Robotic total station for microtopographic mappingan example from the Northern Great Plains.
код для вставкиСкачатьArchaeological Prospection Archaeol. Prospect. 13, 91–102 (2006) Published online 22 August 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.270 RoboticTotal Station for Microtopographic Mapping: an Example from the Northern Great Plains KENNETH L. KVAMME1*, EILEEN G. ERNENWEIN2 AND CHRISTINE J. MARKUSSEN1 1 Archeo-imaging Laboratory, Department of Anthropology, Old Main 330, University of Arkansas, Fayetteville, AR 72701, USA 2 Environmental Dynamics, Ozark Hall 113, University of Arkansas, Fayetteville, AR 72701, USA ABSTRACT Past human activities in cultural landscapes are often expressed by subtle variations in surface topography that reflect buried archaeological features.When seen from the air under low sunlight angles, resultant ‘shadow marks’ form a cornerstone of site detection in aerial archaeology. Past attempts to quantify and map such variations across large archaeological landscapes have resorted to aerial photogrammetry, electronic totalstations, air- and ground-basedlidar, and kinematic globalpositioning systems. The most commonly used surveying instrument is the total station, but its slow rate of data acquisition makes it poorly suited for collecting vast amounts of elevation data over large areas, althoughit isoftenused for that task.A robotic total station, examined here, is arelativelynew technology that provides a rapid survey solution. It requires only a single person to operate the total station by radio linkage from a control pad affixed to a wheeled reflector rod. As the rod is rolled over the landscape it is automatically tracked, and measurements of surface topography may be acquired to subcentimetre accuracy continuously, at a rate of one measurement per second. A case study from the Double Ditch State Historic Site in the Great Plains of North Dakota, a fortified earthlodge village with culturallysignificant surface expressions, exemplifiesthispotential.Thelociofprehistorichouses, borrow pits, fortification ditches, middens and defensive mounds are clearly revealed in the topographic mapping.Copyright 2005 JohnWiley & Sons,Ltd. Key words: DEM; robotic total station; microtopographic mapping; Great Plains USA; earthlodge settlements Introduction The importance of topographic expressions of past human activities in cultural landscapes as signatures of archaeological sites cannot be overstated. Subtle and robust variations in the terrain surface frequently signify the locations of buried * Correspondence to: K. L. Kvamme, Archeo-imaging Laboratory, Department of Anthropology, Old Main 330, University of Arkansas, Fayetteville, AR 72701, USA. E-mail: kkvamme@uark.edu Copyright # 2005 John Wiley & Sons, Ltd. structures, walls, ditches, former field boundaries, roads, trails and even entire villages. Their presence makes possible one of the primary means by which cultural features are made visible to aerial archaeology. Terrain variations caused by past human activity are responsible for the shadow-marking phenomenon where subtle changes in the ground surface become readily visible from the air under low sunlight angles (Wilson, 2000). They also cause differential frost or snow markings across the landscape Received 18 December 2004 Accepted 7 June 2005 92 because melting tends to occur sooner on slopes facing the sun than on shadowed slopes, and surface depressions and leeward slopes trap blowing snow, making such features more visible from an aerial vantage (Wilson, 2000). These same terrain variations also have an impact on aerial thermography when imagery is acquired under sunny conditions (because sunlit slopes tend to be warmer than shadowed ones; Scollar et al., 1990), and have an impact on electrical resistivity surveys where local summits and depressions tend to exhibit high and low resistivities, respectively (caused primarily by correlated moisture variations; Clark, 2000). The quantification of anthropogenic variations in landform has therefore received large interest, and several methods, briefly reviewed here, have been devised to accomplish it. Coupled with this advance is the incorporation of computer graphic techniques for mapping that allow subtle, and even previously unknown, landscape features to be revealed. These methods include (i) extremely dense contouring of the elevation data (using narrow contour intervals of only a few centimetres), (ii) pseudo-three-dimensional views with extreme vertical exaggeration (by several hundred per cent), and (iii) analytical surface shading using light-source angles and directions far surpassing those available in nature (see Newman, 1993; Doody et al., 1995; Barratt et al., 2000; Chapman and Van de Noort, 2001; Kvamme, 2005a). This work has presented high-resolution landscape mapping as a legitimate domain of remote sensing, because it is non-invasive and capable of yielding new information by revealing archaeological features normally undetectable to the human eye. Aerial photogrammetry for terrain characterization Early attempts to quantify culturally induced variations in the landscape surface relied primarily on aerial photogrammetry. Pioneering work in the American Southwest focused on robust standing architecture of Puebloan ruins that were well-mapped using one-foot (30.5 cm) intervals in photogrammetrically generated contour maps (Pouls et al., 1976). These same data sets also revealed minor and faintly perceived Copyright # 2005 John Wiley & Sons, Ltd. K. L. Kvamme et al. circular depressions in surrounding areas that indicated the loci of buried kivas—subterranean ceremonial structures associated with the ruins. Recent aerial photogrammetric work offers mappings of the ground surface of much higher precision—Stone (2003) reports 10 cm— allowing smaller terrain variations to be captured in the results. A photogrammetric example is given below. Electronic total stations for terrain characterization Laser distance measuring systems in electronic total stations, or even antiquated optical transits, have long been used to map anthropogenically altered topographic surfaces to centimetre-level accuracy. Early work of this nature is illustrated by Frankel (1980), in Australia, and Shapiro and Miller (1990), in the USA. Numerous similar studies have since been conducted around the globe, with a significant concentration of work in Ireland. Newman (1993), for example, maps the topographic surface of the prehistoric ritual landscape at Tara (which holds several barrows and other ancient monuments); Doody (1993) presents similar results for a complex of Bronze Age barrows and enclosures at Chancellorsland; and a complete survey of the heavily vegetated bivallate hillfort of Castle Gale, on the promontory of Carrig Henry, is described by Doody et al. (1995). The principal drawback of traditional total station mappings is the tremendous amount of labour required to survey large areas, with the station operator commonly required to set up and work from several datums, a second individual constantly moving and placing a reflector rod, and the difficulty of communication between the two (commonly alleviated by radio headsets). Moving the reflector rod along parallel linear traverses separated by a constant interval (e.g. 1 m), with measurements sampled at a similar interval along each traverse, ensures a uniformly sampled matrix of elevations for creation of a digital elevation model (DEM), but such a field protocol can take an enormous amount of time. Kvamme (2005a) describes the microtopographic mapping of the Fort Clark Trading Post, a mid-nineteenth century outpost of the American Archaeol. Prospect. 13, 91–102 (2006) Robotic Total Station Mapping fur trade in North Dakota. Uniform sampling of the 4000 m2 region at 1 m intervals along x- and yaxes required approximately 4 days for the twoperson crew of station operator and rod holder. In contrast to uniform spatial sampling, irregularly placed samples offer a number of clear advantages because fewer measurements need to be taken in areas of little topographic change, and additional ones may be placed to better characterize complex surfaces or areas of particular interest. The disadvantage of an irregular survey protocol is that it is largely judgmental, depending on the rod holder’s skill and perception in defining or observing significant landscape change points while meeting, at the same time, the need to provide sufficient coverage over large areas. In practice, a combination of uniform and irregular sampling is most effective. This is well demonstrated in a computer simulation by Fletcher and Spicer (1988) that shows the benefits of combining uniform sampling of a region supplemented with additional points judgmentally placed over areas of significant topographic variation. Most workers have since followed this protocol (e.g. Doody 1993; Barratt et al., 2000; Chapman and Van de Noort 2001; Ainsworth and Thomason, 2003; see also below). New technologies for microtopographic characterization Several new technologies have emerged in recent years that are capable of rapidly producing high resolution DEM over large areas. One is airborne lidar (LIght Detection And Ranging) that offers horizontal sampling densities well below 1 m with absolute vertical accuracy as good as 15 cm (although relative accuracy between nearby measurements is considerably higher (Flood, 2001). Barnes (2003) illustrates an impressive archaeological application with a lidar produced DEM of a 2.4 km2 region surrounding the Iron Age Sidbury Hillfort, England, that clearly reveals earthworks, barrows, trails and tracks. Ground-based lidar systems, more commonly referred to as ‘three-dimensional laser scanners’, have also been used to create high resolution DEMs (Neubauer, 2004). Designed primarily for creating three-dimensional models of nearby architecture, numerous stations must Copyright # 2005 John Wiley & Sons, Ltd. 93 be used to produce large-area DEMs and the low oblique scanning angle, even when placed on a raised platform, means that surface vegetation— including blades of grass—must be filtered out from the scans using complex algorithms (Limp, 2004). Surveyor-grade real time kinematic global positioning systems (GPS) have also been explored for mapping topographic surfaces at archaeological sites and producing DEMs holding great detail. Barratt et al. (2000) describe a 35 ha survey at the Roman town of Viroconium (one of the largest urban centers in Roman Britania). Using a series of previously established datums, a direct radio link between a base station and a roving unit allowed differential corrections on the fly, with subcentimetre accuracy. A 5-m traverse separation was generally used, although 2- or 1-m traverse separations were used where more information was desired. Measurements were typically sampled every 2 m along traverses, although minimum intervals as small as 0.1 m were occasionally adopted over important features. Chapman and Van de Noort (2001) used similar GPS methods in two wetlands surveys in England by collecting data points every 8 m in ‘background’ regions and at less than 1 m intervals in areas of high archaeological potential. A recent publication by English Heritage (Ainsworth and Thomason, 2003) illustrates similar case studies. Use and availability All of the foregoing approaches are capable of producing high-resolution DEMs of great accuracy (or relative accuracy in the case of aerial lidar) that reveal culturally generated terrain variations, including those of great subtlety. Yet, depending on location and budget, some methods may not be readily available. Aerial lidar is practically inaccessible to most archaeological projects in the USA, for example, and surveyor-grade kinematic GPS, although growing in popularity, is expensive and therefore not widely used. For this reason, the total station, a common archaeological mapping tool, remains of necessity the primary method for landscape characterization in many regions. The purpose of this paper is to present a relatively new Archaeol. Prospect. 13, 91–102 (2006) 94 development in total station technology that offers very rapid survey coverage at half the labour costs of a conventional unit. The robotic total station rivals or surpasses kinematic GPS in accuracy and speed of survey using far less expensive instrumentation. The robotic total station The robotic total station is a relatively new electronic distance measuring (EDM) system with many automated capabilities. The Trimble 5600 is one such instrument that contains a highly accurate EDM (allowing subcentimetre work), a four-speed servo for rapid and accurate aiming, AutolockTM technology for automatic tracking of the target as it is moved, and robotic capabilities allowing one-person survey through radio linkage (halving crew requirements). The servodriven aiming, Autolock and robotic capabilities mean that all operations may be conducted by a single person through a control pad affixed to the reflector rod that is radio-linked to the total station (Figure 1). A further innovation that K. L. Kvamme et al. enables a tremendous leap in survey speed is a wheel attached to the base of the reflector rod. This allows the operator to simply roll the rod over the landscape, controlling the survey by radio from the control pad, while the total station automatically tracks the roving rod and records data. For expedient surveys data may be acquired in a continuous mode at a rate of one measurement per second. The x, y (positional) and z (elevation) coordinates are transmitted back to the control pad on the roving unit and displayed in map form in real time (Figure 1). Data are easily downloaded as comma delimited, x, y, z data triplets, in an ASCII format for computer processing and map creation. The Trimble 5600 has been used by the University of Arkansas on a number of archaeological sites for the specific purpose of creating dense DEMs over large areas with 1 m sample separations or less and subcentimetre accuracy (e.g. see Herrmann, 2004). Several difficulties have been resolved during this work. The original 15 cm diameter rod wheel that came with the unit was far too small for efficient movement over Figure 1. The Trimble 5600 robotic total station (left), rod-mounted control pad with real-time display (middle), and wheeled reflector rod in use (right). Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 13, 91–102 (2006) Robotic Total Station Mapping terrain—it frequently became stuck between clumps of grass, in surface detritus, or rodent holes. A larger 35 cm diameter wheel was easily installed and remedied this situation. Maintaining the rod nearly vertical while it is in motion remains a constant problem, but one that experience helps to mitigate. With the height of the reflector typically 2 m above the ground surface, a simple calculation reveals that if the rod is angled by only 15 off-vertical, horizontal accuracy can shift by more than a 0.5 m and vertical accuracy by 0.07 m. The Windows CE operating system occasionally ‘crashes’ during survey, causing delays because the instrument must be rebooted and the station set-up procedure rerun. Initially, battery life appeared to be an issue with a ‘low battery’ signal appearing after only 1–2 h of use. It has been established that the gauge is inaccurate, and that batteries last for an entire day. The instrument’s range also poses difficulties. The maximum range using the more accurate miniprism under Autolock tracking is only about 150 m in clear weather, with that distance rapidly diminishing under light rain or even high humidity. A minimum effective range of about 10 m has also been determined, dictated by the inability of the Autolock tracking to keep up with the relatively fast moving rod at close distances. Finally, obstructions to the laser beam, whether caused by signage, vegetation, rising ground, or people, are a regular difficulty that must be closely monitored, because the Autolock tracking may be lost and must be reacquired before survey can recommence (the operator, too, may cause beam blockage, but this circumstance is easily avoided by ensuring that the reflector is above the operator). Case study: mapping microtopography at the Double Ditch State Historic Site The Double Ditch State Historic Site (32BL8) is a large, fortified, earthlodge village located on a high terrace above the Missouri River, near Bismarck, North Dakota (historically, the earthlodge was a dome-shaped dwelling composed of timbers, 15–18 m in diameter, and covered with 0.5 m of earth). When Lewis and Clark of the Voyage of Discovery first reported the site in Copyright # 2005 John Wiley & Sons, Ltd. 95 1804, it had been abandoned by the Mandan about 20 years previously, most probably owing to a smallpox epidemic that hit the region in the early 1780s (Ahler, 2004). As it exists today, Double Ditch is one of the largest and best preserved of the remaining earthlodge village sites. It is distinctive in its many clear terrain expressions of cultural features and by its large size, with an approximately 6 ha area showing significant topographic expressions. Two robust fortification ditches, in places more than 1 m deep, are readily seen in the ground surface (prehistorically, they were lined on the interior side with a palisade of large posts). Nearly 100 shallow depressions 12–50 m in diameter signify the locations of former earthlodges and soil borrowing areas. Above them lie numerous large mounds (probably part of defensive works) and mounded middens, some approaching 3 m in height. Hailey (2005) illustrates many of these features from an aerial vantage (see also Figure 2). Double Ditch has been the focus of an intensive 4-year remote sensing project that has been combined with archaeological excavations to better understand and document this important site to Northern Great Plains prehistory (Ahler, 2004; Kvamme, 2005b). The remote sensing has included aerial imaging and thermography of the entire park, complete geophysical surveys by magnetic gradiometry and electrical resistivity methods, and large areas of groundpenetrating radar and electromagnetic induction surveys. Significant findings (many verified by excavation) include the identification of thousands of subterranean corn storage pits (about 1.5 m deep by 1.5 m in diameter) that testify to high levels of maize horticultural production. Extensive areas of earth borrowing for earthlodge coverings and fortifications were also defined. Most significantly, the discovery of two previously unknown fortification ditches, invisible on the surface, expanded the village’s size to about 10 ha. These and other findings have begun to alter perceptions of the complexity of the village way of life on the northern Great Plains (Ahler, 2004). Near the close of the project in 2004, it was decided to further document Double Ditch’s core village area through creation of a high resolution Archaeol. Prospect. 13, 91–102 (2006) 96 K. L. Kvamme et al. Figure 2. Aerialview of Double Ditch under low sunlight conditions showing anthropogenic variationsin microtopography (photograph credit: Tommy Ike Hailey). DEM. Such a data set would document the state of the site’s surface as it existed in 2004, and could serve as a baseline for subsequent change studies resulting from normal erosion or intensive tourist traffic. The data would also be useful for examining certain aspects of this cultural landscape—for instance, size distributions of ‘house’ (earthlodge) or earth borrowing depressions, middens and mounds. When merged with evidence of the two hidden fortification ditches revealed by the geophysics, a DEM might also allow better understanding of the distribution of the many large and built-up mounds as components of these fortifications (see below). Finally, it is well known that terrain form holds significant relationships with various types of remote sensing, such as electrical resistivity and aerial thermography. A high-resolution DEM would potentially allow study of terrain effects on these data sets. Copyright # 2005 John Wiley & Sons, Ltd. Parkwide ‘background’ DEM through photogrammetry A topographic map with a 25 cm contour interval was available for the entire 700 800 m park area. It was created by photogrammetry in 1994 by a local surveying company, KBM Inc., at the request of the State Historical Society of North Dakota. Although the focus of the robotic total station survey was to lie only within the confines of the approximate 260 280 m area of the village proper, it was decided to first create a parkwide DEM using the relatively crude KBM data as a background into which the results of the more detailed survey could be inserted. The KBM map reveals many of the Double Ditch’s principal topographic features in the elevation contours (Figure 3). Hundreds of isolated elevations are also included on the map at significant change points (e.g. mound summits, low points Archaeol. Prospect. 13, 91–102 (2006) Robotic Total Station Mapping 97 Figure 3. The photogrammetrically produced 25-cm contour map of the entire park by KBM, Inc. (left) was used to create aTINgenerated DEM (upper right), but only after significant editing to close line labelling gaps (indicated by arrows, inset, lower right). in depressions). Fortunately, the spatial component of the KBM data existed in digital form that allowed extraction of elevation contours and points into a geographical information system (GIS) as line and point ‘vectors.’ Unfortunately, the attribute information for each vector was unavailable except in paper copies, necessitating each vector to be manually tagged with its elevation. Significant editing was also required to connect labelling gaps left in contour lines (inset, Figure 3). Finally, hundreds of surplus lines that denoted other park features (fence lines, roads, monuments, trails) had to be removed. These elevation vectors were then registered to the project’s arbitrary plane coordinate system utilizing stone monuments built in the 1930s that surround the park periphery as ground control points. Overall RMS error for the 700 800 m park area was 0.292 m. A DEM was produced by first pre-processing each line vector to add vertices to ensure no data gaps larger than 2 m. This enabled a more detailed triangular irregular network (TIN) describing the park’s surface to be generated with fewer large gaps in the Delauney triangulation, resulting in a smoother surface (Burrough and McDonnell, 1998). Using this TIN surface, a Copyright # 2005 John Wiley & Sons, Ltd. raster DEM was created with a horizontal spatial resolution of a 0.5 m, compatible with several of the geophysical data sets. The result, although illustrating very well Double Ditch’s prominent characteristics (Figure 3), tends to lack many details, inadequately portraying sizes and shapes of many of the site’s features owing to the limited information content offered by 25-cm contour lines. These findings verified the need for the more accurate representation by robotic total station survey. DEM by robotic total station A rapid data collection protocol was established at Double Ditch for the Trimble 5600 survey. It made use of extant 20 m square data collection units, previously located by total station for the geophysical surveys, that were marked across the site by 1 m long PVC pipes placed vertically in the ground at their corners (see background in Figure 1). By utilizing the 20 m units, the topographic survey could be conducted piecemeal, unit-by-unit. In each 20 m unit, survey was accomplished by placing pin flags at 5-m intervals at opposite ends of each unit to serve as guides for the survey transects. With an initial Archaeol. Prospect. 13, 91–102 (2006) 98 sampling density goal of one measurement per metre in both x- and y-axes, traverses were made approximately every metre by zigzagging through each 20 m unit and visually estimating approximate traverse locations vis-à-vis the pin flags, PVC corner markers and the real-time display on the rod’s control unit (Figure 1). Within each traverse, the wheeled reflector rod was moved at a rate of somewhat less than 1 m s 1 to achieve better than one measurement per metre based on the instrument’s continuous mode sampling rate. It is emphasized that additional elevations were also acquired over particularly variable, small and culturally significant landscape features in each 20 m unit. Approximately 15 min were normally required to survey a 20 m unit, including pin flag set-up and the walking of 20 traverses. With fieldwork K. L. Kvamme et al. coming to a close and the entire village core surveyed with 1 m traverses (the area within the innermost ditch; Figures 2 and 3), it was decided to attempt complete survey of the larger village area encompassed by the outer topographically indicated ditch with 4 days remaining to the project (Figures 2 and 3). This was accomplished by changing traverse separation from 1 m to 2 m (although samples within each traverse were maintained at approximately 1 m intervals). This decreased survey time to as little as 10 min per 20 m unit, but resulted in somewhat diminished detail in the resultant DEM that occasionally becomes apparent in certain forms of display. A mapping of a portion of the data postings reveals much about the nature of our surveys (Figure 4). With data collected continuously, the Figure 4. Data postings from the robotic total station showing 1m versus 2 m traverse separations and clustering of data points near 20 m collection unit boundaries, indicated by the backgroundgrid.Holesin the point distributionrepresent the lociofon-going excavations (E) and certain data clusters indicate judgmentally placed measurements (J). Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 13, 91–102 (2006) Robotic Total Station Mapping 99 Figure 5. Digital elevation model (DEM) produced by the robotic total station (left), overlaid on the larger photogrammetrically generated DEM (right). zigzag survey in 20 m collection units caused data points to accumulate at the end of each north–south traverse, or when the operator happened to stop in mid-transect (perhaps to speak with tourists). The 1 m versus 2 m traverse separations are apparent, and the fact that traverses are generally parallel and uniformly separated is encouraging. The few holes visible in the data were caused by the need to avoid open excavations, and some data clustering or additional survey lines point to areas of extraordinary topographic change or special interest (Figure 4). Geographical information system methods were used to thin dense point clusters by removing measurements separated by less than 0.5 m. A TIN surface was then generated between the remaining points, and a 0.5 m horizontal-resolution raster DEM was created from this surface. The result holds remarkable detail, showing not only house and borrow pit depressions, ditches, mounds and middens, but very small cavities throughout the landscape and a generally richer texture. Many of the small cavities reflect looter’s pits from early in the past century but, significantly, some may represent collapsed prehistoric subterranean storage pits (Figure 5). The DEM Copyright # 2005 John Wiley & Sons, Ltd. generated by robotic total station was ultimately mated with the larger photogrammetrically produced DEM using GIS overlay methods. This allowed creation of an ultimate park-wide DEM for display and analysis purposes (Figure 5). Discussion A 60 80 m segment of the photogrammetrically produced DEM shows its inaccuracies and simplification of the surface when compared against the more detailed results obtained by the robotic total station (Figure 6). It is clear that 25-cm contours are inadequate for revealing many features in a cultural landscape, even at a site with such robust terrain expressions as Double Ditch (given vertical precision differences between the two data sets of 1 cm versus 25 cm, further comparisons are pointless). A cross-section across several of Double Ditch’s house, ditch and mound features suggests the potential of the robotic total station’s data for exploring such issues as house size, deposit formation around houses, ditch depth and mound formation (Figure 6). The full analytical worth of these Archaeol. Prospect. 13, 91–102 (2006) 100 K. L. Kvamme et al. Figure 6. Comparison of details and feature content for the same region between DEMs generated by photogrammetry (left) and the robotic total station (right). Note absence of features in the left profile. data has not yet been explored, but probably will include GIS-based studies of viewsheds and visibility from key defensive mounds relative to the site’s fortifications, probable corridors of travel within the village, volumetric studies of mounds and borrow pits, and house size studies. Terrain effects on geophysical data, particularly thermal and electrical resistivity, also may be explored. Some of the DEM’s potential for better understanding Double Ditch is suggested in Figure 7, where the magnetic gradiometry data have been draped over the elevation surface. In this view, Figure 7. A segment of the magnetic gradiometry data overlaid on the park-wide DEM showing relationships between large mounds and fortification ditches (labelled1^4). Ditches 3 and 4 are visible only to magnetometry. Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 13, 91–102 (2006) Robotic Total Station Mapping topographically expressed fortification Ditches 1 and 2 may be seen in the fore- and middleground, while magnetically revealed Ditches 3 and 4 are seen along the background (a series of regularly spaced, U-shaped bastions occur at regular intervals along outer Ditch 4; Kvamme, 2005b). The underlying topographic data allow simultaneous visualization of the magnetic anomalies and the loci of the large mounds, many of which probably served a defensive function such as viewing platforms, bastions or perhaps ramparts. Analysis of imagery such as Figure 7 may ultimately better clarify the role that mounds played in Double Ditch’s defenses. Acknowledgements Eileen Ernenwein, doctoral student at the University of Arkansas, was in charge of directing the Trimble 5600 survey at Double Ditch, although most of it was physically performed by Andrew Gold (after training), a local resident of Bismarck. Christine Markussen, M.A. student at the University of Arkansas, did most of the work to create the photogrammetric DEM. This research was made possible by a grant from the PaleoCultural Research Group, Stanley A. Ahler, Director, of Flagstaff, Arizona, in league with the Historic Preservation Division, Fern Swenson, Director, of the State Historical Society of North Dakota. Stan Ahler is thanked for engineering the transition to a larger survey wheel for the reflector rod. Use of the Trimble 5600 was generously granted by the Center for Advanced Spatial Technologies, University of Arkansas, directed by W. Fredrick Limp. The Trimble 5600 was obtained through a Major Research Instrumentation grant awarded to the University of Arkansas by the U.S. National Science Foundation (grant number BCS-0321286), W. Fredrick Limp, Kenneth L. Kvamme, Sreekak Bajwa, Stephen Beaupre, and Steve Burian, Principal Investigators. References Ahler SA. 2004. Introduction and background for 2003–2004 studies. In Archaeological Investigations During 2003 at Double Ditch State Historic Site, North Dakota, Ahler, SA (ed.). Unpublished report submitted to the State Historical Society of North Dakota, Bismarck; 1–16. Copyright # 2005 John Wiley & Sons, Ltd. 101 Ainsworth S, Thomason B. 2003. Where on Earth are We? The Global Positioning System (GPS) in Archaeological Field Survey. 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