Leveling is the process of determining differences in the elevation of points, i.e., the vertical distance between them.   The instruments employed are the wye level and a graduated rod which is held vertically and is read to feet and tenths.  Leveling is used for determining the exact elevation (vertical distance) of definite points, called bench marks, above the datum (reference plane); these bench marks are used for establishing grades for roads, drainage, buildings, and for other construction work.  Leveling is also employed in obtaining data to depict the shape the of the earth's surface by means of lines of equal elevation (called contour lines).

Unlike the transit, the level does not have to be set up over a definite point; it is set up at any place from which the rod may readily be seen and read.  Therefore no chain with hook is provided for attaching a plumb-bob.

The wye level is so named because the telescope is held in Y-shaped supports which are attached to the ends of a horizontal bar.   The bar is parallel to the telescope.  At the middle of the bar, and attached perpendicular to it on the under side, is a vertical conical axis.  This axis fits into a conical socket in the leveling head, which acts as a bearing in which the axis can turn.  The leveling head is similar to the leveling head of a transit.  When in use, like the transit, the level is placed on a tripod.  The instrument is so constructed that the telescope may be changed end-for-end in the Y's and thus be readily reversed for adjusting.  Attached and parallel to the telescope is a sensitive spirit level for leveling the instrument.  When the level is in adjustment the axis of the bubble tube and the line of sight are parallel and both are perpendicular to the vertical axis.