The Collaborative International Dictionary of English v.0.48:

Geodesy \Ge*od"e*sy\, n. [Gr. ?; ge`a, gh^, the earth + ? to
   divide: cf. F. g['e]od['e]sie.] (Math.)
   That branch of applied mathematics which determines, by means
   of observations and measurements, the figures and areas of
   large portions of the earth's surface, or the general figure
   and dimenshions of the earth; or that branch of surveying in
   which the curvature of the earth is taken into account, as in
   the surveys of States, or of long lines of coast. Geodetic

WordNet (r) 3.0 (2006):

geodesy
    n 1: the branch of geology that studies the shape of the earth
         and the determination of the exact position of geographical
         points