Spectral Characterisation of the Osterseen Lake District - EARSeL ...

Table 3. Input parameter for atmospheric correction of the ROSIS flight lines
flight altitude [km] 5.65
ground altitude ab. sea level [km] 0.59
sun zenith angle [°] 25.2
sun azimuth angle [°] 181.1
flight heading [90°east] 351
day of the year 155
water vapor content [g cm - ²] 2.0
aerosole type rural
visibility [km] 25.0
adjacency box [Pixel] 30
By inflight calibration experiments one can generally check the validity of the laboratory calibration. The
radiometric performance of an airborne sensor may differ from the one in laboratory due to the aircraft environment
and the longer distance between the sensor and the target influencing stray light effects in the blue wavelength
range. Therefore, for the present study different calibration coefficients have been generated and tested using the
inflight calibration module in ATCOR 4 (see Fig. 3). It appeared that the resulting atmospherically corrected
reflectance spectra are very sensitive to variations with these coefficients. The best results compared to modeled
spectra of selected lakes were achieved by applying the coefficients partly derived from inflight calibration with a
shallow water area in Lake Ostersee with logarithmic interpolation between 478 nm and 730 nm (see Fig. 3 h).
Figure 4 shows an intercomparison between modeled spectra (Fig. 4) for four selected lakes (including one shallow
water spectrum modeled for 1 m water depth over sandy sediment) and atmospherically corrected ROSIS spectra of
the same location using 3 x 3 pixel mean values. The shallow water spectrum (No. 13) can be reproduced similarly
using the forward model. The deep water spectra are in the same order of magnitude, however the reflectance
between 450 nm and 550 nm is sometimes higher in the ROSIS spectra.
Arbitrary units
0.004
0.0035
0.003
0.0025
0.002
0.0015
0.001
0.0005
0
400 450 500 550 600 650 700 750 800 850
Wavelength [nm]
Figure 3. Calibration coefficients tested for atmospheric correction a) standard coefficients, b) generated from
inflight calibration at a different test site "Nantes", c) generated from inflight calibration with modeled spectrum
Lake Ostersee shallow water (Fig. 2, No. 13), d) generated from inflight calibration with modeled spectrum Lake
Fohnsee (Fig. 2, No. 7), e) generated from inflight calibration with modeled spectrum Lake Sengsee (Fig. 2, No.
10), f) joined coefficients from 3c below 450 nm and from 3b above 450 nm, g) mean from coefficients 3b and 3c,
and h) coefficients from 3c with logarithmic interpolation between 478 nm and 730 nm.
5 DATA ANALYSIS AND INTERPRETATION
For all lakes mentioned in Tab. 1, the reflectance was modeled based on the concentration of water constituents as
measured in situ (Fig. 5a). In Fig. 4, the corresponding ROSIS spectra were extracted with the mean value of a 3 x 3
pixel matrix. For some lakes like Lake Ostersee (southern basin) or Lake Breitenauer See (western basin), the
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a
b
c
d
e
f
g
h