Construction surveying

Construction surveying or building surveying (otherwise known as "staking", "stake-out", "lay-out" or "setting-out") is to stake out reference points and markers that will guide the construction of new structures such as roads or buildings. These markers are usually staked out according to a suitable coordinate system selected for the project.

Land surveys and surveys of existing conditions are generally performed according to geodesic coordinates. However for the purposes of construction a more suitable coordinate system will often be used. During construction surveying, the surveyor will often have to convert from geodesic coordinates to the coordinate system used for that project.

In the case of roads or other linear infrastructure, a chainage (derived from Gunter's Chain - 1 chain is equal to 66 feet or 100 links) will be established, often to correspond with the centre line of the road or pipeline. During construction, structures would then be located in terms of chainage, offset and elevation. Offset is said to be "left" or "right" relative to someone standing on the chainage line who is looking in the direction of increasing chainage. Plans would often show plan views (viewed from above), profile views (a "transparent" section view collapsing all section views of the road parallel to the chainage) or cross-section views (a "true" section view perpendicular to the chainage). In a plan view, chainage generally increases from left to right, or from the bottom to the top of the plan. Profiles are shown with the chainage increasing from left to right, and cross-sections are shown as if the viewer is looking in the direction of increasing chainage (so that the "left" offset is to the left and the "right" offset is to the right).

"Chainage" may also be referred to as "Station".

In the case of buildings, an arbitrary system of grids is often established so as to correspond to the rows of columns and the major load-bearing walls of the building. The grids may be identified alphabetically in one direction, and numerically in the other direction (as in a road map). The grids are usually but not necessarily perpendicular, and are often but not necessarily evenly spaced. Floors and basement levels are also numbered. Structures, equipment or architectural details may be located in reference to the floor and the nearest intersection of the arbitrary axes.

In other types of construction projects, arbitrary "north-south" and "east-west" reference lines may be established, that do not necessarily correspond to true coordinates.

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Wikipedia