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Developing a Laser Surveying Station for Archeological and Paleontological Sites

Ted Etter, Project Leader,
and James “Scott” Groenier, Civil Engineer

Several years ago, the Missoula Technology and Development Center (MTDC) was asked by a paleontologist in the U.S. Department of the Interior Bureau of Land Management (BLM) to investigate and, if necessary, construct a surveying tool that would require very little training for novice users. Frequently, surveying tasks are performed by seasonal or temporary workers, so project managers are interested in getting reliable measurements with minimal training for new personnel.

The original concept was to develop an ultrasonic range-finding network that would measure the distance between the survey target and several reference points. If the reference point locations were accurately measured, the position of the target could be calculated. The position measurement accuracy of the ultrasonic transponders tested was on the order of +/– 5 centimeters, less accurate than the subcentimeter accuracy the BLM had requested. The only viable option appeared to be a laser rangefinder.

After investigating the availability of suitable components, the project team at MTDC determined that something resembling a total surveying station could be assembled from materials costing less than $4,000, not including the cost of a laptop computer that would convert positions in polar coordinates to rectangular coordinates and store those points in a database. The laser rangefinder selected had an accuracy of 1 millimeter. The angle encoders that detected the laser's aiming point could resolve angles with an accuracy of 1/20 of a degree or less. The resulting accuracy for a point 5 meters from the laser would be better than +/– 6 millimeters, adequate for this application.

A critical component that allowed the MTDC laser surveying station (figure 1) to be constructed relatively inexpensively was a mounting gimbal for the laser. The project's mechanical engineer found a surplus unit with vernier adjustments that could be used when aiming the laser. Although the project team discussed building a second laser surveying tool, the team had the last surplus gimbal. Purchasing or manufacturing a commercial gimbal of the same quality would increase the cost of a station by several thousand dollars, meaning that the MTDC laser surveying station would cost as much as a commercial total station.

Figure 1—The prototype MTDC laser surveying station
(the laptop computer and the box with the electronic
interface to the laptop are not shown).

The MTDC laser surveying station measured locations in a polar coordinate system based on:

• Distance from the laser (or radius)
  
  
• Elevation angle
  
  
• Compass angle (or azimuth)
  
  

Total stations also use a polar coordinate system. Typically, users are interested in rectangular, or NEZ (north, east, and elevation) coordinates. Calculations must be performed to convert a location in polar coordinates into one with rectangular coordinates. For the MTDC device, the calculation was performed by software in a laptop computer. In total stations, the calculation is done internally. MTDC's customer already had a laptop computer for cataloging the surveyed points and keeping records of the excavation sites, so the laptop provided “free” computing.

It is desirable to measure locations of points or artifacts by specifying those locations in reference to a local coordinate system, such as the floor of a rectangular building. Frequently, it is impractical to set up the surveying tool on the exact origin of a coordinate system, so some method is needed to translate the points measured relative to the surveying tool to coordinates based on the local environment. For the MTDC station, the team performed this translation with software in the laptop computer. Most new total stations also perform this task.

The MTDC laser surveying station used a laser range finder that produced a visible red spot on the target. The most expensive total stations (figure 2) the project team surveyed are available with a red dot for aiming. All total stations included a telescope with crosshairs that provides a magnified view of the target. Although the red dot provides some assurance that the correct point is being measured, the red dot is not required with the total stations. In direct sunlight, the red laser dot can be difficult to see from a distance. In such situations, the telescope with its integral crosshairs is a better alternative.

Figure 2—A commercial total station used for surveying
(instrument provided for this study by Selby's Engineers
Service and Supply Co., Missoula, MT).

Regardless of how the surveying measurements are obtained, the data typically need to be stored in a computer database when documenting the site. For the MTDC laser surveying station, the database was in the laptop that translated the coordinates. The interface software for the electronic components was written in Microsoft Visual Basic by MTDC, and the data were stored in a Microsoft Access database.

The total stations surveyed have serial data interfaces that allow the measured coordinates to be transferred to a computer for storage and association with other files. Several programs, such as Terramodel from Trimble, Land Desktop from Autodesk, and Eagle Point from Eagle Point Software Corp., allow the measured coordinates to be used to model surfaces and prepare maps. All three of these programs are being used by archeologists, hydrologists, surveyors, and engineers in the USDA Forest Service.

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since February 28, 2005