For many years, the optical transit was the surveyor's tool of choice to lay out property lines and building sites. By the 1970s, however, the electronic theodolite began to replace the transit since it could measure angles more accurately on both the horizontal and vertical axes. In the early 1980s, "total stations," which measure distances very accurately by using electronic distance meters (EDMs), became the instrument of choice. Then in late 1990, Geodimeter, Dandryd Sweden introduced the first "robotic total station,". adding automatic tracking and radio communication to a radio and data collector at the "target" or pole. Thus, for the first time, no person was required at the instrument--only at the target, reducing the size of a survey crew.

Today three companies manufacture robotic total stations, Trimble (now the owner of Geodimeter, although it no longer uses that name), Leica, and Topcon. Once a tool for surveyors only, robotic total stations are currently being purchased and used in numerous ways by contractors--especially concrete contractors. One large contractor/construction company owns more than 75 robotic total stations.

The fundamental elements

Each manufacturer of robotic total stations has its own features and benefits, but the basic elements are all the same.

All robotic total stations are servo-motor-driven and measure angles both in the X-Y (horizontal) plane and the Z (elevation) axis. These measurements are very precise and accurate: most instruments measure 3 seconds of accuracy or less (5-second instruments are typically required for construction purposes).

Each total station has an electronic distance meter (EDM). Through either laser technology or infrared sensors, it can precisely measure the distance from the instrument to the target within millimeter accuracy. Precision tolerances of 1/100 foot (less than 1/16 inch) in 1000 feet are typical.

Every system uses a target, which has a prism to reflect light back to the instrument for measurement, and an electronic data collector, which communicates with the instrument through radio communication--also known as telemetry.

By using servomotors, prisms, and infrared technology, robotic total stations search for, and then lock onto, the target, automatically following it as the layout person moves it around a jobsite. At the same time, the data collector built into the target is being updated with information radioed from the instrument.

All manufacturers provide computer software to download and upload coordinate information. This information can come from computer-aided drafting (CAD) files downloaded from office computers, PCMCIA cards (flash cards similar to those used for digital cameras), ASCII files, or coordinate points manually entered into the instrument.

Working with them on a jobsite

Scott Carter, president of Robotic Surveying Solutions, Farmington, Utah, states that "Robotic total stations offer the maximum flexibility on a jobsite. While increasing productivity and accuracy, they reduce labor cost and eliminate error." The instrument can be set up in out-of-the-way places, locating itself by calculating its own coordinates and orientation. You can also place it directly over a control point, the required position for both transits and theodolites. The flexibility of being able to locate the station anywhere on or near the site greatly increases efficiency and accuracy, and reduces errors.

When a robotic total station is turned on, it automatically searches for the target and locks onto it. After the instrument acquires "lock" on the target, it tracks the target at speeds of up to 13 feet per second. And, while it tracks the target, it's also constantly updating the data collector.

"Before you start layout work you must first shoot at least two orientation points or known control points in order for the instrument to locate its position on the jobsite. These two control points are usually land survey points. Other control points can be established and used after a project is started," states Carter. "You are now ready to do layout work. The instrument directs you to each point for staking out. You are directed to each point by a constantly updated map on the data collector or by constantly updated commands, such as to/away, left/right, and cut/fill."

To err is human

Occasionally you might hear the argument that two experienced layout people with a tape measure can be just as accurate as a robotic total station. But this isn't true. Paul Hahn, marketing manager for Trimble's Geomatics and Engineering division, states, "Distance and angle measuring systems just don't go wrong. Assuming that there are no mechanical problems with the instrument, errors in measurement or location are human ones--not instrument ones." The primary reasons for errors are the following:

* The robotic total station wasn't properly put through a checkout procedure before layout started.

* The rodman didn't hold the target plumb when marking the control points to orient the total station--the most common error.

* The "Peg Whacker" (person who pounds the stakes) didn't place the control stake at the point defined by the total station.

* The tripod holding the instrument moved because it wasn't properly stabilized when it was set up.

* Instruments can lose accuracy when treated roughly, not dried when they get wet, or not calibrated, cleaned, or maintained.

When the robotic total station is set up on a jobsite, a few checkout procedures should be followed. If the instrument is set up directly on a control point, carefully check to be sure that the instrument is plumb over the control point and that the control points have not been disturbed. Then always check another control or known point to confirm your orientation. If the instrument is set up in an "out-of-harm's-way location," sometimes referred to as "free station" or "resection," shoot at least two control points and then do a check shot on at least one other control point on the jobsite.

Good layout people always check their work as they proceed. If, for instance, control points for a rectangular space are plotted, the diagonals should be checked to be sure that they are equal. Programs for checks like this are found in each data collector.

Limitations

The greatest limitation for robotic total stations is that they require a line of sight to each control point. Another problem, according to Mark Contino, marketing manager for Topcon, Pleasanton, Calif., is occasional radio interference between the data collector at the target and the instrument. This problem is partly solved by multichannel frequencies.

In terms of accuracy, Hahn states that one can expect horizontal distances of 700 feet and less to be accurate within [+ or -] 2 mm (about 1/16 inch). Positional accuracy should be within 1/8 inch. Beyond 700 feet, accuracy drops to within 1/4 inch. Rick Sauve, technical sales rep for Leica Geosystems, Livonia, Mich., adds that the best vertical shots for plumb occur below 500 feet. Above that, light rays can be affected by atmospheric conditions. Wind sway in tall structures can cause problems too.

Going prism free

Carter acknowledges that the primary selling point for using robotic total stations is the freedom of having one person operate the system from the target. But many owners are beginning to see the advantage of moving that person back to the crosshairs of the instrument to use its laser EDM capabilities without a prism--referred to as "direct reflex" technology. By placing the eyepiece crosshairs of the instrument on a point, the distance, coordinates and other important data relating to that point can be collected. Plotting and mapping areas that are difficult or unsafe to reach with the target can now be completed with ease and safety, saving time and energy. Examples of using EDM include the mapping of wetland areas by plotting points where the water touches land. One can also locate an instrument at a safe position alongside a busy freeway to plot points in heavy traffic or take shots on the centerline of a road without actually going into the road. Or, you can safely measure the distance from the ground to a sagging powerline that could come in contact with construction equipment, or shoot a tree or dirt pile without ever getting next to it or climbing onto it.