On a linear project, everything is located by its distance along the alignment and its offset from it. That distance is called chainage (or stationing in the US, mileage on older railways). Learn to read it, and to check a vertical curve from a real drawing.
On a building you locate things by grids and levels. On a road, a railway or a pipeline you locate them by how far along the line they are, and how far to the side. That line is the alignment, and the distance along it is the chainage.
A structure on the line is located as "chainage, offset": for example Ch 2+340, 4.5 m left.
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