Dictionary.net

The reduction of uneven surfaces to a level; the art or process of ascertaining the different elevations of objects on the surface of the earth.

The act of reducing to a plane or flat surface; in surv., the art or operation of finding a horizontal line, or of ascertaining the differences of level between the various points in a survey.

n. The reduction of uneven surfaces to a level or plane;— the art or operation of ascertaining the different elevations of points of the earth’s surface included in a survey.