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In surveying, a baseline is a line between two points on the earth's surface and the direction and distance between them. In a triangulation network, at least one baseline needs to be measured to calculate the size of the triangles by trigonometry.
In the United States Public Land Survey System, a baseline is the principal east-west line (i.e., a parallel) upon which all rectangular surveys in a defined area are based. The baseline meets its corresponding principal meridian at the point of origin, or initial point, for the land survey. For example, the baseline for Nebraska and Kansas is shared as the border for both states, at the 40th parallel north.
More specifically a baseline may be the line that divides a survey township between north and south.
Many communities in the United States have roads that run along survey baselines, many of which are named to reflect that fact. Some examples:
In Canadian land surveying, a base line is one of the many principal east-west lines that correspond to four tiers of townships (two tiers north and two south). The base lines are about 24 miles (39 km) apart, with the first base line at the 49th parallel, the western Canada-US border. It is, therefore equivalent to the standard parallel in the US system. In Ontario, a baseline forms a straight line parallel a geographical feature (mostly a lake, especially Lake Ontario or Lake Erie) that serves as a reference line for surveying a grid of property lots. The result of this surveying is the concession road and sideline system in use today.