Geodesy (Gr. earth, andto divide), the science and art of laying out divisions of the earth's surface upon a large scale. It differs from ordinary surveying in its measurements, being constantly referred to the spherical surface upon which they are made, and reduced to the same horizontal level. Corrections also are made for horizontal parallax in computing the value of instrumental observations, and the calculus of probabilities is applied to resolve their ditferences. Its object may even be the determination of the spherical curvature of portions of the surface, which is accomplished by the aid of extended series of astronomical observations made in connection with the most exact measurements. The methods of conducting these operations, and contending with the numerous causes of error incident to the imperfection of the instruments and powers employed, are treated in the article Coast Survey. The most important results in regard to the figure and dimensions of the earth, deduced from a comparison of the most extensive and accurate geodetical surveys, are given in the articles Earth and Degree.