uncertainties in ocean colour remote sensing …...2014/07/27 · 1.10 1.15 1.20 1.25 1.30 1.35...
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2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
2nd IOCCG Summer Lecture Series
Villefranche-sur-Mer, France
21 July – 2. August
Uncertainties in ocean colour remote sensing
Lecure 2: algorithms, validation
Roland Doerffer
Retired from Helmholtz Zentrum Geesthacht
Institute of Coastal Research
Now: Brockmann Consult
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Algorithms and uncertainties
• Inversion schemes
• Saturation and masking effects
• Out of scope conditions
• Verification
• Validation
• Strategies for validation
• The OC-CCI approach
• Summary and conclusions
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Case 1 water algorithm based on reflectance ratio model R445 / R555
Morel / Antoine MERIS Case 1 water ATBD
Chl =a[R(445) / R(555)]b
Case 1 water:
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Water leaving radiance reflectance spectra in coastal water
300 400 500 600 700 800 900
0.00E+000
5.00E-003
1.00E-002
1.50E-002
2.00E-002
2.50E-002
Pin 6
300 400 500 600 700 800 900
0.00E+000
5.00E-004
1.00E-003
1.50E-003
2.00E-003
2.50E-003
3.00E-003
3.50E-003
4.00E-003
4.50E-003
Pin 5
300 400 500 600 700 800 900
0.00E+000
5.00E-004
1.00E-003
1.50E-003
2.00E-003
2.50E-003
3.00E-003
3.50E-003
4.00E-003
4.50E-003
5.00E-003
Pin 1
300 400 500 600 700 800 900
0.00E+000
2.00E-003
4.00E-003
6.00E-003
8.00E-003
1.00E-002
1.20E-002
Pin 2
300 400 500 600 700 800 900
0.00E+000
1.00E-003
2.00E-003
3.00E-003
4.00E-003
5.00E-003
6.00E-003
Pin 3
300 400 500 600 700 800 900
0.00E+000
2.00E-004
4.00E-004
6.00E-004
8.00E-004
1.00E-003
1.20E-003
1.40E-003
1.60E-003
1.80E-003
2.00E-003
Pin 4
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Variability of water leaving reflectance spectra
300 400 500 600 700 800 900
0.00E+000
5.00E-003
1.00E-002
1.50E-002
2.00E-002
2.50E-002
Water leaving radiance reflectance spectra
MERIS North Sea 20060726
Pin 1
Pin 2
Pin 3
Pin 4
Pin 5
Pin 6
wavelength (nm)
RLw
(sr-
1)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
The inverse problem
• Matrix inversion
• Inversion by optimization
• Inversion by neural network
reflectance
spectrum
sun zenith
view zenith
azimuth diff
pure water
phytoplankton
suspended matter
dissolved org. matter
sun zenith
view zenith
azimuth diff
inverse model
forward model
Success depends on:
• Bio-optical model
• ambiguities
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Inverse Modellierung using Optimization Procedures
start values
model
parameters
IOP / conc.
radiative
transfer
modell
reflectance-
spectra
simulated
reflectance-
spectra
Satellite
do spectra
agree ?
parameters
are the
IOPs / conc.
change
parameters
determine
search direction
downhill in cost function
no
yes
test = ∑(Rsim(i) – Rsat(i))2
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Simplified scheme of NN Algorithm
y s d w s c v s b u xl l kl k jk j ij iijk
( ( ( )))1
4
1
5
1
3
o s bias w xi i ( )incominglinksW iXi
X1
X2
X3
o
X4
R1
R2
R3
W
Chlorophyll
Susp. Matter
Gelbstoff
Hidden layer Input layer:
reflectances
and angles
Output layer:
concentrations
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Sensitivity at different concentration ranges and spectral bands
0 10 20 30 40 50 60 700
0.005
0.01
0.015
0.02
0.025
0.03
SPM [g m-3]
RLw
[sr-1
]
a_gelb_440: 0.2, a_part_440: SPM/25, pig: 2 mg m-3
band 1 = 412 nm
band 5 = 560 nm
band 9 = 708 nm
RLw for MERIS bands 1 (412 nm), 6 (560 nm), 10 (708 nm)
Sensitivity of the
reflectance at a
spectral band
depends on the
concentration
To cover a large
concentration range
many bands from
the blue to NIR
range are necessary
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Sensitivity at different concentration ranges and spectral bands
300 400 500 600 700 800 900
0.00E+000
1.00E-003
2.00E-003
3.00E-003
4.00E-003
5.00E-003
6.00E-003
7.00E-003
8.00E-003
9.00E-003
Remote Sensing reflectance TSM 1
Rsr 5_01_1
Rsr 10_01_1
wavelength (nm)
Rsr
(sr-
1)
300 400 500 600 700 800 900
0.00E+000
1.00E-002
2.00E-002
3.00E-002
4.00E-002
5.00E-002
6.00E-002
Remote Sensing reflectance TSM 100
Rsr 5_01_100
Rsr 10_01_100
wavelength (nm)R
sr
(sr-
1)
Chl. 5/10 mg m-3
TSM 1 g m-3
aYS(443) 0.1 m-1
Chl. 5/10 mg m-3
TSM 100 g m-3
aYS(443) 0.1 m-1
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Searching for minimum: principle, 1D case
Independent variable
Search for minimum: Deviation between measured and simulated spectrum
devia
tion
Acceptance level
Independent variable
devia
tion
Acceptance level
Width can be estimated from the 2nd order derivative (Hessean matrix)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Error due to masking and ambiguities
300 400 500 600 700 800 900 1000 1100
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
simulated reflectance test
error 0.01 stdev
rlw_true
rlw_err
rlw_retr
wavelength [nm]
RL
w [s
r-1
]
• True spectrum simulated
• „measured“ spectrum = true * random error
• Retrieved spectrum when LM has found solution
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Results and errors of retrieval
Variable err. estimated % err true %
1 0.8337 0.09191 9.626 -19.94
1 1.152 0.1684 18.34 13.19
0.1 0.1005 0.03566 3.63 0.4842
1 0.9948 0.006498 0.6519 -0.5238
conc true conc retr. stdev of log_conc
chlorophyll [mg m-3]
detritus [g m-3]
gelbstoff a443 [m-1]
min. SPM [g m-3]
kdmin_true: 0.2096
kdmin_ret: 0.2089
error: - 0.33%
kd490_true: 0.2636
kd490_ret: 0.2609
error: -1.04%
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Uncertainties due to ambiguities
for different concentration mixtures
10 -3 10 -2 10 -1 10 0 10 1 10 2
10 -3
10 -2
10 -1
10 0
10 1
10 2
model chlorophyll µg/l
nn
-de
rive
d µ
g/l
case 2 water chlorophyll retrieval with NN
All cases of turbid water
2 mg m-3
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Ambiguities 2
10 -3 10 -2 10 -1 10 0 10 1 10 2
10 -3
10 -2
10 -1
10 0
10 1
10 2
model µg/l
nn
µg/l
Pigment in North Sea Water (gelb < 0.2 m-1, MSM < 5 mg/l)
Typical North Sea coastal water: ay_443: < 0.2 m-1, TSM < 5 mg /l
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Determine uncertainties on a pixel by pixel basis II
0
50
100
0
50
100
0
50
100
tsm
chl
a(gelb+part)
conc_bp
0.05 - 0.5 -5 -45
conc_pig
0.005 - 0.03 - 0.15 - 1.0
conc_gelb
0.005 - 0.035 - 0.2 –0.15
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 -60
-50
-40
-30
-20
-10
0
wavelength µm
wate
r depth
m
Signal depth at different spectral bands
z90=1/k
coastal:
TSM=5 mg/l
Chlor.=5µg/l
Gelb=a380=1m-1
open ocean:
Chlor.=1µg/l
Signal depth z90
pure water
turbid coastel
ocean
Multiband algorithms: the information for each band may come from a
different water layer
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions
• 2 Procedures have been developed
– Combination of an inverse and forward Neural Network
– Use of an autoassociative Neural Network
• Both produce a reflection spectrum, which is compared with the input
spectrum
• Deviation between input and output spectrum can be computed as a chi2
• A threshold can be used to trigger an out of scope warning flag
Inverse
NN IOPs
Forward
NN R Input R output
autoassociative
NN R Input R output
• Combination
of inverse
and forward NN
• Auto-associative
NN
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions (MERIS processor)
…
…
NN input
NN output
r, r’ – log of reflectances
c – log of concentrations
g– geometry information
q – quality indicator
r
g
r
g
cr’
c
cforwNN
invNN
……
………
NN input
NN output
r, r’ – log of reflectances
c – log of concentrations
g– geometry information
q – quality indicator
NN input
NN output
r, r’ – log of reflectances
c – log of concentrations
g– geometry information
q – quality indicator
r
g
r
g
cr’
c
cforwNN
invNN
If RLw < 0.0009, RLw = 0.0009 ( ρ ≈ 0.003)
flag PCD1_16,17
q true if
sum (r(i) / r‘(i)) > 4.0
C:
log(b_tot)
log(a_ys+a_btsm)
log(a_pig)
All at 443 nm
Reflectance
spectrum
Angles:
Sun zenith
View nadir
Azimuth diff
IOPs or
concentrations
Reflectance
spectrum
Output of NN
Comparator
to trigger out
of scope flag Out of
scope
flag
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions (MERIS processor)
Top of atmosphere
radiance spectra
at normal and
critical locations
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions (MERIS processor)
Exeptional bloom,
Indicated by high Chi_square value
Chi_square is computed by
comparing
The input reflectance spectrum
with the output of the forward
NN
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions using an aaNN
• Important to detect toa radiance specta which are not in the simulated
training data set
• These are out of scope of the atmospheric correction algorithm
• Autoassociative neural network with a bottle neck layer
Input
layer Hidden 1
Bottle-
neck
Hidden 3 output
layer
Functions also
as nonlinear PCA
i.e. bottle neck number of
neurons
Provide estimate of
Independent components
For the GAC training data
Set of ~ 1Mio. Cases
Bottleneck minimum was 4-5
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions aaNN:
example for L1 (TOA) data
Transect
High
SPM Sun
glint
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Detection of out of scope conditions aaNN: example
120 121 122 123 124 125 126 127 128 1290.85
0.90
0.95
1.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
AutoNN test 12x5x12 Yellow Sea transect, MERIS band 7, 664.3 nm
longitude
ratio
0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30 1.35 1.400
2
4
6
8
10
12
14
16
18
AutoNN test Yellow Sea transect 12x5x12, MERIS band 7, 664.3 nm
rel. radiance reflectance ratio to true
rel. f
req
ue
ncy120 121 122 123 124 125 126 127 128 129
-0.01
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
AutoNN test 12x5x12 Yellow Sea transect, MERIS band 7, 664.3 nm
longitude
RL
toa
rltoa
rltoa_aNN
difference
significant deviation in area with
high SPM concentrations, but not in
sun glint area
rel. deviation
Histogram of deviations
shows 2 maxima,
around 1 in sun glint
0.9 in high SPM area,
which out of scope
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Verification
• Using simulated test data
• You can detect ambiguities
• Non linear behaviour
• Concentration ranges with failures
• It might be necessary to change bio-optical model
• Or range and frequency distribution of the training data set
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Test of NN I 1
443 nm
1020 nm
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Test of NN I 3
Kd_min Kd_490
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Test of NN 17x27x17, training with 5% random noise on RLw
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Validation
• NOMAD data set
– Compiled, quality checked and maintained by OC group of NASA
– In situ observations from different cruises, different teams, instruments,
procedures, sky and wave conditions
– Includes RLw at 6 MERIS bands (412,443,490, 510, 560,665)
– a_total, b_total / bb_total at443
• Note: in situ data have their own variabilities and uncertainties!
Relationship
between
chlorophyll a
concentration
and the
absorption
coefficient of
phytoplankton
pigments
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Total absorption at 443 nm (water + constituents)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
a443
log10_a443_nn = log10_a443_measured * 0.977 - 0.0167, stdev = 0.141
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Frequency distribution
Frequency distributions of measured and derived a443
after removing outliers with sum_sq > 1.0 e-5
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Measured and nn-derived a443 for all cases with sd <1.0e-5
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Differences and rel.deviation
Mean difference: 0.0086102 m-1, stdev: 0.129
Mean ratio: 0.9717098, stdev: 0.334
Log10 difference Log10 ratio
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Sum_sq of measured and nn derived reflectances
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Test of NN based on measurements for chlorophyll
Log10 scale, red: 1 by 1 line
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Comparison of histograms: measured, NN computed
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
NN for kd489
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Histogram kd489 measured and NN derived
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Uncertainties related to comparison with in situ data
• Error in method and handling, e.g. HPLC for chlorophyll determination
• Sample not representative for water volume of pixel
• Vertical distribution: water comes from a certain depth, e.g. 4 m for
FerryBox
• Temporal difference between sample and satellite overpass
• Sub-pixel patchiness
• Scatter in bio-optical data, e.g. relationship between concentration and IOPs
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Uncertainties: Ocean Colour CCI Approach
• Definiton of optical water classes from reflectance spectra (9 classes)
• Determination of the membership of in situ reflectance data
• Determination of the uncertainties from statistics of available in situ data for
each class
• Determination of the membership of a pixel to different optical water classes
• Weighting of the uncertainties by Fuzzy Logic
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Ocean Colour Climate Change Initiative
Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
OC-CCI Processing System
Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer Courtesy of PML
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Strategies to determine uncertainties for coastal water:
Solutions
• Out of scope detection
– Check of auxilary variabels, e.g. windspeed -> whitecaps
– Check of reflectance in particular bands: NIR reflectance for floating material
– Auto-associative neural network
– Combination of backward and forward neural network (standard MERIS
processing)
– Convergence of optimization procedure on high deviation level
• Ambiguities in bio-optical / reflectance model
– Analysis of simulated data using the bio-optical model
• Uncertainties on a pixel by pixel basis
– Empirical from observations, optional for different optical water classes
– Variations in optimization procedure
– Determination using variations of simulated data set -> look up table, NN
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Summary and Conclusion: uncertainties
• There are a lot of factors, which determine top of atmosphere radiance spectra
• Vice versa the information content of TOA spectra is much too low to determine all of these factors independently
• In complex water the signal can be very low in the blue spectral range
• Atmospheric correction then extremely critical
• In complex water the dominant component might mask the effect of all other components
• In this case the uncertainty range for the subdominant components increases significantly
• Saturation effects limit the accuracy and may cause a shift in the importance of bands
• There are constellations of atmosphere / water which leads to failure in the algorithms
• These out of scope conditions have to be detected and marked per pixel using flags and uncertainty indicators
• Expected errors can be determined by sensitivity studies
• Of high importance is a continuous validation using in situ observations of high quality
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Colour Remote Sensing of complex water is possible!
But:
• Restrict to a small number of components with similar optical properties
• Detection of special cases such as red tides, cyanobacteria
– Exclude or develop special algorithms
• General knowledge about vertical distribution at different seasons
• Bathymetry to estimate possible bottom effects
• Determine penetration depth / z90 depth
• Determine scope of algorithm
• Develop algorithm to determine / flag out of scope conditions
• Determine uncertainties for each product
Atmospheric correction most challenging issue
• Develop special procedures for atmospheric correction over complex waters
• Problems: adjacency effects, floating material
• Determine conditions when AC leads to too large uncertainties
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
MERIS 20070505
Top of atmosphere radiance reflectance RLtoa RGB
New York
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Path radiance+ Fresnel reflectance RLpath MERIS band 5 (560 nm)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Water leaving radiance reflectance RLw MERIS band 5 (560 nm)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Water leaving radiance reflectance RLw MERIS band 2 (443 nm)
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Chlorophyll
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Acknowledgemets
• Supported by various projects
– ESA Case 2 Water Regional Algorithm
– ESA Glint correction
– ESA Water radiance
– ESA CoastColour
– ESA Dragon
– ESA Ocean Colour CCI
– DLR DeMarine
• Neural Network Training software H. Schiller, GKSS
• MERIS data provided by ESA
• Implementation of C2R and Glint correction in BEAM: M. , Brockmann-Consult
2nd IOCCG Summer Lecture Series July 21- Aug.2. 2014, R. Doerffer
Summary and conclusions
• Uncertainty in coastal water products can be large due to the large number of
factors in atmosphere and water, which determine the reflectance spectrum
• Conditions where algorithms (AC & water) fail
• Prerequisite for a successful retrieval are optical models of the atmosphere and the
water, which meet the actual conditions
– Regional models might be necessary
• Reflectance spectra have to be tested if they are within the scope of these models
– Out of scope spectra have to be flagged, treated with special algorithms or
excluded from further processing
• Limited sensitivity of reflectance spectrum and ambiguities lead to an uncertainty
even for spectra, which are in scope
– Uncertainties have to be quantified on a pixel-by-pixel basis
• Validation in coastal waters by match up in situ samples can be difficult due to
patchiness and fast changes
– Uncertainties in in situ match up data have to be quantified
– Validation should be complemented by statistical analysis of larger areas,
transects and time series