Land elevations are measured for a variety of reasons, such as land surveys completed for the purposes of topography and in the planning of proposed constructions. Land elevations are also used to conduct scientific experiments on the effects of climate change on the earth.
Land surveying techniques for measuring land elevation include the use of a technique called levelling, which uses various methods to measure the elevation of a piece of land or a structure constructed on a piece of land. Levelling as part of a land survey can be completed using a method known as direct or differentlal levelling, using a graduated rod and a surveying tool, such as the dumpy level, transit or theodolites.
Indirect levelling is a method of levelling using trigonometry techniques, including vertical angles and the horizontal distances between certain points. To complete a levelling technique correctly, a point must be found that has a known elevation and is not likely to be disturbed or to move. For the process of levelling, a concrete post or peg inserted into a tree or root can be used as the starting point for the process of levelling to be completed. These permanent objects used in levelling are known as benchmarks.
In June 2003, NASA launched a satellite equipped with three lasers, with the mission of calculating land elevations around the globe by projecting pulses of green laser and infrared light onto the surface of the earth at a rate of 40 times per second. The NASA satellite, known as ICESat, travels in orbit around the earth at a rate of 17,000 miles per hour with a mission of measuring the vertical characteristics of the earth. The calculations on land elevation are made from information created by measuring the time the laser light takes to travel to and from the earth combined with the position of the satellite in orbit and the orientation of the satellite's instruments, according to NASA.
The use of ICESat is designed to measure land elevations and the elevation of polar ice sheets and forests to measure the effects of humans and nature on the climate of the earth. As the first satellite system for calculating land elevations, ICESat is also capable of measuring the elevation of clouds in the atmosphere.