NDVI or Normalized Difference Vegetation Index: all plants both absorb and reflect radiation. The chlorophyll in the leaf will reflect Near Infrared. This reflectance is invisible to the human eye, as we cannot see NIR. The more NIR that a plant reflects indicates a robust, healthy plant.
Outside of this, is a plant ‘under stress’ ! An NDVI image can identify those problem areas.
With various robotic aircraft (UAV's, UAS, drones) becoming available to farmers and agronomists, it is important to carefully weigh the available options. Just as farmers have more than one tractor on the farm where each one is better suited for specific applications, robotic aircraft come in various configurations and sizes. For example, you wouldn't think of using your large John Deere 9R 560 engine hp, 20,000-pound hitch-lift capacity on a small test plot. Nor would you use your garden tractor to work a section of ground. Each machine is better suited for certain applications.