History of construction surveying
- The nearly perfect squareness and north-south orientation of the Great Pyramid of Giza, built c. 2700 BC, affirm the Egyptians’ command of surveying.
- A recent reassessment of Stonehenge (c.2500 BC) suggests that the monument was set out by prehistoric surveyors using peg and rope geometry.
- In the sixth century BC geometric based techniques were used to construct the tunnel of Eupalinos on the island of Samos.
- Modern technology advanced surveying’s accuracy and efficiency. For example, surveyors used to use two posts joined with a chain to measure distance. This technology could only account for distance and not elevation. Current technology uses the Global Positioning System (GPS) that can measure the distance from point A to point B as well as differences in elevation.
Elements of the construction survey
- Survey existing conditions of the future work site, including topography, existing buildings and infrastructure, and underground infrastructure whenever possible (for example, measuring invert elevations and diameters of sewers at manholes)
- Stake out lot corners, stake limit of work and stake location of construction trailer (clear of all excavation and construction)
- Stake out reference points and markers that will guide the construction of new structures
- Verify the location of structures during construction
- Provide horizontal control on multiple floors
- Conduct an As-Built survey: 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 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.
Chainage or station
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.
Other coordinate systems
In other types of construction projects, arbitrary “plan north” reference lines may be established, using Cartesian coordinates that may or may not necessarily correspond to true coordinates. The technique is called localized grid. This method uses the plan building grids as their own ordinates. A point of beginning is established at the southwest cross grid. IE [N1000.000,E3000.000] The grids are added together heading north and east to make each line its own ordinate.
Equipment and techniques used in construction surveying
Surveying equipment, such as levels and theodolites, are used for accurate measurement of angular deviation, horizontal, vertical and slope distances. With computerisation, electronic distance measurement (EDM), total stations, GPS surveying and laser scanning have supplemented (and to a large extent supplanted) the traditional optical instruments.
The builder’s level measures neither horizontal nor vertical angles. It simply combines a spirit level and telescope to allow the user to visually establish a line of sight along a level plane. When used together with a graduated staff it can be used to transfer elevations from one location to another. An alternative method to transfer elevation is to use water in a transparent hose as the level of the water in the hose at opposite ends will be at the same elevation. A double right angle prism verifies grid patterns, isolating layout errors.