"Ground Truthing" is the process whereby the data collected by some remote sensing technique is verified in situ. It is important to know that the data you collect is correct! Several techniques used on the programme require ground truthing.

Remotely sensed satellite data for sea surface temperature and chlorophyll content must be correctly calibrated, and this is done by comparing the value determined from satellite data with actual measurements of these values on the sea itself (usually from a ship). Satellite imagery can determine temperature by carefully analysing the frequencies of infra-red radiation given off by the sea surface; unfortunately, satellites can only "see" the temperature of the top few millimeters of water, so to understand what is going on beneath the surface, we still need to conduct oceanographic surveys from ships or deploy moorings. Chlorophyll values are determined from analysis of ocean colour.

Some geophysical data requires ground truthing. For example, it is possible to infer the type of bottom (sand, shell, coral, rock, mud etc.) from acoustic (sonar) data. However, it is important to ensure that you have correctly identified the bottom type from the acoustic data, and to initially determine what acoustic characteristics denote which bottom types. To do this, you can take a sediment sample using a grab or corer, or you can inspect it visually with SCUBA divers, submersibles or ROVs.