What are total stations?
Total stations once were complex and were only used by surveyors. Surveying equipment-manufacturers, such as Leica, Topcon, Trimble, and Sokkia, realized, though, that contractors could increase efficiency by doing their own layouts. This inspired the development of much more user-friendly instruments that meet the needs
of contractors. Since employing a surveying team every time a contractor needs to check a location is impractical, survey teams are typically brought in only to locate the points marking the property limits. Contractors then use those points to locate their instruments and begin the process of locating points on the jobsite.
In the progression of optical layout equipment, the classic levels could transfer vertical dimensions from one point to another, then with a steel tape measure and some trigonometry the surveyor could locate any point. With the development of theodolites in the 1960s and 70s, surveyors could lay out precise angles in both the vertical and horizontal directions. Total stations evolved from theodolites, adding electronic distance measurement (EDM), completely doing away with tape measures.
Robotic total stations take total station technology one step further. By adding radio communication to receivers on the rod and servo motors to the instrument, it's now possible for the instrument to follow the rod and send information about the location of points to the person at the end. The big difference between a total station and a robotic total station for most applications is that two people are needed to mark points using a total station, while one person can easily operate a robotic total station.
Data collectors and software
To make the job of layout easier, the manufacturers of total stations have created data collectors. Looking like a calculator with a small computer screen, the data collector plugs into the total station, providing information about point locations. You can download computer aided design (CAD) files into the data collector or load coordinates for each point that you wish to layout in the field. Manufacturers make this process even easier today with computer software that allows you to identify the points you need to layout from plans that may have many sheets, and then download the points and draw shapes in your data collector.
When the data collector is plugged into the instrument, the person with the rod is directed by the total station where to move the rod in order to find the point.
How accurate can you be?
Depending on the unit, points can be located as far away as 8200 feet from the instrument. Instruments for industrial concrete work, however, are rarely used to shoot points at these distances—the accuracy of the point location is more important, especially for locating anchor bolts.
Rods have a prism on top of them which is what the instrument sees. Holding the rod plumb is a key factor in getting highly accurate locations. Contractors have discovered that the best point location can be achieved using a short rod—only about a foot long—with bipod legs to hold it in a plumb position. Roland Raffin, a superintendent for Raffin Construction, Chicago, says that using a short rod they can locate points within 1/16 inch of the true point.
Total station technology for the construction industry was first adopted by general contractors. Residential foundation contractors were the next group to see the advantages for quick, accurate layout work. Robotic total stations have become standard equipment now for 3-D laser screeding and stringless paving work.
Industrial concrete contractors are the latest contractor group to start using the equipment. Their primary reason is due to the accuracy required for anchor bolt and metal embeds layout in concrete. Here are some reports from contractors about how they are using their equipment.
- Raffin says they bought their robotic unit when they got the contract to build a steel tube mill in the Chicago area. “We have 1000 anchor bolts to locate on this project and this was the perfect solution. Accuracy is very important and mistakes are costly. My father, the owner of the company, believes in pencil and paper. But we jumped on this opportunity, wanting to learn how to use the technology so that we can increase our productivity and accuracy.”
- Jim Raffin, a project manager for Raffin Construction, loads the points from the plans to the data collector. He does this using software from the manufacturer. He says that their surveyor will set property control points that can be used by their robotic instrument to locate itself on the property. And until they feel more confident about their layout work, they will also ask their surveyor to revisit the job to double-check their work. He adds that when you own a tool such as this, you also use it for other purposes because it makes the job easier. On this project they will lay out foundation perimeters, caissons, and cooling system troughs.