Construction surveying is generally performed by specialised technicians. Unlike land surveyors, the resulting plan does not have legal status. Construction surveyors perform the following tasks:
- Survey existing conditions of the future work site, including topography, existing buildings and infrastructure, and even including underground infrastructure whenever possible (for example, measuring invert elevations and diameters of sewers at manholes);
- Construction surveying (otherwise “lay-out” or “setting-out”): to stake out reference points and markers that will guide the construction of new structures such as roads or buildings for subsequent construction. These markers are usually staked out according to the arbitrary coordinate system used for the project;
- Verify the location of structures during construction;
- As-Built surveying: a survey conducted at the end of the construction project to verify that the work authorized was completed to the specifications set on plans.
“Coordinate Systems used in Construction”
Land surveys and surveys of existing conditions are generally performed according to [geodesic] coordinates. However for the purposes of construction an arbitrary construction coordinate system will often be used. During construction surveying, the surveyor will often have to convert from geodesic coordinates to the arbitrary coordinate system used for that project.
In the case of roads or other linear [infrastructure], a “chainage” will be established, often to correspond with the center line of the road. 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”).
In the case of buildings, an arbitrary system of axes is often established so as to correspond to the rows of columns and the major load-bearing walls of the building. The axes may be identified alphabetically in one direction, and numerically in the other direction (as in a road map). The axes 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.