(1) A sight to a previously established point of a survey, or the reading or measurement obtained by that sight. (2) In leveling from an initial point to a final point through a sequence of intermediate points, a sight to (or the reading on) a leveling rod held on a preceding point. The sight to, or reading on, a leveling rod held on the succeeding point is called the foresight. In leveling, a backsight is sometimes called a plus sight because its value is added to the elevation of the point on which the leveling rod is placed, to obtain the elevation of the leveling instrument. But if the sight is to a mark on a wall or is in the roof of a mining tunnel, with the instrument at a lower elevation than the mark, the backsight will be subtracted from the known elevation to obtain the height of the instrument. The term backsight is preferred over plus sight. Neither of these definitions requires that the point or leveling rod to which the sight is made be one whose coordinates have been defined or determined previously. If the sequence consists of only two points, it is a matter of indifference which is called the backsight and which the foresight. (3) The visual intersection of the horizontal crosshair of a leveling instrument with a staff or leveling rod held vertically over or under the preceding point when the line of sight is horizontal or when the vertical angle of the line of sight is known or will become known from actual measurement. (4) The angle measured on the horizontal circle of a surveying instrument (set or to be read and recorded) when the vertical crosshair of the instrument coincides visually with the preceding point and the center of the instrument.