Construction surveying



Surveying is the process by which a surveyor measures certain dimensions that generally occur on the surface of the Earth. Surveying equipment, such as levels and theodolites, are used for accurate measurement of angular deviation, horizontal, vertical and slope distances. With computerization, electronic distance measurement (EDM), total stations, GPS surveying and laser scanning have supplemented (and to a large extent supplanted) the traditional optical instruments. An important distinction is made between land surveying by a certified land surveyor, and construction surveying.

Land Surveying
In the United States, Canada, the United Kingdom and most Commonwealth countries land surveying is considered to be a distinct profession. Land surveyors have their own professional associations and licensing requirements. The services of a licensed land surveyor are generally required for boundary surveys (to establish the boundaries of a parcel using its legal description) and subdivision plans (a plot or map based on a survey of a parcel of land, with boundary lines drawn inside the larger parcel to indicated the creation of new boundary lines and roads).

Construction Surveying
Construction surveying is generally performed by specialized 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 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.

Geodesic Coordinates
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.

Chainage
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).

Building Axes
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.

Reference Lines
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.