@ecn.purdue.edu

• A Historical Perspective• Data and Analysis Factors• Hyperspectral Data Characteristics• Examples• Summary of Key Factors

Outline

2

1957 - Sputnik

REMOTE SENSING OF THE EARTH

Atmosphere - Oceans - Land

Brief History

1958 - National Space Act - NASA formed

1960 - TIROS I

1960 - 1980 Some 40 Earth Observational Satellites Flown

3

Image Pixels

Enlarged 10 Times

Thematic Mapper Image

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Three Generations of Sensors

Band No.

Re

lativ

e R

esp

on

se

05

101520253035404550

1 2 3 4

Green Veg.

Bare Soil6-bit data

MSS1968

Band No.

Re

lati

ve

Re

sp

on

se

0

50

100

150

200

1 2 3 4 5 6 7

Green Veg.

Bare Soil

8-bit data

TM1975

Wavelength (µm)

Re

lativ

e R

ad

ian

ce

Re

sp

on

se

0

500

1000

1500

2000

2500

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

Water

Emerging Crop

Trees

Soil

10-bit data

Hyperspectral1986

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Systems View

Sensor On-BoardProcessing

PreprocessingData

AnalysisInformationUtilization

Human Participationwith Ancillary Data

Ephemeris,Calibration, etc.

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Scene Effects on Pixel

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Data Representations

Image Space Spectral Space

0

1000

2000

3000

0.40 0.80 1.20 1.60 2.00 2.40

Wavelength (µm)

Water Trees Soil

Feature Space

0

200

400

600

800

1000

1200

1400

1600

0 500 1000 1500 2000

Band 0.60

Water

Trees

Soil

Sample

• Image Space - Geographic Orientation

• Feature Space - For Use in Pattern Analysis• Spectral Space - Relate to Physical Basis for Response

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Data Classes

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SCATTER PLOT FOR TYPICAL DATA

30

60

90

120

150

180

210

17 34 51 68 85 102

BiPlot of Channels 4 vs 3

Channel 3

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10

BHATTACHARYYA DISTANCE

B 1

81 2

T 1 2

2

1

1 2 1

2Ln

1

21 2

1 2

Mean Difference Term Covariance Term

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Vegetation in Spectral Space

Laboratory Data: Two classes of vegetation

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Scatter Plots of Reflectance

0.720.710.700.690.680.670.660.6510

12

14

16

18

20

22

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Class 1 - 0.67 µm

Class 2 - 0.67 µmClass 1 - 0.69 µm

Class 2 - 0.69 µm

Scatter of 2-Class Data

Wavelength - µm

Refl

ect

ance

- %

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Vegetation in Feature Space

15141312111016

17

18

19

20

21

22

23

Class 1

Class 2

Samples from Two Classes

% Reflectance at 0.67 µm

% R

efl

ecta

nce a

t 0

.69

µm

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Hughes Effect

m=25

10

20

50100

200

1000

500

m =

1 100050020010050201052

MEASUREMENT COMPLEXITY n (Total Discrete Values)

0.50

0.55

0.60

0.65

0.70

0.75M

EA

N R

EC

OG

NIT

ION

AC

CU

RA

CY

G.F. Hughes, "On the mean accuracy of statistical pattern recognizers," IEEE Trans. Inform. Theory., Vol IT-14, pp. 55-63, 1968.

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A Simple Measurement Complexity Example

16

Classifiers of Varying Complexity

• Quadratic Form

gi(X) = 1

2(X i )

T i 1(X i )

1

2ln i

• Fisher Linear Discriminant - Common class covariance

gi(X) = 1

2(X i )

T 1(X i )

• Minimum Distance to Means - Ignores second moment

gi(X) = 1

2(X i )

T (X i )

17

Classifier Complexity - con’t• Correlation Classifier

gi(X) XT i

XTX iT i

• Spectral Angle Mapper

gi(X) cos 1 XTiXTX i

Ti

• Matched Filter - Constrained Energy Minimization

gi(X) XTCb

1i iTCb

1 i• Other types - “Nonparametric”

Parzen Window Estimators Fuzzy Set - based Neural Network implementations K Nearest Neighbor - K-NN etc.

18

Covariance Coefficients to be Estimated

• Assume a 5 class problem in 6 dimensions

• Normal maximum likelihood - estimate coefficients a and b• Ignore correlation between bands - estimate coefficients b

• Ignore correlation between bands - estimate coefficients d

Class 1 Class 2 Class 3 Class 4 Class 5b b b b ba b a b a b a b a ba a b a a b a a b a a b a a ba a a b a a a b a a a b a a a b a a a ba a a a b a a a a b a a a a b a a a a b a a a a ba a a a a b a a a a a b a a a a a b a a a a a b a a a a a b

• Assume common covariance - estimate coefficients c and d

Common Covar.dc dc c dc c c dc c c c dc c c c c d

19

EXAMPLE SOURCES OFCLASSIFICATION ERROR

Decision boundary defined by the

diagonal covariance classifier

class 2

class 1

Decision boundary defined by Gaussian ML classifier

20

Number of Coefficients to be Estimated

• Assume 5 classes and p features

No. ofFeatures p

Class Covar.

(a & b above)5{{ p+1)p/2}

Diagonal ClassCommon Covar.

(b above)5p

CommonCovar.

(c & d above){ p+1)p/2}

Diagonal CommonCovar.

(d above)p

5 75 25 15 510 275 50 55 1020 1050 100 210 2050 6375 250 1275 50

200 100,500 1000 20,100 200

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Intuition and Higher Dimensional Space

Borsuk’s Conjecture: If you break a stick in two, both pieces are shorter than the original.

Keller’s Conjecture: It is possible to use cubes (hypercubes) of equal size to fill an n-dimensional space, leaving no overlaps nor underlaps.

Science, Vol. 259, 1 Jan 1993, pp 26-27

Counter-examples to both have been found for higher dimensional spaces.

22

The Geometry of High Dimensional Space

The Volume of a Hypercube concentrates in the corners

0.6

1 2 3 4 5 6 70

0.2

0.4

0.8

1

dimension d

The Volume of a Hypersphereconcentrates in the outer shell

1 2 3 4 5 6 7 8 9 10 110

0.2

0.4

0.6

0.8

1

dimension d

Vd (r ) Vd (r )

Vd (r)rd (r )d

rd1 1

r

d

d 1

V hypersphere

Vhypercube

d

2

d2d 1 d2

d 0

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Some Implications

High dimensional space is mostly empty. Data in high dimensional space is mostly in a lower dimensional structure.

Normally distributed data will have a tendency to concentrate in the tails; Uniformly distributed data will concentrate in the corners.

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Volume of a hypersphere =2rd

dd / 2

(d / 2)

How can that be?

dVdr

2d / 2

(d / 2)r (d 1)

Differential Volume at r =

0 1 2 3 4 50

20

40

60

80

Distance from Class Mean, r

1

2

3 4 5

Surface of Hypersphere

Volumn of shell

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How can that be? (continued)

rd 1e r

2

2

2d2 1d2

The Probability Mass at r =

0 1 2 3 4 50

0.2

0.4

0.6

0.8

Distance from Class Mean, r

1

2 3 4 5 10 15 20

Probability Density of Distance r

Probability mass in shell

26

MORE ON GEOMETRY

• The diagonals in high dimensional spaces become nearly orthogonal to all coordinate axes

cos d 1d

Implication: The projectionof any cluster onto anydiagonal, e.g., by averagingfeatures could destroy information

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STILL MORE GEOMETRY

• The number of labeled samples needed for supervised classification increases rapidly with dimensionality

In a specific instance, it has been shown that the samples required for a linear classifier increases linearly, as the square for a quadratic classifier. It has been estimated that the number increases exponentially for a non-parametric classifier.

• For most high dimensional data sets, lower dimensional linear projections tend to be normal or a combination of normals.

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A HYPERSPECTRAL DATA ANALYSIS SCHEME

200 Dimensional Data

Class ConditionalFeature Extraction

FeatureSelection

Classifier/Analyzer

Class-SpecificInformation

29

Finding Optimal Feature Subspaces

• Feature Selection (FS)

• Discriminant Analysis Feature Extraction (DAFE)

• Decision Boundary Feature Extraction (DBFE)

• Projection Pursuit (PP)

.Available in MultiSpec via WWW at: http://dynamo.ecn.purdue.edu/~biehl/MultiSpec/Additional documentation via WWW at: http://dynamo.ecn.purdue.edu/~landgreb/publications.html

30

Hyperspectral Image of DC Mall

HYDICE Airborne System1208 Scan Lines, 307 Pixels/Scan Line210 Spectral Bands in 0.4-2.4 µm Region155 Megabytes of Data(Not yet Geometrically Corrected)

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Define Desired Classes

Training areas designated by polygons outlined in white

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Thematic Map of DC Mall

Legend Operation CPU Time (sec.) Analyst TimeDisplay Image 18Define Classes < 20 min.Feature Extraction 12Reformat 67Initial Classification 34Inspect and Mod. Training ≈ 5 min.Final Classification 33

Total 164 sec = 2.7 min. ≈ 25 min.

Roofs

Streets

Grass

Trees

Paths

Water

Shadows

(No preprocessing involved)

33

Hyperspectral Potential - Simply Stated

• Assume 10 bit data in a 100 dimensional space.• That is (1024)100 ≈ 10300 discrete locations

Even for a data set of 106 pixels, the probability

of any two pixels lying in the same discrete location

is vanishingly small.

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Summary - Limiting Factors

PreprocessingData

AnalysisInformationUtilization

Human Participationwith Ancillary Data

Sensor On-BoardProcessing

Ephemeris,Calibration, etc. • Scene - The most complex

and dynamic part

• Sensor - Also not under analyst’s control

• Processing System - Analyst’s choices

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Limiting Factors

Scene - Varies from hour to hour and sq. km to sq. km

Sensor - Spatial Resolution, Spectral bands, S/N

Processing System -

• Classes to be labeled

• Number of samples to define the classes

• Complexity of the Classifier

• Features to be used

- Exhaustive,

- Separable,- Informational Value,

36

Source of Ancillary Input

Possibilities

• Ground Observations

• “Imaging Spectroscopy”

- From the Ground

- Of the Ground

• Previously Gather Spectra

• “End Members”

Image Space

Spectral Space

Feature Space

.

37

Use of Ancillary Input

A Key Point:

• Ancillary input is used to label training samples.

• Training samples are then used to compute class quantitative descriptions

Result:

• This reduces or eliminates the need for many types of preprocessing by normalizing out the difference between class descriptions and the data