Observations made on the ground at a site that is being imaged from space for the purpose of verifying either the absolute radiometric and/or geometric calibration of the imagery or the classified product from the image. These data which are acquired from field checks, high-resolution remote sensing data, or other sources of "known" data are used as the basis for making decisions on training areas and evaluating classification results. Instrument ground truthing made during field trips is often called vicarious calibration when experimental measurements are made of such factors as solar irradiance, atmospheric transmittance, and reflectance of either natural or calibrated homogeneous or gridded targets. Classification ground truthing can be done (1) by visiting the sites to identify what is on the ground, (2) by referring to classification of the area from other sources such as thematic maps, or (3) by classifying higher spatial resolution imagery from satellites or aircraft into classes that can be observed in the coarser resolution imagery. Also see ground data and vicarious calibration.