Sahelian savannah is composed of an annual herbaceous layer and a sparse tree community. The tree has a strong impact on the biomass and the species composition of the herbaceous layer due to microclimate and increase in fertility. In this work, we evaluated the distance of the tree impact on the herbaceous layers. We used an RGB UAV to produce biomass map and then evaluated the distance of the tree impacts. In 2020 in a rangeland in northern Senegal, three grass measurements were made every second day during the growth season combined with a UAV flight made with a parrot Anafi drone. At each date, we produced a biomass map and evaluated the distance of the tree impact using geostatic method. We obtained a calibration between UAV and field measurement with an R² equal to 0.64. The impacts of the tree were ranging from 5 m in the beginning of the wet season to 15 m at the end of the wet seasons. This work shows the distance of the tree impact on the grass layer in savannah. The evaluation of this impact could be helpful for the management of the tree layers to increase the quantity of grass for the pastoralism.