In Senegal, the official Annual Agricultural Survey (AAS) is a list frame survey that covers the entire country. In 2018, 2000 holdings were selected through a stratified sampling approach from the 526,000 holdings in Senegal, representing approximately 0.4% of the total holdings. Within each holding, farmers were interviewed (crop type, area and production for the main season crops). GPS coordinates are gathered at the parcel-level and parcel area is measured.
This map relied on the AAS 2018, adding a small buffer around each GPS point at parcel-level. Around 3000 additional samples of non-cropland classes were collected by visual interpretation of Very High Resolution imagery.
Accurate crop type mapping was only possible over a smaller region where parcels boundaries were recorded (instead of GPS points).
A specific in situ data collection took place in the department of Nioro in 2020, with the support of DAPSA with the following objectives:
A new cropland map was produced, with still very high accuracy. More importantly, this in situ dataset allowed delivering an accurate crop type map over the department. The accuracy of maize was not good, mainly due to the small number of samples but it is quite satisfactory for millet and groundnut.
Millet and groundnut acreage statistics were estimated by coupling the crop type map and the in situ dataset and these estimates were compared to the official statistics obtained by only the in situ data. This comparison demonstrates how EO can help reducing the standard error and thus improving the acreage estimates.
Having a crop type map also allows estimating acreage statistics at the arrondissement (district) level, which is not possible by relying only on ground survey because there is not enough samples.