Complementarity of Sentinel-1 and Sentinel-2 Data for Mapping Agricultural Areas in Senegal

The small size of agricultural plots is the main difficulty for crops mapping with remote sensing data in the Sahelian region of Africa. The study aims to combine Sentinel-1 (radar) and Sentinel-2 (Optic) data to discriminate millet, maize and peanut crops. Training plots were used in order to analyse temporal variation of the three crops’ signals. The NDVI (Normalized Difference Vegetation Index) was able to differentiate crops only at the end of the rainy season (October). The optical data as well as the radar ones could not easily discriminate the three crops during the growing season, because in that period vegetation cover is low, and soil contribution to the signals (due to roughness and moisture) was more important than that of real vegetation. However, the ratio of VH/VV (VH: incident signal in vertical polarization and reflected signal in horizontal polarization;VV: incident signal in vertical polarization and reflected signal in horizontal polarization) gave a difference between millet and the two other crops at the beginning cultural season (July 11). Difference appears from the second third of September when the harvest of cereals crops (millet and maize) began. From middle of October, the peanut signal dropped sharply thus facilitating the differentiation of peanut from the two other crops. This analysis led to the identification of data that have could be used to discriminate these crops (useful data). Classification of the combined useful data gave an overall high accuracy of 82%, with 96%, 61% and 65% for peanut, maize and millet, respectively. The non-agricultural areas (water, natural vegetation, habit, bare soil) were well classified with an accuracy greater than 90%.

Copernicrinus zamorae gen.et sp.nov.,the oldest thiolliericrinid crinoid(Crinoidea,Echinodermata) from the Bajocian strata of Algeria,Africa

The thiolliericrinids(Thiolliericrinidae) are comatulids(Comatulida),which retained their stalks as adults.Here,we report a centrodorsal from the Bajocian strata(Middle Jurassic) of the Djebel Kérdacha area,northwestern Algeria(Africa).It is the first thiolliericrinid report from outside of Europe and also the oldest representative of the Thiolliericrinidae whose first occurrence is from the Oxfordian.The crinoid is assigned to Copernicrinus zamorae gen.et sp.nov.The reported new taxon shows strong similarities with the stemless ‘true’comatulids,Solanocrinites;the only difference is the presence of a facet to the stem in Copernicrinus.The closest comparable thiolliericrinid to Copernicrinus is the Oxfordian Thiolliericrinus,interpreted as a descendant of the latter.The origin and overview of all thiolliericrinid representatives are also discussed herein.

The unexpected occurrence of enigmatic'percevalicrinids'(Echinodermata,Crinoidea)in the Lower Jurassic strata of North Africa—Implications for their stratigraphic and palaeogeographic distribution and discussion on their belonging to the subfamily Balanocrininae

The marl and limestone alternations of the Lower Jurassic Ain Ouarka and Ain Rhezala formations(Pliensbachian-Toarcian)in the western Saharan Atlas,Northwest Algeria,yield a diverse micro-and mac-rofauna,including moderately numerous crinoids,which are represented by remains of isocrinids,i.e.,Bal-anocrinus ticinensis Hess and columnals of the genus Percevalicrinus.So far,the latter genus has been observed from the Upper Jurassic-Lower Cretaceous strata of Eurasia,North America,and the African continent.Thus,the present find is the oldest record of this crinoid genus,and the second one from the southern Tethyan margin.In this paper,it is shown that Percevalicrinus,which is traditionally regarded as a representative of the subfamily Balanocrininae,displays several features of the subfamily Isocrininae.The crinoid assemblage and associated facies and invertebrate fauna are typical of a low-energy deep outer shelf/ramp(below the storm wave-base)setting.