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Surveying refers to a kind of technology employing instruments to accurately determine the terrestrial or three dimensional space position relationships between points on land and express shapes, areas and other aspects by figures or drawings (Bell, 1991). Surveying dates back to the ancient times (Keay, p. 182, 2000), and were it not for this technology, famous pyramids surviving to this day could not have been built. These pyramids would be composed of four sides with the same length and ultimately same surface areas converging at a single point while maintaining a uniform angle (Cazier, 1993). This justifies the fact that accurate surveying methods must have been employed to achieve these accurate dimensions. Surveying can be said to have been invented with advent of construction and has continued to evolve to match up with the ever dynamic field of construction (Roberts, Jack p. 228, 1995). “Construction surveying is an integral part of the construction management technology program, it serves as an introduction to a variety of surveying equipment and techniques and encompasses the process of gathering information about a proposed job site, laying out the location of structures on the sites, checking the dimension of the structures during construction and documenting the completed work” (McCormac, p. 227, 1991).
Introduction
This essay intends to establish the main differences between GPS surveying and Laser total station surveying within construction and single out their respective applications. The essay would also point out when it is necessary to use either of the two surveying methods. “Building surveying emerged in the 1970s as a profession in the United Kingdom by a group of technically minded general practice surveyors” (Wilkinson, 1994).
GPS (Global Positioning System)
GPS is a passive, all weather, 24-hr global navigation satellite system which came into being in late1960’s and early 70’s primarily for navigational purposes by the military. The technology was seen to be very helpful in many different ways such that the president of USA, Ronald Reagan promised free availability to the public but people became aware of the technology after its successful application during the Persian Gulf War in 1990s (Bennett, 1987). Since then, the technology has been embraced in other fields including; automotive industry, light air, marine navigation, surveying, mapping, environmental research and recently in construction (Lewis, 2007). The technology refers to a satellite based navigation system that engages a number of twenty four to twenty seven satellites orbiting the earth to provide triangulation and accurate navigational signals through a receiver on the ground which is capable of accurately tracking the information from the satellites (Lewis, 2007). GPS can also be defined as a, “system of satellites, computers and receivers that is able to determine the latitude and longitude of a receiver on earth by calculating the time difference for signals from different satellites to reach the receiver” (Genovese, p. 314, 2005). It is believed that a single GPS receiver can find its own position in seconds from GPS satellite signals to an accuracy of one meter, a capability which has reduced the cost of acquiring spatial data for making maps while increasing cartographic accuracy (Thompson, 1979). The technology works best with clear skies, thus there should not be any obstruction probably from tall buildings since this would block the satellites ultimately lessening the accuracy, it is believed that there should be an unobstructed view of four or more GPS satellites for effectiveness to be established (Robert & Elizabeth, p. 8, 1982).
During constructions like for example, a highway construction, there could be so many obstructions including: changing terrains, rivers, streams, bridges and so on, which can be easily dealt with by employing the GPS technology to map the area. For heavy highway contractors, this technology enables one to quickly survey the geographical area while increasing the accuracy and speed of the survey information, qualities which were missing in the past since a line of sight would have to be established for precise positioning (Rayner & Schmidt, pp. 82 – 85, 1969).
Studies across United States have shown that this technology increases the productivity of conventional surveying crews, improves survey accuracy, and allows crews to work under a broad range of weather conditions, furthermore the technology requires less expertise to operate the GPS surveying unit that is needed to operate conventional surveying technologies (Brown, 2004). GPS is quickly taking center stage in highway community and it can be used to provide traveler information and even for mapping since it can be integrated easily with geographic information systems and even better the technology eliminates the need for constant surveying and mapping updates on the job site because the GPS data in conjunction with computer aided drafting (CAD) and geographic information system (GIS) technology, can keep track of a projects progress; this would also important in initial project study and evaluation (William & Finlay, 2005). Due to the fact that the technology uses satellites, it can be used across long distances with minimal setups since after the system has been placed, roving can be performed within a radius of 10 kilometers of the stationary base unit; this makes the technology fast, to require less labor, less training and relatively more accurate compared to conventional surveying technology (Riches & Allie, 1966). “GPS technology on a large a large earth repositioning jobsite cuts down on surveying costs because surveyors are not needed to set up thousands of grade stakes for surface work, but only to set control for GPS, certify pads, and stake for underground utilities” (Johnson, p. 1, 2007).
GPS technology and wireless communication can be used by contractors to acquire real time location of machines, historical data (its performance over recent times), tracking and monitoring information, this makes it useful for structural placement and bridge construction (Michelsen, p.1, 2000). Bridge construction, especially the ones which need large structural pieces can be placed with precision and accuracy using GPS positioning to follow strict design blueprints, this improves on quality and ultimately ensure durability of the structures. This technology can also be used to monitor verticality of tall structures since its accuracy does not lessen with heights thus it can be used to compare actual position with design location, calculating any shifts or changes from external forces such as wind ultimately ensuring faster project completion, reduced construction costs, and more efficient monitoring upon completion of a project (Michelsen, p. 2, 2000).
GPS technology in conjunction with geographical information system (GIS), has also been very applicable in groundwork construction, with onboard Gi database, including information on location of existing pipelines and utility cables, trenches can be dug without any danger to existing underground networks; this would pose a great challenge in the past before the advent of GPS technology where it would be a matter of try and error and sometimes it would lead to other unprecedented costs on repair works.
GPS technology can be used during constructions to safeguard construction equipments from theft and vandalism; theft and vandalism in buildings and construction sites remains to be one of most growing problems today, vices which could be curbed by application of GPS tracking devices to protect theft and even help recover stolen equipments. The GPS unit sets up a curfew on someone’s equipment to ensure the latter can only be used with the owner’s authorization and if anyone attempts to move the equipment, the unit will either call, relay an email or disable the equipment.
GPS is mostly applied in the following construction survey works which would allow a typical accuracy of 1 – 5cm:
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Grading, guided according to a predefined terrain model surface,
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Elevation determination during instillation of utilities (e.g. pipelines, power lines, cables,
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Staking out of road marks, footings, pipelines, utilities, landscapes, and so on,
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Mapping,
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Checking of the as-built with the designs,
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Site explorations for new projects (EL-Mowafy, p. 4, 2004).
“GPS is turning out to be very applicable in constructions as it is being used to guide the operation of the construction equipments during the construction process itself; this has been made possible by the incorporation of 3D controls which involves the use of one or more fixed base stations, located in and around the construction site, coupled with one or more mobile units respectively disposed on the various pieces of construction equipment that are to be controlled via the system” (Colvard, p. 9).
Incorporation of GPS and GIS has simplified so many things, For example; in highway application, roadway information, alignment, and condition are supplied by the system to engineers to assists them in planning Railroads stand to benefit greatly from incorporation of GPS to track roadbed information such as tilt, soil stability, or rockslide likelihood (ASCE, p. 231, 1998).
The main advantage of a GPS over laser total station is the fact that the base station broadcasts over an area with radius of around 10km, depending on the radio used, it also broadcasts omnidirectionally, without needing a direct line of sight from the reference station to the rover unit and it would broadcast through dust and around obstructions, additionally, GPS can support an unlimited number of roving units within the broadcasting area (Jonasson, p. 368, 2000).
Shortcomings
The system relies on a connection to the satellites for positioning, and on the radio link for real time kinematics which may be a greater challenge in some sites probably due to various interruptions for both signals not ruling out the possibility of a multipath GPS signal. Multipath can cause the GPS receiver to compute inaccurate GPS positions or slow down the performance, but with the evolving technology it can be mitigated by use of a choke-ring antenna though its use is limited by the fact that it is very expensive and too heavy for convenient field use (Manson, p. 8, 1997).
The technology compared to the laser total station especially the more sophisticated 3D systems, does not offer automatic height control for the blade, moreso, for motor grade operators (Sickle, 2001).
Because GPS surveys are optimized to satisfy specific needs of a particular survey, other applications at the site may not be easily captured, further, GPS surveys may need to be carried out, as the need arises, for few applications implying that one cannot have a permanent monumental control network altogether, and reestablishment of coordinates by GPS would be necessitated each time a need arises (Rizos, 1999).
Due to the fact that intervisibility is not necessary with GPS technology, the technology is attractive and sufficiently applicable in rugged and inhospitable terrain, however, logistical problems of transporting and supporting several field staff at these areas are still formidable thus requiring complex means of haulage like the use of helicopters, which may elevate the costs.
“Environmental sources of error such as satellite geometry (PDOP) may lead to a poor computation of the GPS position; when satellites are spread evenly across the sky, a set of pseudo-range measurements to these satellites has a good geometry for trilateration, the mathematical operation of computing a position on the ground given the position of the satellites and the pseudo-range distances to these satellites, when satellites are close together in the sky, the trilateration geometry is not so good, and measurement errors tend to compound ultimately resulting to this kind of error” (Manson, p. 2, 1997). Real time kinematics have increased the speed of surveying, but the technology mostly still remain horizontally accurate to about 20mm and vertically accurate to about 30 – 40mm (2002, p. 40).
Illustration
GPS has been in use and there are successful instances where the technology was used intensively, though not exclusively, and a good example is the Vasco de Gama Bridge (novaPonte) in Lisbon, Portugal where real-time kinematics GPS was used for the following:
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Marine and land stages,
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Hydrographic survey,
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Dredging,
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Control survey,
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Topo survey/earthworks,
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Bridge pile positioning,
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Approach road setting out (Manson, p. 6, 1997).
Laser Total Station Surveying
“This refers to a modern surveying technology which uses electronic or optical technology to read distances to a particular point, the total station is an electronic transit integrated with an electronic distance meter to read distances from the instrument to a particular point” (Flexline, 2008). This technology was first introduced in the late 1950s and has since those days undergone continual refinement, the early instruments were capable of very precise measurement over very long distances, were large, heavy, complicated and expensive but with technology advancements these machines have been made lighter, smaller, simpler and less expensive models (Roberts, p. 228,1995). The technology is very efficient as it can allow the operator to control the instrument from a distance using a remote control, a capability which has been made possible by introduction of Robotic total stations; this has led to reduction in surveying costs because it eliminates the need for an assistant since the operator can hold the reflector and at the same time control the total station from an observed distant point. Robotic total station is operated using a remote positioning unit (RPU) from as far as 1, 500 feet away and it is positioned over a station and oriented to a back sight station from which it can be controlled from the RPU, after moving to the point to be surveyed, the operator pushes a button and a signal is transmitted by radio link, to the robot which finally begins to search for the retro prism on the RPU (Roundtree, p. 230, 1998). Robotic total station has simplified surveying exercise since the exercise can now be done in the dark thus saving on time, it has led to increased productivity, increased efficiency and customer satisfaction, and it has made the whole exercise easy, faster and more efficient.
Using laser total station one can determine angles and distances from the instrument to points to be surveyed and finally by application of trigonometry use the same angles to determine the actual positions (coordinates x, y and z) of surveyed points in absolute terms. The technology incorporates two components; electronic transit and electronic distance measuring device (EDM), the former is basically a telescope with cross-hairs for sighting a target and it is then attached to scales for measuring the angle of rotation of the telescope and the angle of inclination, the electronic transit offers a digital read-out of the angles instead of following on the scale while the latter, EDM measures the distance from the instrument to its target; the component sends out an infrared beam which is reflected back to the unit, and the unit uses timing measurements to calculate the distance traveled by the beam (Eiteljorg, II, Vol. VII, 1994). In the past the technology used to employ prisms which had to be placed at the measurement point; this would be limiting in the sense that it would be difficult to measure distances to high locations, diagonal surfaces or inaccessible locations but with application of reflectorless method, one would be able to survey areas from distant locations including areas of possible danger such as landslides. This aspect has made it possible to do recordings on upstanding buildings and ruins, areas which would pose great risk in the past due to the nature of measurements which would require access to literary inaccessible or dangerous locations. The technology allows one or two people to do the surveying job contrally to the past where the exercise would require very many people; this has increased accuracy to within one hundredth of a foot which is a great contribution to quality surveying (Schofield & Breach, 2007).
Total station allows surveyors to measure slope distances as well as horizontal and vertical angles making the technology appropriate for differential leveling and for stadia readings for topographic work (Arumala, p. 230, 2000).
“This kind of technology is mostly used by land surveyors, archaeologists to record excavations and by police, though recently it has been used by crime scene investigators, private accident reconstructionists and even insurance companies to take measurements of scenes” (Flexline, p. 4, 2008).
Total stations are commonly used in mining survey to assess the ground before the drifts of underground mine are driven, this ensures absolute locations of tunnel walls, ceilings, and floors are established and guarantees safety of the miners since this information is downloaded into a computer and an application software used to compute results and generate a map which is then compared to the designed layout of the tunnel.
Shortcomings
In order to maintain line of sight between the prism and the robotic tracking station, the prism is usually elevated above what it is measuring or controlling; this makes it difficult to hold the center of the prism at the point you want to measure or locate, considering the fact that, the point being measured is always offset from the center of the prism, manual point location is normally achieved by placing the prism on a pole, of known length, and then by aligning and holding the poll plumb to the earth ultimately making the whole process to be time consuming and more susceptible to human error ( Colvard, p. 12, 2005).
A prism would only locate a single XYZ point in space, implying that a stationary prism, by itself, would not directly or sufficiently show any direction or orientation making the use of laser total station limited to an extend that the technology is not sufficiently applicable without being aided by some other technologies.
With robotic tracking station, it would only track one prism at a time, if one chooses to use more than one prism, then this would allow direction or orientation to be determined but it would necessitate use of more additional robotic stations which have to be set up and sufficiently calibrated.
This technology has so far been modernized to incorporate imaging technology such that telescope images can be viewed directly on the instruments full color windows display, an aspect which has extended its range of application. The imaging technology has the following advantages:
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Instant visual record of control points to aid relocation,
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Staking out points superimposed on real-time views,
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Use site observed digital images to show recorded positions,
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Easy identification and simple visualization of surveyed points,
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Quick and precise identification of corner positions through the use of auto edge extraction (Capelle, 2005).
Sources of Error for a Laser Total Station (instrumental errors)
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Trunnion axis error,
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Eccentricity of scan center,
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Angle of incidence as well as surface properties (Schulz and Ingensand, 2004a, 2004b).
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Collimation axis error,
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Horizontal axis error (Deumlich and Staiger, 2002).
Conclusion
Global positioning System (GPS) does not require line of sight as it is the case with laser total station survey making it more convenient where long distances are involved, although some total stations nowadays have a global navigation satellite system interface which does not require a direct line of sight to determine coordinates, however, GPS has been used for instantaneous determination of a total station location working on the site by mounting the antenna directly on top of the total station alidade; thus eliminating the need for establishing permanent horizontal control stations. This method is however appropriate for medium accuracy construction surveys, such as grading and staking out of road marks, footings, pipelines, utilities, landscapes and fences (EL-Mowafy, p. 1, 2004).
The two types of surveying are used according to the nature of application and level of accuracy, for example, total stations would be most appropriate when extremely high accuracy is required such as building and bridge construction, though relatively expensive, while GPS systems would be more convenient where errors on the order of millimeters to centimeters are permitted (Nikon, 2009), when no reference point is near, or when the accuracy does not have to be perfect. Laser total station can be used to complete GPS surveys in situations where the latter would not be sufficiently applicable due to obstructions such as trees or high buildings though, its range of operation is less as compared to GPS guidance systems.
Regular use of GPS and Laser total station would guarantee increment of quality productivity, reduction of reworking, and frees the grade checker at least for parts of the day; these attributes should endear the two forms of technology to people’s day to day activities especially on surveying (Pugh, p. 230,1975). GPS surveying technology would be highly ideal for construction of a motor way in an open space due to the fact that, it operates best in clear skies or open environments since there would be uninterrupted transmission of GPS signals from the satellites to the receivers at the sites, improving on quality and efficiency of the surveying process. However for construction of a multistorey building in a town center, the technology falls short and if adopted its accuracy would be compromised due to blockage of the satellites thus the use of laser total station surveying technology which can operate best in this kind of settings. Overall, application of both technologies has helped cut costs and increased production for companies and organizations which have put the technologies in practice ultimately benefiting clients (Spier, 1970).
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