Measurement
Ground truthing
Ground truthing is checking what a remote measurement claims by going and looking. A satellite says this hectare is green. Somebody walks it and records what is actually growing, how tall it is and whether it is the planted species or something else.
It is what turns a vegetation index from a pattern into a statement about a place, and it is the step that makes the remote layer trustworthy at scale.
A small number of ground points, taken consistently at the same locations each round, calibrates a very large area of imagery.