MERIS Lakes
Version 1.0
Validation protocol
Project Title:
Development of MERIS lake water algorithms
Document Title:
Validation protocol (WP 2.4)
Version:
1.0
Author(s):
Kari Kallio, Sampsa Koponen, Antonio Ruiz-Verdú, Thomas Heege, Kai Sørensen, Timo Pyhälahti & Roland Doerffer
Affiliation(s):
SYKE, TKK, CEDEX, EOMAP, NIVA, GKSS
Modification History:
Feb. 1, 2007 Draft v0.1 by Kari Kallio & Sampsa Koponen
Feb. 7, 2007 Comments from Antonio Ruiz-Verdú
Feb. 12, 2007 Comments from Thomas Heege
Feb. 27, 2007 Modifications by Sampsa Koponen
March 15, 2007 Modifications by Kari Kallio & Timo Pyhälahti
March 17, 2007 Draft v0.2 by Kari Kallio & Sampsa Koponen
March 19, 2007 Comments from Antonio Ruiz-Verdú
March 29, 2007 Comments from Kai Sørensen
April 2, 2007 Draft v0.3 by Kari Kallio & Sampsa Koponen
April 3-4, 2007 Discussions on the open questions with all participants at the 1st Progress Meeting (GKSS, Germany).
April 12, 2007 Comments from Thomas Heege & Antonio RuizVerdú
April 18, 2007 Version 1.0 by Kari Kallio & Sampsa Koponen
Distribution:
Kallio et al. (2007)
Peter Regner, Project team
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Validation protocol
Contents:
1
Introduction ................................................................................................................................ 3
2
Validation of atmospheric correction ....................................................................................... 3
2.1
Water leaving radiance reflectance from subsurface measurements ................................... 4
2.2
Water leaving radiance reflectance from above water measurements ................................. 5
2.3
Ship position relative to sun during radiation field measurements ...................................... 6
2.4
Atmospheric measurements during field campaigns ........................................................... 6
3
Inherent optical properties (IOPs) ........................................................................................... 6
3.1
Particle absorption................................................................................................................ 7
3.2
CDOM absorption ................................................................................................................ 7
3.3
Total absorption and scattering ............................................................................................ 7
3.4
Backscattering ...................................................................................................................... 7
4
Specific inherent optical properties (SIOPs) and determination of water quality
constituents ......................................................................................................................................... 8
4.1
Specific inherent optical properties...................................................................................... 8
4.2
Determination water constituents......................................................................................... 8
4.2.1
Concentration of Chlorophyll-a (Chl-a) ....................................................................... 8
4.2.2
Concentration of total suspended matter (TSM) .......................................................... 8
4.2.3
acdom .............................................................................................................................. 9
4.2.4
abp ................................................................................................................................. 9
4.2.5
Other optical measurements ......................................................................................... 9
5
Measurement of vertical and spatial distribution of water quality within a MERIS pixel. 9
5.1
Sampling depth of water samples ........................................................................................ 9
5.2
Determination of the vertical distribution of water constituents.......................................... 9
5.3
Determination of spatial distribution of water constituents within a MERIS pixel ........... 10
6
Miscellaneous ............................................................................................................................ 10
6.1
Storage of water samples ................................................................................................... 10
6.2
Environmental conditions and measurements at validation stations ................................. 10
6.3
Temporal difference between sampling and overflight ..................................................... 11
6.4
MERIS viewing angle and solar zenith angle .................................................................... 11
6.5
Criteria for site selection for validation ............................................................................. 11
7
Calibration of the instruments ................................................................................................ 11
8
Validation data of Lake Victoria, Africa ............................................................................... 12
References ......................................................................................................................................... 12
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1 Introduction
The algorithms development in the project are validated based on measurements in Finland (4
lakes), Germany (1 lake), and Spain (3 lakes) in 2007. In Lake Victoria, Africa, the validation is
based on the data collected before the start of the project, in 2003 and 2004.
Three types of validation measurements are made:
1. atmospheric correction
- water leaving radiance reflectance
2. Inherent optical properties (IOPs):
- total absorption coefficient atot()
- total scattering coefficient btot()
- total backscattering (bbtot)
- absorption coefficient of phytoplankton aph()
- absorption coefficient of bleached particles abp()
- absorption coefficient of CDOM acdom()
3. Water constituents
- concentration of total suspended matter(TSM)
- concentration of chlorophyll a (Chl-a)
- CDOM indicated as acdom(443)
- bleached particle absorption indicated as abp(443)
In addition, concentration of phycocyanin will be measured in the Spanish lakes for studying the
possibilities to estimate this cyanobacteria specific pigment from MERIS data. The specific inherent
optical properties are calculated from the validation measurements and they are used to convert the
IOPs to concentrations of water constituents.
The validation methods applied here are mainly based on the following protocols:
- NASA's ocean optics protocols (NASA 2003)
- MAVT (MERIS and AATSR Validation Team) protocol (Doerffer 2002)
MERIS protocols, particularly for coastal Case-2 waters, are also discussed in the REVAMP EUproject reports (Tilstone et al. 2002a and 2002b).
Inter-calibration of optical instruments/methods is not possible in a project of this size. Therefore,
special attention is paid to the calibration of the individual instruments and to standardize the reflectance and IOP measurement methods.
The final aim of this document is to ensure that 1) the methods in different countries are, as much
possible, the same and 2) the measurements made by different instruments or methods are comparable. Although the MERIS processor requires only MERIS wavelengths the optical measurements
are measured for the full spectrum whenever possible.
2 Validation of atmospheric correction
The validation of the atmospheric correction is performed by measuring in situ water leaving radiance reflectance (remote sensing reflectance, RRS). RRS can be estimated by measuring the radiance
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upwelling from water and downwelling irradiance above water. The preferred protocols described
here follow the recommendations of NASA (2003) (Vol. 3, Chapters 2 and 3, pages 7-31). The
spectrometers available for the measurement of water leaving radiance reflectance in this project are
summarized in Table 1.
Table 1. Spectrometers available for the measurement of water leaving radiance reflectance.
Finland
ASD-FR spectrometer
Two TriOS RAMSES irradiance
spectrometers.
ASD-FR spectrometer with underwater foreoptics
Above water
Subsurface
Germany
TriOS RAMSES
spectrometer
Three TriOS
RAMSES spectrometers (two irradiance and one
radiance )
Spain
ASD-FR spectrometer
Two Ocean Optics
HR4000 and one
Ocean Optics
SD2000 (two irradiance and one
radiance)”
2.1 Water leaving radiance reflectance from subsurface measurements
The preferred method for estimating RRS is to first measure the upwelling radiance below surface
towards nadir (L(z,λ)), extrapolate it to just below surface (L(0-,λ)) and convert that to water leaving radiance (Lw(λ)) with
Lw(λ) = L(0−, λ) t n−2 ,
where t is the upward Fresnel transmittance of the air-sea interface (~0.975) and n is the refractive
index of (sea)water. The term t n−2 has a value of 0.543 for seawater and is not sensitive to wind
speed (NASA 2003). The value for freshwater is only slightly different (t n−2 = 0.55 instead of
0.543 according to Palmer & Williams 1974).
RRS (λ) can then be derived with
RRS (λ) = Lw(λ) / ES(λ),
where ES(λ) is the downwelling irradiance just above water surface towards zenith.
Correction for self shading has to be applied to the subsurface radiance measurement. According to
Gordon and Ding (1992) the correction can be performed with
Lcor(0−,λ) = L(0−, λ) / [1- ε(λ)],
where Lcor(0−,λ) is the corrected upwelling radiance just below the surface and
ε(λ) = 1 – e-’a()r,
where a() is the absorption, r is the radius of the instrument and
’ = y/tan(0´),
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where y is an empirical factor (y  2) and 0´ is the refracted solar zenith angle.
2.2 Water leaving radiance reflectance from above water measurements
Water leaving radiance can be estimated from above water measurements of upwelling radiance.
However, in this case the radiance detected at the sensor (Lsfc(λ)) contains the blue-sky radiation
reflected by the surface. A correction has to be applied to the measured data that removes the sky
radiation. This can be done with equation
Lw(λ) = Lsfc(λ) -  Lsky(λ),
where  is the reflectance of skylight from water surface and Lsky(λ) is the radiance measured from
the sky. The remote sensing reflectance is derived with
RRS (λ) = Lw(λ) / ES(λ).
 can be estimated from wind speed using the method of Mobley (1999, Fig. 9).
Due to the uncertainties in the correction of the reflected sky radiation the accuracy of the estimation of water leaving radiance reflectance from subsurface measurements is poorer than from subsurface measurements (NASA 2003).
Lsfc (λ) is measured with an angle  (usually between 30 and 50, recommendation 40) away from
the nadir and away from the sun (azimuth  from the sun between 90 and 180, recommendation
135). Lsky(λ) is measured with an angle Sky (from zenith) is numerically the same as  and sky = .
The method is also described in Mobley (1999).
An example of the measurement set-up for an above water spectrometer is presented Fig. 1.
Optic fibre
Control of observation zenith angle
 = 40º
 = 135º
Spectralon 25%
Control of azimth angle with respect to sun
Fig. 1. Measurement set-up for above water spectrometer used in Spanish lakes (Ruiz-Verdú et al.
2007).
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2.3 Ship position relative to sun during radiation field measurements
The radiometric measurements must be made so that the ships shadow and reflections from the
ships hull do not affect the upwelling radiance. Thus, the distance between the ship and the area that
is being measured must be large enough.
With the downwelling irradiance measurements the sensor head must be located so that antennas
and other structures do not cast shadows on the sensor or otherwise affect the measurement.
2.4 Atmospheric measurements during field campaigns
In Germany the Microtops sunphotometer is used according to the availability of the instruments at
time of the validation measurements.
In Spain the atmospheric optical measurements during the validation measurements will be made
with the Cimel 318NE sky and Microtops II sunphotometers. The following variables will be retrieved:
- Direct Sun irradiance
- Sky radiance in almucantar and principal plane
- Aerosol Optical Thickness
- Columnar water vapor content
- Other aerosol properties
- Columnar ozone amount
The Cimel 318NE sky sunphotometer is calibrated by means of a lamp (for the sky radiance) and by
Langley plots or calibration transfer from other calibrated sunphotometers (for direct sun measurements). It measures direct component for 8 channels (340, 380, 440, 500, 670, 870, 936, 1020nm)
and sky radiance for 5 channels (440, 500, 670, 870, 1020nm).
From wavelengths 340, 380, 440, 500, 670, 870 and 1020nm the aerosol optical thickness can be
retrieved. Aerosol size distribution functions can be also retrieved from sun and sky measurements.
Columnar water vapor content is retrieved using the 936nm wavelength. Finally, an inversion code
(SKYRAD4.2) can be employed for retrieving other aerosol properties, such as the refractive index
or the single scattering albedo.
Microtops II is a manual sunphotometer which gives direct measurements of the columnar ozone
amount, the precipitable water vapour content and the aerosol optical depth at 1020 nm. It has 5
channels. Three of them are used to find the columnar ozone, one for the water vapour content and
the other for the aerosol optical depth.
3 Inherent optical properties (IOPs)
A summary of instruments available and methods used in the determination of IOPs are presented
in Table 2.
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Table 2. The main optical instruments and determination methods for the IOP measurements.
Variable
Finland
Germany
Spain
aTot
WET Labs ac-9
-
WET Labs ac-s
bTot
WET Labs ac-9
-
WET Labs ac-s
bbTot
-
-
WETLabs ECO
BB9
aph and abp
Filter pad method
acdom
All: Spectrophotometric measurement of filtered sample
Filter pad method
3.1 Particle absorption
Absorption by phytoplankton aph() and bleached particles abp() is measured in laboratory with the
filter pad method using a spectrophotometer and an integrating sphere (Ferrari & Tassan 1999, Tassan & Ferrari 2002). The MAVT protocol recommends a beta factor of 2 for the filter pad method.
Here we use the equation of Tassan & Ferrari (2002), which takes into account the dependence of
beta factor from the absorption level. The recommended (minimum) wavelength range of the particle and cdom absorption measurements is 400-750 nm.
3.2 CDOM absorption
Absorption by CDOM acdom() is measured with a spectrophotometer relative to pure water. The
water sample is filtered before the measurement using a membrane filter (e.g. Nuclepore polycarbonate) with a 0.2 µm pore size. The protocol is described in NASA (2003), Vol 4, p. 47-51. The
measurement procedure is also described in Tilstone et al. 2002a, pp 22-24.
3.3 Total absorption and scattering
Total absorption and scattering coefficients using the WET Labs' absorption-attenuation meters: ac9 (Finland, 9 bands in the 412-715 nm range) and ac-s (Spain, continuous spectra in 400-730 nm
range). Scattering coefficients are obtained by subtracting absorption from attenuation.
3.4 Backscattering
For the in situ measurement of total backscattering (bbtot), the WET labs' ECO BB9 will be used in
Spain. This instrument measures the optical scattering at 117 degrees and at nine wavelengths: 412,
440, 488, 510, 532, 595, 660, 676 and 715 nm. The angle is determined as a minimum convergence
point for variations in the volume scattering function (VSF), induced by suspended materials and
water. The instrument is deployed together with a CTD and thus provides vertical profiles of the
backscattering coefficients.
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4 Specific inherent optical properties (SIOPs) and determination of
water quality constituents
4.1 Specific inherent optical properties
SIOPs are calculated from the IOPs and the concentrations of water constituents. SIOPs are needed
in the calculation conversion factors or functions, which are used in the MERIS processor to compute the concentrations from IOPs.
4.2 Determination water constituents
4.2.1
Concentration of Chlorophyll-a (Chl-a)
Two methods are used for the determination of Chl-a:
1) Spectrophotometric determination (GF/F filter, ISO 10260)
2) HPLC (Jeffrey et al. 1997, NASA 2003, Vol. 5)
The spectrophotometric method is used in Finland and Germany, while HPLC is applied in Spain.
In addition, a comparison of these two methods will be tried to be carried out in during the validation measurements in Germany.
The spectrophotometric method normally gives the sum of Chl-a and some degradation products
(such as phaeophytin-a), while the HPLC makes possible the separation of Chl-a and phaeophytina. In an inter-comparison between 20 validation teams with water samples from Norwegian coastal
waters (Chl-a range 1-12 mg m-3, Sørensen et al. 2007) the spectrophotometric method yielded 1520% higher Chl-a concentrations (e.g. due to influence of different degradation products) than the
HPLC. According to the standard (ISO 10260) phaeophytin corrected Chl-a is possible to estimate
with the spectrophotometric method through acidification of the sample extracts, but the accuracy
of the estimation is poor (e.g. Stich & Brinker 2004). Therefore, it is recommended that Chl-a not
corrected for phaeophytin-a is used for validation.
One major source of error in the Chl-a determination is the extraction procedure. The efficiency of
the solvent and extraction method varies foe example with the phytoplankton community (e.g.
Webb et al. 1992) and therefore a complete extraction can be difficult to achieve. Here the main
recommended extraction solvent is hot ethanol, which is part of the standard Chl-a determination
procedure in Finland and Germany. The use of ethanol rather than acetone (which is the most common extraction method in marine applications) can be important in lakes with cyanobacteria or
green algae. If acetone extraction is used in such lakes one must use sonification or grinding during
extraction and even that is not always enough.
4.2.2
Concentration of total suspended matter (TSM)
Concentration of TSM is determined by the gravimetric method using 0.4 m polycarbonate filter
(e.g. Nuclepore) as recommended by NASA (2003), Vol. 5, p. 3 (procedure otherwise as in EN
872). The MAVT protocols recommend GF/F filter (with a nominal pore size of 0.7 m) , but here
the 0.4 m polycarbonate filter is used because it filters small particles more effectively than the
GF/F.
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4.2.3 acdom
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Absorption coefficient of CDOM at a standard wavelength of 443 nm is obtained from the acdom ()
measurement is used as a measure of CDOM.
4.2.4
abp
Absorption coefficient of the bleached particle absorption at a standard wavelength of 443 nm is
obtained from the abp() measurement. abp(443) is added to the CDOM absorption at 443 nm as an
expression for the yellow substance products of MERIS (YSBPA). In this project the possibilities to
make acdom(433) as a separate product is studied.
4.2.5
Other optical measurements
Secchi disk depth is measured at all validation stations.
5 Measurement of vertical and spatial distribution of water quality
within a MERIS pixel
5.1 Sampling depth of water samples
The water samples will be taken as an integrated sample from the surface down to the to the first
optical depth, from which 90% of the remote sensing signal comes from (Gordon and McCunley
1975). The first optical depth is the same as attenuation depth Ze:
Ze = 1/Kd(PAR),
where Kd(PAR) is mean of diffuse attenuation coefficient in the PAR (400-750 nm).
The MERIS processor assumes vertically homogeneous water in the first optical depth and that
floating material does not exist. Therefore, validation measurements should be preferably made at
the time when these condition are fulfilled. In lakes the water may not be homogeneous in the first
optical depth e.g. during cyanobacterial blooms. The vertical distribution of water quality constituents at the validation stations is determined by profiling instruments (see 5.2 Determination of the
vertical distribution of water constituents). These measurements can be used to explain possible
discrepancies between validation measurements and MERIS products.
Ze can be estimated in the field from the Secchi disk (m) measurement, using an empirical relationship that should be defined for each lake region or lake type based on simultaneous Kd and Secchi
measurements. In Spanish reservoirs, for example, Ze = 0.6*Secchi (Peña-Martínez et al. 2003).
The integrated samples can be taken with a simple device consisting on PVC tubes of 10 cm Ø and
a rubber stopper (like that of a Van Dorn sampler, e.g. KC Denmark, 2007), which closes the tube
by pulling, once it is at the desired depth.
5.2 Determination of the vertical distribution of water constituents
Vertical variation of water quality in the surface layer (down to Secchi depth) at validation stations
is measured with profiling instruments (turbidity and Chl-a fluorescence). The use of turbidity proKallio et al. (2007)
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files is preferred since chl-a fluorescence can be highly variable due to the photocynthetic activity
of the plankton. Additional variables, such as phycocyanin, phycoerytrin and CDOM, can be measured with more advanced fluorometers, for example CTD SBE 19plus equipped with Chelsea and
Seapoint fluorometers of the Spanish team.
5.3 Determination of spatial distribution of water constituents within a MERIS
pixel
Two different approaches are possible depending on the availability of instruments and measurement set-up.
Method based on point sampling (Spanish lakes)
At each water body, at least two MERIS sampling areas will be defined. Each area will be defined
as a square of 500 x 500 m in which a number of measurements will be taken and averaged (at least
three measurement points). A Z-shaped transect will be made covering the whole area with the following strategy:
-
-
Vertical profiles including Ed and Lu; PAR; chlorophyll-a; phycocyanin, CDOM, turbidity, phycoerytrin; phytoplankton fluorescence and total absorption coefficients (ac-s), total backscattering coefficient (ECO BB9)
Measurements of Secchi disk depth
Samples for laboratory measurements of IOP’s (absorption coefficients) and TSM
Samples for HPLC analysis of photosynthetic pigments
Method based on transect measurements (Finnish lakes)
Spatial distribution of atot and btot in the surface layer is measured with ac-9 installed in a flowthrough system, which is operated from a moving boat (Lindfors et al. 2005). This system produces
ac-9 transects with atot and btot measured with about 6 m intervals. From atot and btot CDOM, Chl-a
and TSM can be estimated and the transects through a MERIS pixel are used as an estimation of
overall water quality in the pixel.
In addition, continuous measurements of above water reflectance can be recorded in both approaches. This measurement is sensitive to the sun position and disturbance due to waves, but can be usable for validation in optimum conditions (sun position in relation to the boat, calm weather).
6 Miscellaneous
6.1 Storage of water samples
Storage of bottles should take place in a dark and cold (at about 4ºC) container and analysis preferably carried out during the same day (or within 24 hours). If possible, filtration and freezing of Chla samples (including particle absorption filters) and filtration of TSM and CDOM samples should
be made in the field.
6.2 Environmental conditions and measurements at validation stations
Radiometric measurements shall be performed under clear sky only (Doerffer 2002).
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Measurements shall be avoided in the presence of any surface material such as floating algae or
foam (Doerffer 2002). The areas being measured should not contain spatial variations. The water
samples shall be collected from the area that is observed by the optical field instruments.
Wind direction and speed, and wave height shall be recorded
6.3 Temporal difference between sampling and overflight
The in situ measurements shall be performed within  0.5 hours of the MERIS overflight in order to
minimize the errors caused by water transport and other dynamic phenomena (Doerffer 2002).
However, this requirement is difficult to follow due to the time required for the measurements and
travel from one station to another. Thus, we recommend that the measurements are planned so that
one station at each lake is visited during the MERIS overflight and the others as close to the overflight as possible.
6.4 MERIS viewing angle and solar zenith angle
Doerffer (2002) recommends that in Case 2 water the in situ measurements are matched with
MERIS pixels near the swath center (i.e. nadir pixels). However, sun-glint may be a problem with
small viewing angles especially at lower latitudes. Also, cloud cover may prevent MERIS data acquisition when the geometry is good. Thus, the campaigns should be planned so that the viewing
angle is minimized while avoiding sun glint and cloudy days.
During the surface measurements of the water leaving radiance it is recommended that the solar
zenith angle is > 20 (preferably >30) in order to avoid sun glint.
In Spain, solar zenith angle is <30º from mid May to mid July (mid August in the south) at the time
of MERIS overpass. When the field measurements extend towards solar noon, angles can be <20º.
Therefore, it will be difficult to avoid measuring with low zenith angles.
It is also recommended that sun-glint probabilities are calculated from predicted image geometry
when the measurements are planned (e.g. according to Koepke 1985).
6.5 Criteria for site selection for validation
The MERIS validation protocol (Doerffer 2002) recommends that the distance from an in situ station to land is greater than 5 km in order to minimize the adjacency effect. This is feasible only in
very large lakes. In this protocol the recommendation is to have one station at the center of the lake
(or at the point that is farthest from shore) and to time the measurements so that they take place during the MERIS over flight (See Chapter 6.3). In any case the stations should not be closer than 1 km
to the shore. The adjacency effect correction module (ESA project) will be added BEAM during
2007.
The water depth at the stations must be > 2*Secchi depth (Doerffer 2002) to avoid reflection from
bottom.
7 Calibration of the instruments
Instruments shall be calibrated according to the recommendations of each instrument manufacturer.
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8 Validation data of Lake Victoria, Africa
The main data collection at Lake Victoria took place between July 2003 and January 2004. Water
samples were taken at six stations along a coast – offshore transect, and further analysed for CDOM
and bleached particle absorption, phytoplankton absorption, and the concentrations of Chl-a and
total suspended matter, following MERIS validation protocols recommended by the MAVT. Subsurface upwelling and downwelling irradiances were measured by the NILU-UV irradiance meter
(Norwegian Institute for Air Research). The latter measures at five wavelengths: 305, 312, 320,
340, 380, 555 nm and the PAR. Absorption and attenuation coefficients were measured using WET
Labs ac-9. The measurements and methods are described in Okullo et al. (2005).
References
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Mobley, C. D., 1999. Estimation of the remote-sensing reflectance from above-surface measurements. Applied Optics 38 (36), 7442-7454.
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http://oceancolor.gsfc.nasa.gov/DOCS/
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Okullo, W., T. Ssenyonga, B. Hamre, Ø. Frette, K. Sørensen, J.J. Stamnes, A. Steign, K. Stamnes,
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validation activities in Spain. Internal report for the Development of MERIS lake water algorithmsproject. 9 p.
Stich, H. B. and Brinker A. 2004. Less is better: Uncorrected versus pheopigmentcorrected
photometric chlorophyll-a estimation. Arch. Hydrobiol. 162: 111–120.
Sørensen, K., Grung, M., and R. Röttgers 2007. An intercomparison of in vitro chlorophyll a determinations for MERIS level 2 data validation. International Journal of Remote Sensing, 28: 537-554.
Tassan, S & Ferrari, G.M. 2002. A sensitivity analysis of the 'Transmittance-Reflectance' method
for measuring light absorption by aquatic particles. Journal of Plankton Research 24(8): 757-774.
Tilstone G.H., Moore G.F., Sorensen K., Doerffer R., Röttgers R., Ruddick K.G., Pasterkamp R. &
Jorgensen, P.V. 2002a. REVAMP Protocols (Regional validation of MERIS chlorophyll products in
North Sea coastal waters). EU Contract: EVG1- CT-2001-00049. 77 pp.
Tilstone G.H., Moore G.F., Sorensen K., Doerffer R., Röttgers R., , Jorgensen, P.V., Martinez Vicente V. & Ruddick K.G. 2002b. REVAMP Inter-calibration Report (Regional validation of
MERIS chlorophyll products in North Sea coastal waters). EU Contract: EVG1- CT-2001-00049.
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