Linear referencing

Linear referencing (also called linear reference system or linear referencing system or LRS), is a method of spatial referencing, in which the locations of features are described in terms of measurements along a linear element, from a defined starting point, for example a milestone along a road. Each feature is located by either a point (e.g. a signpost) or a line (e.g. a no-passing zone). The system is designed so that if a segment of a route is changed, only those milepoints on the changed segment need to be updated. Linear referencing is suitable for management of data related to linear features like roads, railways, oil and gas transmission pipelines, power and data transmission lines, and rivers.

A system for identifying the location of pipeline features and characteristics is by measuring distance from the start of the pipeline. An example linear reference address is: Engineering Station 1145 + 86 on pipeline Alpha = 114,586 feet from the start of the pipeline. With a reroute, cumulative stationing might not be the same as engineering stationing, because of the addition of the extra pipeline. Linear referencing systems compute the differences to resolve this dilemma.

Linear referencing is one of a family of methods of expressing location. Coordinates such as latitude and longitude are another member of the family, as are landmark references such as "5 km south of Ayers Rock." Linear referencing has traditionally been the expression of choice in engineering applications such as road and pipeline maintenance. One can more realistically dispatch a worker to a bridge 12.7 km along a road from a reference point, rather than to a pair of coordinates or a landmark. The road serves as the reference frame, just as the earth serves as the reference frame for latitude and longitude.

Linear referencing can be used to define points along a linear feature with just a small amount of information such as the name of a road and the distance and bearing from a landmark along the road. This information can be communicated concisely via plaintext. For example: "State route 4, 20 feet east of mile marker 187." Giving a latitude and longitude coordinate to a work crew is not meaningful unless the coordinate is plotted on a map. Often work crews work in remote areas without wireless connectivity which makes on-line digital maps not practical, and the relatively higher effort of providing offline maps or printed maps is not as economical as simply stating locations as offsets, or ranges of offsets, along a linear feature.

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