ground truth

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[verb] To gather data onsite in order calibrate a model or determine whether information captured remotely (such as imagery or measurements taken by satellite) is being accurately interpreted. This often involves going to a location "on-the-ground" or in the field to compare actual characteristics, such as vegetation cover or temperature readings, with characteristics predicted by a model or interpreted from an image. This technique is commonly used with aerial and satellite imagery, remote sensing, meteorology models, and GIS.

[noun] Data collected at a study site, as opposed to information collected remotely or predicted by a model.