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The Use of Laser Tools in ArchaeologyJohn W. Rick |
As an archaeologist, taking site measurements accurately and efficiently are staples of my fieldwork, and I am both excited by new developments in measurement technology, and frustrated by limited access to information about advances in this field. For the past few years I have been sporadically researching this type of technology, and have amassed a small, opportunistic arsenal of measurement devices and information about them. Without any pretense that I have comprehensive, or even the most up-to-date information, I'd like to share some of my findings and experiences. My goal is to describe laser-based devices that can increase our accuracy and/or decrease the amount of time we spend gathering data, all within reasonable budget limits. Most devices either allow rapid and accurate distance or angle measurement involved in traditional surveying: azimuth, declination, or radius from instrument to measured point. Here I will explore a variety of such devices, as well as those important in defining lines, planes, or orthogonal axes, but I am intentionally not covering total stations, or laser-based theodolite systems, the possible subject of a future column.
split the beam five times to obtain plumb, square, and level reference. There is a one-inch offset between vertical and horizontal beams. Most importantly, the device is self-leveling, as long as it is placed within 8deg. of level. Thus, when positioned in a site, the device establishes basic elements of a precision grid--three horizontal axes, and plumb lines that can be aligned to datums above or below the instrument. The PLS5, the model number of the instrument, has a standard camera 1/4-20 threaded socket, allowing it to be mounted on an inexpensive photographic tripod for rapid positioning. Its beams are visible under a variety of weather conditions, and its range is listed at about 100 ft, although bright outdoor light may limit this somewhat. Maximum error of each beam is listed as less than 3 mm at 15 m, however the manufacturer advises that the error is closer to +/- 1.5 mm at 15 m. To give anidea of this accuracy, each subsequent setup of the instrument, relying on the previous setup's base points, would have a maximum of around .02deg. angular error. In other words, considerable repositioning could be done before any significant error would accumulate, by field archaeology standards.
An obvious use of the device's orthogonal beams is to establish excavation grids or other right angle site standards. The PLS5 could also allow efficient mapping work within standing architecture. By marking the endpoint positions of the left-right laser beams, the PLS5 can be laterally displaced, realigned to the endpoints, and new lateral measurements taken. Similarly, the device could be moved through doorways, or down corridors, fairly easily, while maintaining a common horizontal alignment through the endpoint marks on walls. Wall positions can be measured laterally from the laser beams, and in combination with a portable EDM, quite rapid and accurate measurement of architecture is possible without the elaborate and slow setup procedures of theodolites, transits, or alidades. To measure wall features at different heights, the device could simply be elevated, maintaining plumb position above a floor datum, and aligned to plumbed positions above a previous wall mark.
The PLS5 is about the size of a small hand drill, weighs .77 kg with batteries, and uses three AA alkaline or rechargeable nicad cells, giving continuous use for about 15-20 hours. It will not operate if tilted out of auto-leveling range, and it is easy to check the accuracy of level and beam orthogonality. The device sells for about $1,495, and comes with magnetic wall bracket and a target for transfer of vertical positions [contact the manufacturer, PLS Pacific Laser Systems, 449 Coloma St., Sausalito, CA 94965, (800) 601-4500].
The Disto (Figure 2), made by Leica, is apparently the only HHEDM yet available, retailing for $1,495 without serial port, $1,995 with serial port [contact the manufacturer for distributors at 3155 Medlock Bridge Rd.,
Norcross, GA 30071 (800)-367-9453]. It weighs about half a kg, measures 23.5 cm x 10.4 cm x 5.9 cm (Figure 3), and can measure distance to the target from either its front or rear edge. It can measure distances from 20 cm to 100 m with an accuracy of +/- 3 mm, but non-reflective, low angle, or other difficult surfaces beyond 30 m require the use of a white or brown target plate. I have found that very bright light conditions (such as I experience in the sierra of Peru) may make measurement difficult and limit the distance that can be measured, and, particularly, the visibility of the laser sighting dot. A bright light attachment helps solve the latter problem, but does not help too much with the former. In practical terms, it is difficult to hold the target beam
steady on small surfaces more than 30-50 m away, and if the beam wanders over a surface of varying distance, the Disto will not return a measurement. Measurement can take from two to 10 seconds, and the internal, nonremovable nicad batteries will provide about 400 measurements before recharging. It can also continuously track distance to an object, and has three memory registers that allow it to do simple addition and subtraction of distances, and calculation of simple areas and volumes based on measured distances. Recharge time is about an hour using either the 110 v line transformer or the supplied 12 v charging cord.
This device has proved very rapid and accurate, and is particularly valuable in difficult measurement situations. For instance, it excels in measuring high ceilings, long otherwise-impenetrable shafts, or distances across broken terrain. We found it indispensable for taking accurate and rapid measurements of the extensive internal galleries and ventilator shafts permeating the monumental architecture at Chavín de Huantar, Peru, parts of which cannot be entered at all. This particular model comes sheathed in a shock-absorbing rubber jacket, and our rigorous use suggests it to be very durable under field conditions, and tolerant of dust, rain, and general abuse. A HHEDM can be used very effectively in conjunction with other instruments for efficient mapping. Using one with a transit can simulate a total station, albeit with much lowered convenience and efficiency. Coupled with the PLS5 device described above, cartesian coordinate mapping can be quick, avoiding the use of sometimes confusing surveying angles. Alternatively, an HHEDM can be used with a plane table, or mounted in a protractor device to quickly produce polar coordinate measurements. This latter technique, quite practical for a single fieldworker over moderate distances, is one of the most rapid dimensional data recovery field methods, although not necessarily the most accurate.
Distantly related to HHEDMs are a number of ultrasonic distance-measuring units from a number of manufacturers such as Stanley. They typically are much less expensive, but have shorter ranges (typically 30 m or less), have limited accuracy (+/- 2 percent of the distance measured is considered good for these), and most importantly, it is difficult to know which of many possible surfaces or objects is actually being measured by the ultrasound, since a focused light beam is not involved. They might have limited application in archaeology, but caution would be advised before confidence is placed in resulting data.
The utility of this device in archaeology is not clear, since it offers no easy method of full three-dimensional measurement. In industry it is most often used for precision level measurements on known points. A number of models are available from different manufacturers; fully functioning packages from Leica run approximately in the $4,500-$6,500 range.
The Advantage is hand-held, weighs about 2 kg, and is operated by lining the target up with cross hairs in a head-up display. Running on rechargeable nicad batteries, the device can run all day, taking measurements constantly. It is priced at $2,995 for the version with both distance and angle-measuring capabilities; more information on the Advantage can be obtained from Laser Atlanta, 2827 Peterson Pl., Norcross, GA 30071, (770) 446-3866.
This is the only device in this listing that can produce, on its own, three-dimensional provenience. Given its portability, speed, and somewhat limited accuracy, this device would be best suited for survey situations where rapid, reasonably accurate measurement is needed, especially over long distances. The angular accuracy would allow point location within about +/- 1 percent of the distance to the point measured, while the distance error is more fixed, becoming relatively minor compared to the angular error at distances over 30 m. I have not had a chance to use this instrument, but care should be taken to make sure the internal compass is not affected by metallic objects or electromagnetic fields in the operating environment. Coupled with GPS devices for measuring global site location, this device could allow for rapid site documentation for many field projects.
My supplier, Haselbach Instruments of San Mateo, Calif., (800) 462-8181, has been a valued consultant across many years of fieldwork and a major source of new technology and ideas about how to use it. Along with the companies listed above, their provision of much of the information included here is gratefully acknowledged.