Most important excavations are the result of a prepared plan—that is to say, their purpose is to locate buried evidence about an archaeological site.
This work allowed him to determine by mathematical computations the unit of measurement for the construction of the monument.
But many excavations, particularly in the heavily populated areas of central and northern Europe, are done not from choice but from necessity.
Gravel digging, clearing the ground for airports, quarrying, road widening and building, the construction of houses, factories, and public buildings frequently threaten the destruction of sites known to contain archaeological remains.
Emergency excavations then have to be mounted to rescue whatever knowledge of the past can be obtained before these remains are obliterated forever.
Partial destruction of cities in western Europe by bombing during World War II allowed rescue excavations to take place before rebuilding.