Manipulation of the data in hand to get the best results for our needs.
For example, multispectral cameras are really expensive but we can utilize normal RGB vision of stock cameras with standard UAVs to get NDVI calculated. But beware, this is an impossible task. Because on NDVI you have to get RGB and NIR bands to get NDVI results. NDVI literally is NIR-RED/NIR+RED but there is no NIR band on RGB imagery.