A DEM is a numerical and mathematical sketch of an area in terms of elevation (Charleux , 2001). Thus this source of information is used in many areas of daily life (mapping, defense, development and urban planning, civil engineering, telecommunications, geomorphology, hydrology, etc.).The results of its use often contain errors that are not generally perceived by the user. In this study, contours of topographic map were used to create two test digital elevation models (DEM) by using two interpolation methods the TIN (Triangulated Irregular Networks) method and the IDW (Inverse Distance Weight) method. These two models were then compared to a reference DEM, product of interferometry radar technology (SRTM images) to detect major errors on our test DEMs. It is clear from this analysis that: On the interpolated DEMs, summit areas are affected by underestimation of altitude and thalweg areas are affected by overestimation of altitude. However, these errors are not impacted on the overall quality of the DEM.
The present study is led on the Gourou watershed, in the autonomous district of Abidjan. Many quantities of sands and macroelements stream to this watershed's exutory in rainy season, blocking drainage canals and invading decantation basin. The objective of this work is to determine the origins of the sands in order to make map. To do it, we update the watershed's limits according to water separation lines. Then, we make a map of erosion zones susceptible to give up sands. The processing of all these data reveals a plurality of this sand blocking origins; the most producing zones being the ones situated near the road arteries. Sands come mainly from numerous bare zones of the watershed, from disorderly installed human activities and from the canals of the drainage network poor maintenance; some canals are being broken by place.
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