In many disciplines, cartography included, it is a requirement to discover or to measure what is on or in the Earth. Detailed investigation on the surface by itself is clearly non-optimum, for logistic and other reasons. Even in Roman times, when labour was cheap, it is learned that mineral deposits in this country were located by observation from high ground of places where the early morning frost first cleared under the rising sun; presumably this was followed up by detailed work on the identified spots.