GEOGG121: Methods Inversion I: linear approaches Dr. Mathias (Mat) Disney UCL Geography Office: 113,...

GEOGG121: MethodsInversion I: linear approaches

Dr. Mathias (Mat) Disney

UCL Geography

Office: 113, Pearson Building

Tel: 7670 0592

Email: mdisney@ucl.geog.ac.uk

www.geog.ucl.ac.uk/~mdisney

• Linear models and inversion– Least squares revisited, examples– Parameter estimation, uncertainty– Practical examples

• Spectral linear mixture models• Kernel-driven BRDF models and change detection

Lecture outline

• Linear models and inversion– Linear modelling notes: Lewis, 2010– Chapter 2 of Press et al. (1992) Numerical Recipes in C (online

version http://apps.nrbook.com/c/index.html)– http://en.wikipedia.org/wiki/Linear_model– http://en.wikipedia.org/wiki/System_of_linear_equations

Reading

Linear Models

• For some set of independent variables

x = {x0, x1, x2, … , xn}

have a model of a dependent variable y which can be expressed as a linear combination of the independent variables.

110 xaay

22110 xaxaay

ni

iii xay

0

xay

Linear Models?

ni

iiiii xbxaay

10 cossin

ni

iiii bxaay

10 sin

nn

ni

i

ii xaxaxaaxay 0

202010

00 ...

xaeay 10

xay

Linear Mixture Modelling

• Spectral mixture modelling:– Proportionate mixture of (n) end-member spectra

– First-order model: no interactions between components

11

0

ni

i iF

1

0

ni

i iiFr Fr

Linear Mixture Modelling

• r = {r 0l , r 1l , … rlm, 1.0} – Measured reflectance spectrum (m wavelengths)

• nx(m+1) matrix:

1

2

1

0

112111101

11210101

10201000

1

1

0

0.10.10.10.10.1 n

nmmmm

n

n

m

P

P

P

P

r

r

r

Fr

Linear Mixture Modelling

• n=(m+1) – square matrix

• Eg n=2 (wavebands), m=2 (end-members)

Fr

rF 1

Reflectance

Band 1

Reflectance

Band 2

r1

r2

r3

r

Linear Mixture Modelling

• as described, is not robust to error in measurement or end-member spectra;

• Proportions must be constrained to lie in the interval (0,1) – - effectively a convex hull constraint;

• m+1 end-member spectra can be considered;• needs prior definition of end-member spectra; cannot

directly take into account any variation in component reflectances

– e.g. due to topographic effects

Linear Mixture Modelling in the presence of Noise

• Define residual vector• minimise the sum of the squares of the error e,

i.e.

eFr

ee

eeFrFrFrml

l

21

0

Method of Least Squares (MLS)

Error Minimisation

• Set (partial) derivatives to zero

021

0

21

0

ml

lii

ml

l

F

FFr

P

Fr

eeFrFrFrml

l

21

0

iiFF

1

0

1

0

1

020

ml

l i

ml

l i

ml

l i

Fr

Fr

Error Minimisation

• Can write as:

PMO

1

0

1

0

ml

l i

ml

l i Fr

1

1

0

1

0

111110

111110

010100

1

0

1

1

0

n

ml

l

nlnlnllnll

lnlllll

lnlllll

ml

l

nll

ll

ll

F

F

F

r

r

r

Solve for P by matrix inversion

e.g. Linear Regression

mxcy

PMO

m

c

xx

x

xy

y nl

l ll

lnl

l ll

l1

02

1

0

1

m

c

xx

x

yx

y2

1

x

xyy

xx

xy

xx

xyxx

2

2

2

22

1

1 2

2

1

x

xxM

xx

222 xxxx

RMSE

1

0

22nl

lii mxcye

mnRMSE

2

y

xx x1x2

Weight of Determination (1/w)

• Calculate uncertainty at y(x)

m

c

xPQxy

1

QMQw

T 11

we

1

2

2

11

xx

xx

w

P0

P1RMSE

P0

P1RMSE

Issues

• Parameter transformation and bounding• Weighting of the error function• Using additional information• Scaling

Parameter transformation and bounding

• Issue of variable sensitivity– E.g. saturation of LAI effects– Reduce by transformation

• Approximately linearise parameters• Need to consider ‘average’ effects

Weighting of the error function

• Different wavelengths/angles have different sensitivity to parameters

• Previously, weighted all equally– Equivalent to assuming ‘noise’ equal for all observations

Ni

i

Ni

imeasured ii

RMSE

1

1

2modelled

1

Weighting of the error function

• Can ‘target’ sensitivity– E.g. to chlorophyll concentration– Use derivative weighting (Privette 1994)

Ni

i

Ni

imeasured

P

iiP

RMSE

1

21

2

modelled

Using additional information

• Typically, for Vegetation, use canopy growth model– See Moulin et al. (1998)

• Provides expectation of (e.g.) LAI– Need:

• planting date• Daily mean temperature• Varietal information (?)

• Use in various ways– Reduce parameter search space– Expectations of coupling between parameters

Scaling

• Many parameters scale approximately linearly– E.g. cover, albedo, fAPAR

• Many do not– E.g. LAI

• Need to (at least) understand impact of scaling

Crop Mosaic

LAI 1 LAI 4 LAI 0

Crop Mosaic

• 20% of LAI 0, 40% LAI 4, 40% LAI 1. • ‘real’ total value of LAI:

– 0.2x0+0.4x4+0.4x1=2.0.

LAI 1

LAI 4

LAI 0

)2/exp())2/exp(1( LAILAI s

visible: NIR 1.0;2.0 s

3.0;9.0 s

canopy reflectance

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49

LAI

refl

ect

ance

visible

NIR

canopy reflectance over the pixel is 0.15 and 0.60 for the NIR.

• If assume the model above, this equates to an LAI of 1.4. • ‘real’ answer LAI 2.0

Linear Kernel-driven Modelling of Canopy Reflectance

• Semi-empirical models to deal with BRDF effects– Originally due to Roujean et al (1992)– Also Wanner et al (1995)– Practical use in MODIS products

• BRDF effects from wide FOV sensors– MODIS, AVHRR, VEGETATION, MERIS

Satellite, Day 1 Satellite, Day 2

X

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

13

6

14

3

15

0

15

7

16

4

17

1

17

8

18

5

19

2

19

9

20

6

21

8

22

6

23

3

24

0

24

7

25

4

26

1

26

8

27

5

28

2

Julian Day

ND

VI

original NDVI MVC BRDF normalised NDVI

AVHRR NDVI over Hapex-Sahel, 1992

Linear BRDF Model

• of form:

,,,, geogeovolvoliso kfkff

Model parameters:

Isotropic

Volumetric

Geometric-Optics

Linear BRDF Model

• of form: ,,,, geogeovolvoliso kfkff

Model Kernels:

Volumetric

Geometric-Optics

Volumetric Scattering

• Develop from RT theory– Spherical LAD– Lambertian soil– Leaf reflectance = transmittance– First order scattering

• Multiple scattering assumed isotropic

Xs

Xl ee

12

cossin

3

2,1

2

LX

Volumetric Scattering

• If LAI small:

Xe X 1

Xs

Xl ee

1

2cossin

3

2,1

2

LX

2

12

2cossin

3

2,1 LL

sl

sl L

2

2cossin

3

2,1

Volumetric Scattering

• Write as:

sl L

2

2cossin

3

2,1

,,, 10 volthin kaa

2

2cossin

,

volk

slL

a

60

31

lLa

RossThin kernel

Similar approach for RossThick

LBL

exp2

exp

Geometric Optics

• Consider shadowing/protrusion from spheroid on stick (Li-Strahler 1985)

h

b

r

A()

Projection (shadowed)

Sunlit crownshadowed crown

shadowed ground

h

b

r

A()

Projection (shadowed)

Sunlit crownshadowed crown

shadowed ground

Geometric Optics

• Assume ground and crown brightness equal• Fix ‘shape’ parameters• Linearised model

– LiSparse– LiDense

Kernels

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

-75 -60 -45 -30 -15 0 15 30 45 60 75

view angle / degrees

ke

rne

l va

lue

RossThick LiSparse

Retro reflection (‘hot spot’)

Volumetric (RossThick) and Geometric (LiSparse) kernels for viewing angle of 45 degrees

Kernel Models

• Consider proportionate (a) mixture of two scattering effects

,,1

1,,

11

00

geogeovolvol

multgeovol

kaka

aa

Using Linear BRDF Models for angular normalisation• Account for BRDF variation• Absolutely vital for compositing samples

over time (w. different view/sun angles)• BUT BRDF is source of info. too!

MODIS NBAR (Nadir-BRDF Adjusted Reflectance MOD43, MCD43)http://www-modis.bu.edu/brdf/userguide/intro.html

MODIS NBAR (Nadir-BRDF Adjusted Reflectance MOD43, MCD43)http://www-modis.bu.edu/brdf/userguide/intro.html

BRDF Normalisation• Fit observations to model• Output predicted reflectance at standardised

angles – E.g. nadir reflectance, nadir illumination

• Typically not stable

– E.g. nadir reflectance, SZA at local mean

KP ,,

geo

vol

iso

f

f

f

P

,

,

1

geo

vol

k

kK QMQw

T 11

And uncertainty via

Linear BRDF Models to track change

• Examine change due to burn (MODIS)

FROM: http://modis-fire.umd.edu/Documents/atbd_mod14.pdf

220 days of Terra (blue) and Aqua (red) sampling over point in Australia, w. sza (T: orange; A: cyan).

Time series of NIR samples from above sampling

MODIS Channel 5 Observation

DOY 275

MODIS Channel 5 Observation

DOY 277

Detect Change

• Need to model BRDF effects• Define measure of dis-association

wee

predictedobservedpredictedobserved

11

22

MODIS Channel 5 Prediction

DOY 277

MODIS Channel 5 Discrepency

DOY 277

MODIS Channel 5 Observation

DOY 275

MODIS Channel 5 Prediction

DOY 277

MODIS Channel 5 Observation

DOY 277

So BRDF source of info, not JUST noise!

• Use model expectation of angular reflectance behaviour to identify subtle changes

5454Dr. Lisa Maria Rebelo, IWMI, CGIAR.

Detect Change

• Burns are:– negative change in Channel 5– Of ‘long’ (week’) duration

• Other changes picked up– E.g. clouds, cloud shadow– Shorter duration – or positive change (in all channels)– or negative change in all channels

Day of burn

http://modis-fire.umd.edu/Burned_Area_Products.htmlRoy et al. (2005) Prototyping a global algorithm for systematic fire-affected area mapping using MODIS time series data, RSE 97, 137-162.