The data provided by the sensors can also be used to make an estimate on the future crop yield. By calculating the Normalised Difference Vegetation Index (NDVI), or the Soil-Adjusted Vegetation Index (SAVI) when vegetation cover is low, we can get information on the crops' vigour. A lot of research has been done to derive relationships between a vegetation index of a crop, measured at a particular time, and the final crop yield. These relationships can be used to help estimate the final crop yield, using the data collected by the sensors on-board the tractors. Each of these relationships, however, is usually only applicable for a particular crop, grown at a specific region and the remotely sensed data need to have been collected at a particular moment during the growth period.