Mapping microphytobenthos in the intertidal zone of Northern ...

2.4.1. CASI data
CASI overflights occurred in September 2000, under poor weather conditions (overcast skies), preventing
concurrent atmospheric measurements. Field reflectance spectra were nonetheless acquired over four reference
sites. Empirical atmospheric corrections were performed using the empirical line technique and two of the reference
sites. Three flight lines were necessary to cover the entire Bay but we tested the algorithms on a subscene
corresponding to the area were the reference sites were located and the biofilm was present.
The CASI instrument was flown in a “MERIS like” configuration with an additional band centered at 673.9 nm to
map Chl a. Table 1 summarizes the main characteristics of the CASI data.
Table 1. Characteristics of CASI data
2.4.2. ROSIS data
Band # Central wavelength (nm) Band width (nm)
1 411.9 14.0
2 443.3 14.2
3 489.8 14.2
4 510.3 14.2
5 560.1 12.4
6 619.7 14.4
7 665.4 10.6
8 673.9 8.8
9 681.9 8.8
10 704.5 12.6
11 733.2 10.6
12 760.8 6.8
13 776.1 18.4
14 865.2 24.0
Spatial resolution: 2 m
ROSIS data were acquired in the framework of the 2001 HySens Campaign, in August 2001. The weather
conditions were good, allowing for concurrent atmospheric and reflectance measurements. Aerosol optical depth
derived from atmospheric measurements was used by DLR in their atmospheric correction procedure. Data were
also georeferenced using aircraft and ground GPS information. Field reference sites were spatially located using
GPS.
ROSIS is an airborne imaging spectrometer, making use of a two-dimensional CCD array for imaging
simultaneously 115 spectral bands of 512 picture elements perpendicular to the flight direction. The spatial
resolution was 2 m, the spectral range 416.5 – 872.5 nm with a spectral sampling interval of 4 nm.
The data appeared fairly noisy so additional preprocessing was necessary. MNF transform was performed to
remove most of the noise, the first 5 channels were removed and a reference site (dry sand) was used to refine
atmospheric corrections.
3. RESULTS
3.1. Field reflectance spectra
Results for the regression of simple ratio, normalized ratio and scaled band area versus field samples Chl a
concentration are presented in Figure 3. The dependency of absorption depth (simple ratio) or normalized
absorption depth (normalized ratio) to Chl a concentration can be approximated by an exponential function fit
(Beer-Lambert-like law) (Figure 4a and b). The functional relationship between scaled band area and Chl a
concentration was assumed a linear function (Figure 4c).
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