Definition:

Also referred to as Label Data or Target Data in the Machine Learning context, Reference Data is a set of measurements that accurately describe a phenomenon of interest, to be predicted as the output of a ML model. Other names for reference data in the context of EO include ground truth data or Calibration/Validation data. The reference data for a given problem in EO should describe fully all the possible values in the “problem space”.  Reference data can come from a wide range of sources including in-situ measurements, interpreted EO very high resolution imagery, IoT devices and others.