Leveling is a surveying procedure used to determine elevation differences between distant points. Elevation refers to the vertical distance above or below a reference datum, typically mean sea level (MSL). In the United States, elevations are often referenced to the mean sea level station at Father Point Rimouski along the St. Lawrence Seaway. To make the datum accessible, permanent markers are established throughout the region. These markers, called benchmarks, have known elevations. If the elevation of one point in the region is known, the relative elevations of other points within the region can be determined through leveling.
Leveling methods include trigonometric, barometric, and spirit leveling. To determine the elevation difference between two points using trigonometric leveling, either the horizontal distance or slope distance between them, along with the vertical or zenith angle, must be measured. Barometric leveling determines elevations by measuring air pressure changes using aneroid barometers, also known as altimeters. Multiple barometers improve accuracy in determining the elevation of a given point by placing one at a higher known elevation and another at a lower known elevation, in addition to one at the point for which the elevation is to be determined. Spirit leveling, or direct leveling, is the most common method. It involves measuring vertical distances in relation to a horizontal line of known elevation to compute elevation differences.