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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.
Consequently, a major limitation of linear referencing is that specifying points that are not on a linear feature is troublesome and error-prone, though not entirely impossible. Consider for example a ski lodge located 100 meters to the right of the road, traveling north. The linear referencing system can be extended by specifying a lateral offset, but the absolute location (i.e. coordinates) of the lodge cannot be determined unless coordinates are specified for the road; that process is prone to error particularly on curved roads.
Another major drawback of linear referencing is that a modification in the alignment of a road (e.g. constructing a bypass around a town) changes the measurements that reference all downstream points. The system requires an extensive network of reference stations, and constant maintenance. In an era of mobile maps and GPS, this maintenance overhead for linear referencing systems challenges its long-term viability.
Nonetheless, travel along a road is a linear experience, and at the very least, linear referencing will continue to have a conversational role: GPS-based navigation systems will still advise: "turn right 300 meters ahead."
Linear referencing is supported for example by several Geographic Information System software, including:
- GE Global Transmission Office
- GEOMAP GIS
- GRASS GIS
- Intergraph. "An LRS Model Supporting Event Location Stability and Temporal Data Management" (PDF).
- Intergraph. "White Paper: An Automated Approach to Managing Components of a Linear Reference System Network and Event Data" (PDF).
- Benefits "Smallworld Global Transmission Office" Check
- Esri (24 February 2009). "ArcGIS 9.3: An overview of linear referencing". Esri. Retrieved 8 March 2011.
- Geomap Services (6 June 2011). "GEOMAP GIS 2012: Solution for linear referencing and dynamic segmentation over Autodesk, ESRI or MapInfo products". Geomap Services. Retrieved 11 November 2011.
- Radim Blazek (March 2005). "Introducing the Linear Reference System in GRASS" (PDF). International Journal of Geoinformatics. 1 (3). Retrieved 2007-05-01.
- PostGIS team (2010). "PostGIS 1.5.2 Manual". Retrieved 8 March 2011.
- Noronha, Val; Church, Richard L. (2002), Linear Referencing and Alternate Expressions of Location for Transportation (PDF), Santa Barbara, California: Vehicle Intelligence and Transportation Analysis Laboratory, National Center for Geographic Information and Analysis, retrieved 8 March 2011
- Curtin, K; Arifin, R; Nicoara, G (2007), A Comprehensive Process for Linear Referencing (PDF), URISA Journal 19 (2), 41-50