LEVELING. In surveying, the operation of ascertaining the different elevations of objects on the surface of the earth. Leveling is employed to find the relative elevation of points a con siderable distance apart, to obtain the profile of a line. or to establish a grade. These objects may he more or less intermingled in any piece of work. For convenience of discussion, leveling operations may be broadly classed into common spirit-leveling, trigonometrical leveling, and pre cise spirit-leveling. The difference between the first and the third classes lies chiefly in the de gree of accuracy with which the work is per form6d. Spirit-leveling operations are essentially the same whatever their object may be. For ex ample, suppose it is required to determine the difference in elevation between two distant points. The instruments are the level and the level rod, forms of which are illustrated on the Plate of ENGINEERING INSTRUMENTS. The rod is set up on the starting-point and the level is set up a convenient distance away in the direction of the other point. and where a plain view is had of the level-rod. When the level is adjusted. the levelman takes a reading on the graduated level rod and records it in his note-book. The rodman then moves his rod to a point ahead of the level, and time levelman takes another reading, which he likewise records. This reading is called a fore eight. in contradistinction to the first reading, which is called a hacksight. The levelman then removes his level to a point ahead of the seemed position of the rod, sets it up, adjusts it, and takes a backsight reading on the rod. This being recorded, the rodman moves to a new position ahead of the level, where he sets up the rod on which the levelman takes a foresight reading, and then moves again to a new position ahead of the rodman. By means of sufficient repetitions of these operations the distance between the two points whose differences in elevation are to be determined is traversed, the last reading being taken with the rod set, up on the finishing-point.
When the final reading has been taken a simple calculation in addition and subtraction per formed on the recorded readings gives the level man the result he desires, which is the difference in elevation between the starting and the finish ing points. In conducting such a series of read
ings as has been described. the attempt is always made to make the lengths of the backsight and foresight for each setting of the level as nearly equal as possible. and also to make them as long as possible consistent with clearness of sight.
It is in this way that the profile of the route of a railway line or highway is determined. Having obtained the natural profile, the engineer in railway or road building has to cut down the irregularities by means of one or more uni form gradients. These lie determines first on paper, figuring out the rate of ascent or descent. Supposing that he has determined a grade of one foot in a hundred feet, then each succeeding intervening point is one foot higher or lower, according to direction, than the preceding point. The levelman then re-runs his line of levels according to this adopted gradient, and marks on the stake at each intervening point the eleva tion at which its top is above or below the adopted grade-line.
Precise spirit-leveling is performed in exactly the same way, except that larger and more per fect instruments are employed and more care is taken in making the readings and performing other necessary operations.
In trigonometric leveling the difference of ele vation between two different points is determined by measuring the vertical angle, the horizontal distance between the two points having been pre viously measured by triangulation. (See Suit VEYINO.) This distance and the vertical angle give the necessary data for the calculation of the difference in elevation by the familiar rules of trigonometry. This method of leveling is liable to error from refraction. and is far less precise than spirit-leveling when the latter method is practicable. Elevations of different points, as the tops of mountains, are often measured by barom eter or by noting the temperature at which water boils. These are not exact methods, and are used only in exploration work. See Ilvesomrray.