Aurélie Davranche

Radiometric Normalization of SPOT-5 Scenes: 6S atmospheric Model versus Pseudo-invariant Features

We compared the efficiency and robustness of two radiometric correction techniques applied to six SPOT-5 scenes used for assessing environmental changes of Camargue wetlands: the 6S atmospheric model and 86 pseudo-invariant features (PIFs) found in deep water, pine trees, roofs and sand. The few PIFs were selected subjectively following the low number of potentially invariant sectors available on the scenes. Both approaches provided a similar radiometric variation (6S 4.3 percent; PIFs 4.0 percent). The latter increased from water to pine trees, to roofs and sand, with five reference points per feature being identified as cost effective. The withdrawing of variant features among the PIFs across dates or points caused a significant decrease in radiometric variation, especially with 6S (6S 2.8 percent, PIFs 3.4 percent). As many as 31 point per type of PIFs would be necessary to provide a radiometric variation that is not significantly different from that obtained with 6S, whereas nearly 300 and 4,000 points per feature would be required to provide similar or better results than the 6S code, respectively. Use of a few PIFs remains a valid approach, as long as the invariant sectors cover a wide range of brightness and are represented by objects of which the radiometric variation has preliminarily been tested.

Variability of surface and underwater nocturnal spectral irradiance with the presence of clouds in urban and peri-urban wetlands

Artificial light at night (ALAN) is an increasing phenomenon worldwide. It causes a wealth of biological and ecological effects that may eventually affect populations and ecosystems. Despite the growing concern about ALAN, little is known about the light levels species are exposed to at night, especially for wetlands and underwater habitats. We determined nocturnal irradiance in urban and peri-urban wetlands above and under water, and assessed the effect of cloud cover on the variability of ALAN across the urban gradient. Even in aquatic habitats, cloud cover could increase irradiance beyond values observed during clear full moon nights. We report a negative relationship between baseline irradiance and the increase in irradiance during overcast nights. According to this result and previous studies, we propose that the change in the variation regime of ALAN between the urban center and rural land at its periphery is a usual feature. We discuss the ecological and evolutionary implications of this spatial variation in the urban and peri-urban environment.