The geometric observables in geodesy, which can be determined by eg surveying and photogrammetry, are all simple, or more complex functions of relative position of two points. They are influenced by various physical fields, like the earth's gravitational field and the field of atmospheric refraction. The relation between the geodetic observables and the physical fields can conveniently be expressed in stochastic models. These, linearized and interpreted stochastically, provide covariance functions (invariant with respect to suitable groups of parameter transformations), with which one can obtain the "stochastic signal' present in the observables. Stochastic models may also concern geometric parameters, like heights or variations of position, and generally digital terrain models.