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Archaeology and Technology

The Use of Laser Tools in Archaeology

John W. Rick


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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.

Laser Pointers

Although familiar to all of us in the context of slide lectures, the penlight-battery-powered laser pointer can also be a valuable, durable, lightweight, and inexpensive part of site measurement. Most of these pointers produce a quite small diameter, reasonably bright beam that can be visible for at least 50 m, and considerably further under low light conditions. Pointers have different degrees of beam spread at distance, so it's worth checking the size of the beam at the distance you will be working before purchasing. Some will project a fairly sizable spot, but of a consistent round or oval shape, such that the center of the beam can be consistently found, allowing considerable alignment accuracy. These pointers can be attached to a leveled tripod head, giving a horizontal line from which to take vertical or lateral measurements to objects or surfaces of interest. With a little help from a local machine shop, a mount, bearing a high-precision bubble level, can be prepared that can replicate, at much lower cost, some of the high-end laser levels mentioned below. In combination with laser distance-measuring tools (see below), a fairly rapid survey can be made of sites with moderate slope and low surface features. Within standing architecture, laser lines are much more convenient and accurate to measure from than leveled strings, a staple of low-end architectural measurement. Laser pointers can be obtained from a wide variety of electronic and office supply companies, and prices below $50 are becoming common. Battery life for two AAA batteries seems to be on the order of 30+ hours.

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Automatic Plumb Level Square

This is perhaps the most unique instrument of those listed here, with a number of practical applications in archaeology. In essence, this device emits five visible light laser beams, all at right angles to the others: up, down, left, right, and forward (Figure 1). The manufacturer has taken one laser diode and
Figure 1
Figure 1: Pacific Laser System's PLS5, a device emitting orthogonal laser beams.

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].

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Hand-Held Electronic Distance Measuring Devices (HHEDMs)

Just now making its appearance is a highly portable, versatile device capable of measuring linear distance with considerable accuracy. Analogous to the invisible-laser-based distance measuring devices incorporated into total station theodolites, the HHEDM instead uses a visible red beam both indicating the target and measuring the distance to it. For most reasonably reflective surfaces, such as dry earth, stone, or light-colored vegetation, there is no need to place a reflector at the target. In operation, a single person can aim the HHEDM's laser, shoot the distance, and record the measurement. The distance of the shot is displayed on a LCD readout, and an advanced model can pass information to a recording device through a serial port.

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.,

Figure 2
Figure 2: Leica's Disto, a hand-held laser distance measuring device.

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

Figure 3
Figure 3: Laser Atlanta's Advantage, a hand-held rangefinder.

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.

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Automatic Laser Levels (ALLs)

To my knowledge these instruments have rarely been used in archaeological applications, but they might prove useful. Essentially they are relatively lightweight (5 kg), tripod-mounted devices that create a plane of light by rotating a laser-emitting prism. Usually projected as an absolutely level, horizontal plane, many such devices can be rotated 90deg. to provide a vertical plane, or even to other planes lying between horizontal and vertical. While some levels use a visible laser that can be seen on a vertical staff or other height-registering tool, both these and other invisible laser levels use a hand-held or staff-mounted detector that both visually and/or audibly will indicate when it is in the plane of the laser. ALLs have relatively short ranges compared with total stations, varying between a working radius of about 30 m and 250 m from the device. By themselves, laser levels may have limited applicability, although they could be used to establish highly accurate depth datums in sites, or other cases where establishing an accurate plane is important. Coupled with an HHEDM, however, a laser level could be used for mapping or point proveniencing. Using the HHEDM to measure from the depth rod to two known target datums, the position of the rod base could easily be calculated. If a variety of target datums were available, a quite versatile if somewhat complex measurement system could be established, allowing measurement over standing walls and other obstructions that might defeat other measuring methodologies. The planar nature of measurement using levels may not be compatible with many archaeological situations, but they should be kept in mind as a potential tool within the archaeologist's arsenal. Many will operate for a considerable time (30-70 hours) on a single set of alkaline batteries, or on a single charge of a nicad battery pack. A wide variety of precisions and prices exist for these levels (these are made by a number of companies; the Leica line runs between $1,695 and $2,195 for level plus detector; for more information see the HHEDM address above).

Digital Levels

Hybrids of laser levels and total stations are the digital levels, which utilize a leveled, tripod-mounted, vertically fixed head that can be rotated on the horizontal plane. Rather than aiming a visible or detected beam at the rod, the device uses a bar-coded stadia rod to produce both distance to the rod and rod height above measured point. This gives two of the measures (radius, height) necessary for cylindrical coordinate measurement, but not the third, which is horizontal angle. A 360deg. dial on the base of the instrument can give an approximate azimuth reading, but would not approach the accuracy of the height (+/- 1-2 mm) or distance (+/- 0.05 percent of distance) measures. The distance-measuring capability is limited to 100 m maximum. Distance and height can be recorded on a memory module, or output to a linked computer on most models. The device generally runs on a rechargeable nicad battery that will give about eight hours of use.

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.

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Hand-Held Laser Rangefinders (HHLFs)

HHLFs offer a different mix of advantages, and approximate total stations in some respects. They essentially depend on a version of an EDM that measures distance using the time of flight of laser light to a target and back. In the models I am familiar with, the target can be most reflective surfaces or objects (for distances of two to about 750 m), but use of a reflector can extend the range to about 5,000 m. Because of the clock speed of the devices, they have more limited distance accuracy than EDMs (+/- 15 cm), but produce distance readings almost instantaneously. Higher end models can cut this error in half. The more advanced devices, such as Laser Atlanta's Advantage, can also measure horizontal and vertical angles from user to target, using an internal compass; the horizontal angle readings have an accuracy of +/- 0.5deg., while the vertical ones fall within +/- 0.2deg.. The distance and angles are displayed on a digital readout, and data can be transferred out through a serial port or a data-capturing PCMCIA card.

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.

Perspectives

For under $3,000 there is a wide variety of innovative equipment available to the field archaeologist. We are starting to see devices that take advantage of the straight-line, non-dispersing, and distance-measuring capabilities of laser beams. As with computers, but at a lesser pace, the prices of this equipment are dropping. Anyone on a limited budget should establish a working relationship with a reliable and sympathetic survey instrument dealer in their area; I have found that significant discounts are available for educational institutions. In addition to the manufacturers listed above, it would be useful to look at a variety of companies, including products from Topcon and Nikon. Used equipment may also be around, although for these smaller, recent devices it will be a rare find. Finally, contact the manufacturers, both to see if they might take an interest in innovative use of their products, and also to find out the latest information on new technology they are pursuing.

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.

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John W. Rick is in the Anthropology Department at Stanford University.


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