Mapping microphytobenthos in the intertidal zone of Northern ...

Resulting Chl a maps (Figure 6) appear qualitatively correct but the retrieved concentration range seems to show an
overestimation of Chl a based on our knowledge of the area. Unfortunately, it was impossible to validate these
concentrations because field samples collected at the time of the overflight were not properly frozen, preventing an
accurate estimate of Chl a concentration.
Figure 6. Chlorophyll a surface concentration maps derived from CASI data: a – “false color composite” (red: 865 nm; green:
673 nm; blue: 489 nm); b – simple band ratio; c – normalized ratio; d – scaled band area.
3.2.2. ROSIS
As shown in Figure 7, regressions show a better fit than for CASI data. This is probably due to the fact that ROSIS
spectral resolution is closer to field spectra resolution. All three approaches appear again equally robust (R 2 = 0.86).
When applied to ROSIS images (Figure 8), as for CASI maps, Chl a spatial distribution is qualitatively correct but
the concentration range is narrower.
Validation through comparison with field samples (Table 2) shows a strong underestimation of Chl a concentration.
This discrepancy could be explained in part by the noise in the data, assumption which seems to be confirmed by
the fact that the Scaled Band Area gives better results than the other approaches. Instrument calibration can also be
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