Photo Interprétation

Local scale land-use mapping from Google Earth images: application of the methodological approach to village lands in Senegambia

Ibrahima DIÉDHIOU, Catherine MERING, Oumar SY, Tidiane SANÉ — Thu, 22 Dec 2022 00:00:00 +0000

Local scale land-use mapping from Google Earth images: application of the methodological approach to village lands in Senegambia. Optical remote sensing remains one of the most powerful tools for small scale and medium scale monitoring of land-use and land-cover changes. However, this performance is limited, when it comes to assessing phenomena at the local level, where interactions at the origin of the general dynamics observed on medium scale mainly occur. It is this constraint that this work seeks to go beyond, by proposing a reproducible methodological approach allowing to characterize, from images with very high spatial resolution, both the spatial pattern of woody plant cover and the different land-uses within the village territories. The procedure developed here is mainly based on the tools of mathematical morphology and unsupervised classification. In a region such as southern Senegambia, mapping of land-use/land-cover through analysis of Google Earth images made it possible to count the woody plants, to calculate their density, to measure the areas according to land-use categories, and their spatio-temporal evolution, and thus highlight the agricultural practices in progress in these village lands of southern Senegambia.

Evaluation by remote sensing of marine pollution by hydrocarbon in Gabon from 2015 to 2017: case-sudy of Port-Gentil and Mayumba

Kouadio Eugène KONAN, Sanny Jasmina NZIENGUI HAMAMATA — Thu, 22 Dec 2022 00:00:00 +0000

Evaluation by remote sensing of marine pollution by hydrocarbon in Gabon from 2015 to 2017: case-sudy of Port-Gentil and Mayumba. Gabon is the fifth largest oil producer in sub-Saharan Africa. Nearly 70% of its exploited reserves are in the offshore, and it is not uncommon to record oil spills in the marine environment, caused by this oil exploitation. This is not without consequences for the coastal ecosystem, and for man. Pollution monitoring can be addressed through remote sensing. The Sentinel-1A mission has the potential to detect oil spills in the marine environment and the advantage of being able to operate at night and in cloudy conditions. The results obtained reveal for the facades of Port-Gentil and Mayumba the presence of 154.65 km2 of surface oil pollution, including 53% in Port-Gentil and 47% in Mayumba. The origin of the pollution has been identified: unidentified operator (38%), Total Gabon SA (25.66%), Perenco Gabon SA (16.61%), Canadian Natural Resources Ltd (11.35%), Stream Oil Holding (6.54%), Vaalco Gabon (ETAME) Inc. (1.84%). This study confirmed the presence of hydrocarbon spills coming from from oil platforms in the facades of Port-Gentil and Mayumba for the period from 2015 to 2017. We highlight the limits of Sentinel-1A in the detection of hydrocarbons, which represents an obstacle for operational monitoring, and for the identification
of polluters in order to protect the marine and land ecosystems.

Remote sensing assessment of anthropogenic pressure and landscape transformations in a former cocoa growing area: case of Agani-N’Denie kingdom (Ivory Coast)

Armand KANGAH — Thu, 22 Dec 2022 00:00:00 +0000

The regression of forest formations due to expansion of export crops in Africa has been the subject of numerous publications. This study is focused on
the case of the Agani-N’Denie king-dom, which was one of the first regions of Ivory Coast where cocoa and coffee plantations were established during
the colonial era. The development of these plantations, and of other more recent cash crops, lead to an increasing anthropogenic pressure. Our purpose
is to assess pre-sent land cover pattern (2018) and to make a comparative analysis with the situation 30 years ago (1987). Data source is Landsat imagery
(TM, 1987 and OLI, 2018). We obtained color composite images using shortwave infrared, near infrared and red channels respectively in red, green,
blue. In addition, 3 spectral indices have been produced: Normalized Difference Vegetation Index NDVI, Soil Brightness Index SBI, Normalized
Difference Water Index NDWI. A supervised classification procedure (pixel by pixel basis) has been used to obtain 2 land-cover maps (5 classes) for
1987 and 2018. Statistical results illustrate the increasing anthropogenic pressure in the studied area. Already in the 1980s, more than 66% of the area
was occupied by crops, from which 50% devoted to coffee or cocoa plantations. In 2018, anthropogenic pressure is higher and more than 75% of the
area exhibits a high degree of artificialization. This trans-formation of the landscape has resulted in a decrease of forested areas, which are now restricted
to protected areas. Even these protected areas are submitted to increasing anthropogenic pres-sure: they are constantly infiltrated by the surrounding
populations in order to establish culti-vated plots. Furthermore, clearing and cultivation of young fallows, but also the renewal of plots of cultures, make it fear a saturation of cropland in the Agani- N’Denie kingdom.

Satellite monitoring of coastal dynamics: case study of beach management effects at the ‘‘Petite Côte’’ (Senegal, West Africa)

Thu, 22 Dec 2022 00:00:00 +0000

The aim of this study is to show the evolution of the coastline on the beaches of Rufisque, Ndayane and Saly, located on the ‘‘Petite Côte’’ of the
Senegalese coast. The methodological approach includes a multitemporal analysis of Landsat 5, 7 and 8 and Sentinel 2 satellite images, using the
‘‘CoastSat’’ tool, over a 38-year time sequence (1984-2022). The first order linear regression applied to these data reveals that the accretion trend
predominates the erosion trend between 1984 and2022. The study highlights a seasonal cycle of the coastline: coastline advances during the dry season,
and retreats during the rainy season – a period when the swell is more energetic. Our results also illustrate the impact of protection structures on coastal
morphodynamics. The site where the impact of coastal protection structures is most visible by detecting changes in the position of the coastline
is the city of Saly. Thus, our results underline that the breakwaters have allowed a lateral extension of the beaches at this site. On the other hand, on
the beaches where the presence of a groin is observed, it is possible to see the impact of the groin on the blocking of longitudinal sediment transport:
the beach undergoes a significant accretion upstream of the structure, while the sections located downstream undergo erosion. This situation is observed
at Rufisque and Ndayane. According to ourresults, the construction of protection structures has caused disparate and even asymmetrical evolution
dynamics of the ‘‘Petite Côte’’, marked by sectors well served in sediment giving rise to morphotypes such as tombolos in Saly, and others that are under-
fed with the consequence of a retreat of the coastline (Ndayane). In the medium term, the use of soft management measures, in particular sediment
cell nourishment, appears highly desirable.