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.
Leveling is a technique to determine the difference in elevation between various points on the Earth.
Elevation is the vertical distance above or below the region’s mean sea level.
Specific points of known elevation, called benchmarks, are established within a region to carry out leveling in that region.
The general methods of leveling are trigonometric, barometric, and spirit leveling.
The elevation of an inaccessible point P can be determined using trigonometric leveling by measuring the horizontal distance and angle to point P. Then, using trigonometric principles, point P’s elevation is determined.
Barometric leveling can be used to determine the elevation of point C using three altimeters. One is positioned at point C, and the other two are at locations A and B of known elevation.
Air pressure readings from these altimeters are used to calculate point C’s elevation.
Spirit or direct leveling determines the elevation of point D using leveling instruments by taking vertical measurements to point D with reference to a horizontal line of predetermined elevation.
Subtracting the vertical measurement from the horizontal line’s elevation gives point D’s elevation.
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