journal of archaeological science...2. archaeological thermography the principles behind...

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Archaeological aerial thermography: a case study at the Chaco-era Blue J community, New Mexico Jesse Casana a, * , John Kantner b , Adam Wiewel a , Jackson Cothren c a Department of Anthropology, University of Arkansas, Main 330, Fayetteville, AR 72701, United States b University of North Florida, 1 UNF Dr., Jacksonville, FL 32224, United States c Department of Geosciences and Center for Advanced Spatial Technologies, University of Arkansas, JBHT 320, Fayetteville, AR 72701, United States article info Article history: Received 18 September 2013 Received in revised form 17 February 2014 Accepted 20 February 2014 Keywords: Thermal imaging LWIR UAV Photogrammetry Archaeo-geophysics Ancestral Puebloans abstract Despite a long history of studies that demonstrate the potential of aerial thermography to reveal surface and subsurface cultural features, technological and cost barriers have prevented the widespread appli- cation of thermal imaging in archaeology. This paper presents a method for collection of high-resolution thermal imagery using an unmanned aerial vehicle (UAV), as well as a means to efciently process and orthorectify imagery using photogrammetric software. To test the method, aerial surveys were con- ducted at the Chaco-period Blue J community in northwestern New Mexico. Results enable the size and organization of most habitation sites to be readily mapped, and also reveal previously undocumented architectural features. Our easily replicable methodology produces data that rivals traditional archaeo- logical geophysics in terms of feature visibility, but which can be collected very rapidly, over large areas, with minimal cost and processing requirements. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Archaeologists have recognized since the 1970s that aerial im- ages which record thermal infrared wavelengths of light (7.5e 13 mm) could be a powerful means for recognizing both surface and subsurface cultural remains (e.g., Tabbagh, 1977; Fourteau and Tabbagh, 1979; Berlin et al., 1977), yet technological and cost bar- riers have largely prevented the widespread application of ther- mography in archaeological contexts. This paper presents interim results of a National Endowment for the Humanities-funded proj- ect to develop techniques for collection of thermal imagery using a UAV, to design workows for efcient photogrammetric processing of imagery, and to assess the effectiveness of the technology at archaeological sites located in a variety of different environments. Herein we present a case study that provides an illustration of our methodology, conducted at a Chaco-period archaeological com- munity known as Blue J, located in northwestern New Mexico about 70 km south of Chaco Canyon (Fig. 1). Results demonstrate the enormous potential of aerial thermography in archaeology, revealing the organization of occupation at Blue J and aiding in the discovery of many previously undocumented subsurface architectural remains, as well as providing a blueprint for similar investigations elsewhere. 2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density and mois- ture content, materials on and below the ground surface absorb, emit, transmit, and reect thermal infrared radiation at different rates. Experimental data illustrates the complexity of the variables that affect the potential visibility of archaeological features in a thermal image (Scollar et al., 1990: 593e611). However, in general, such features should be resolvable if when compared to the back- ground matrix they have sufcient contrast in thermal inertia, a product of a materials thermal conductivity and its volumetric heat capacity (Perisset and Tabbagh, 1981), and if the image is acquired at a time in the diurnal cycle when such differences are pro- nounced. Several studies have demonstrated that thermography can detect features at or near the ground surface such as pits, ditches and eld boundaries as well as buried architectural features up to a half meter below the ground (e.g., Perisset and Tabbagh, 1981; Lunden, 1985; Bellerby et al., 1990). Despite its potential, archaeological applications of the tech- nology are scarce, largely because few archaeologists had access to * Corresponding author. Tel.: þ1 479 595 9569. E-mail address: [email protected] (J. Casana). Contents lists available at ScienceDirect Journal of Archaeological Science journal homepage: http://www.elsevier.com/locate/jas http://dx.doi.org/10.1016/j.jas.2014.02.015 0305-4403/Ó 2014 Elsevier Ltd. All rights reserved. Journal of Archaeological Science 45 (2014) 207e219

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Page 1: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

Archaeological aerial thermography: a case study at the Chaco-eraBlue J community, New Mexico

Jesse Casana a,*, John Kantner b, Adam Wiewel a, Jackson Cothren c

aDepartment of Anthropology, University of Arkansas, Main 330, Fayetteville, AR 72701, United StatesbUniversity of North Florida, 1 UNF Dr., Jacksonville, FL 32224, United StatescDepartment of Geosciences and Center for Advanced Spatial Technologies, University of Arkansas, JBHT 320, Fayetteville, AR 72701, United States

a r t i c l e i n f o

Article history:

Received 18 September 2013

Received in revised form

17 February 2014

Accepted 20 February 2014

Keywords:

Thermal imaging

LWIR

UAV

Photogrammetry

Archaeo-geophysics

Ancestral Puebloans

a b s t r a c t

Despite a long history of studies that demonstrate the potential of aerial thermography to reveal surface

and subsurface cultural features, technological and cost barriers have prevented the widespread appli-

cation of thermal imaging in archaeology. This paper presents a method for collection of high-resolution

thermal imagery using an unmanned aerial vehicle (UAV), as well as a means to efficiently process and

orthorectify imagery using photogrammetric software. To test the method, aerial surveys were con-

ducted at the Chaco-period Blue J community in northwestern New Mexico. Results enable the size and

organization of most habitation sites to be readily mapped, and also reveal previously undocumented

architectural features. Our easily replicable methodology produces data that rivals traditional archaeo-

logical geophysics in terms of feature visibility, but which can be collected very rapidly, over large areas,

with minimal cost and processing requirements.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Archaeologists have recognized since the 1970s that aerial im-

ages which record thermal infrared wavelengths of light (7.5e

13 mm) could be a powerful means for recognizing both surface and

subsurface cultural remains (e.g., Tabbagh, 1977; Fourteau and

Tabbagh, 1979; Berlin et al., 1977), yet technological and cost bar-

riers have largely prevented the widespread application of ther-

mography in archaeological contexts. This paper presents interim

results of a National Endowment for the Humanities-funded proj-

ect to develop techniques for collection of thermal imagery using a

UAV, to designworkflows for efficient photogrammetric processing

of imagery, and to assess the effectiveness of the technology at

archaeological sites located in a variety of different environments.

Herein we present a case study that provides an illustration of our

methodology, conducted at a Chaco-period archaeological com-

munity known as Blue J, located in northwestern New Mexico

about 70 km south of Chaco Canyon (Fig. 1). Results demonstrate

the enormous potential of aerial thermography in archaeology,

revealing the organization of occupation at Blue J and aiding in

the discovery of many previously undocumented subsurface

architectural remains, as well as providing a blueprint for similar

investigations elsewhere.

2. Archaeological thermography

The principles behind archaeological thermographic imaging

are relatively simple: due to their composition, density and mois-

ture content, materials on and below the ground surface absorb,

emit, transmit, and reflect thermal infrared radiation at different

rates. Experimental data illustrates the complexity of the variables

that affect the potential visibility of archaeological features in a

thermal image (Scollar et al., 1990: 593e611). However, in general,

such features should be resolvable if when compared to the back-

ground matrix they have sufficient contrast in thermal inertia, a

product of a material’s thermal conductivity and its volumetric heat

capacity (Perisset and Tabbagh, 1981), and if the image is acquired

at a time in the diurnal cycle when such differences are pro-

nounced. Several studies have demonstrated that thermography

can detect features at or near the ground surface such as pits,

ditches and field boundaries as well as buried architectural features

up to a half meter below the ground (e.g., Perisset and Tabbagh,

1981; Lunden, 1985; Bellerby et al., 1990).

Despite its potential, archaeological applications of the tech-

nology are scarce, largely because few archaeologists had access to* Corresponding author. Tel.: þ1 479 595 9569.

E-mail address: [email protected] (J. Casana).

Contents lists available at ScienceDirect

Journal of Archaeological Science

journal homepage: http : / /www.elsevier .com/locate/ jas

http://dx.doi.org/10.1016/j.jas.2014.02.015

0305-4403/� 2014 Elsevier Ltd. All rights reserved.

Journal of Archaeological Science 45 (2014) 207e219

Page 2: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

the highly specialized radiometers used by researchers in the 1970s

(Scollar et al., 1990: 614e18), while thermal images acquired from

civilian satellites such as Landsat and ASTER are of too low spatial

resolution to reveal most archaeological features. Even the aircraft-

acquired TIMS (Thermal Infrared Multispectral Scanner) imagery

that Sever and colleagues famously utilized to document ancient

roadways in the Chaco Canyon area (Sever and Wagner, 1991) and

in Costa Rica (Sheets and Sever, 1991) was only 5 m resolution.

While a handful of other more recent archaeological investigations

have used higher-resolution, aircraft-acquired thermal imagery

(e.g., Challis et al., 2009), the exorbitant cost of imagery acquisition,

requiring tasked flights of specialized aircraft equipped with

advanced sensors, puts the technology well beyond the reach of

most archaeologists.

Several recent studies have instead explored the archaeological

potential of much higher resolution thermal images collected with

conventional handheld cameras. Ben-Dor et al. (2001) use imagery

collected from a helicopter to detect subsurface stone architecture

in Israel, while Buck et al. (2003) use thermal images collected by a

camera mounted on a platform to reveal differing concentrations of

surface artifacts. A few avocational archaeologists have similarly

collected images of archaeological sites using handheld thermal

cameras attached to kites, revealing many subsurface remains

(Wells, 2011). One of the most systematic studies was undertaken

at late prehistoric mound center in Mississippi, where thermal

imagery acquired with a handheld camera suspended from a heli-

um blimp was used to successfully document subsurface house

remains as well as the diurnal variation in their thermal values

(Haley et al., 2002; Giardino and Haley, 2006). Kvamme (2006,

2008) was the first to deploy such an approach on a site-wide

scale, flying a handheld thermal camera aboard a manned pow-

ered parachute (see, Hailey, 2005), at Fort Riley, Kansas and Double

Ditch, North Dakota. His results are inspiring in their possibilities as

they reveal much of what can be seen in more traditional

geophysics, as well as distinct features not recorded by other

methods. However, implementing Kvamme’s approach would be

very challenging, requiring a microlight aircraft, an experienced

pilot, ideal weather conditions, and extensive work to rectify and

mosaic the images into a usable map.

In recent years, several technologies have emerged that make

possible the cost-effective acquisition of high-resolution thermal

data at the scale of sites and landscapes. Firstly, unmanned aerial

vehicles (UAVs) are rapidly improving in sophistication and reli-

ability, enabling archaeologists to collect aerial imagery from spe-

cific altitudes with precise camera orientation at any time of day

and in a range of weather conditions. The possibilities for UAVs to

aid in archaeological research have recently generated much in-

terest, particularly for documenting sites, monuments and exca-

vations (e.g., Verhoeven and Docter, 2013; Hill, 2013; Chiabrando

et al., 2011; Seitz and Altenbach, 2011; Sauerbier and Eisenbeiss,

2010; Eisenbeiss and Zhang, 2006). At the same time, advances in

photogrammetric image processing software packages, such as

AgiSoft’s PhotoScan and Microsoft’s Image Composite Editor (ICE),

now make quite straightforward the once laborious process of

mosaicking, orthorectifying and georeferencing images, a fact

which is rapidly transforming archaeological documentation more

broadly (e.g., De Reu et al., 2013; Verhoeven et al., 2012a, 2012b;

Eisenbeiss and Sauerbier, 2012; Verhoeven, 2011; Koutsoudis

et al., 2007). Finally, commercially available thermal cameras have

been significantly improved upon over the past several years, with

increased spatial resolution (from 320 � 240 to 640 � 512 pixels)

and thermal sensitivity (from 0.2 to <0.05 K), while also being

reduced substantially in both size and cost. Through a case study at

the Chaco-period Blue J Community, New Mexico, research

Fig. 1. Locator map showing the Blue J Community and Chaco Canyon in northwestern New Mexico, USA.

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219208

Page 3: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

presented herein integrates these various technologies and dem-

onstrates an efficient methodology for their deployment in

archaeological investigations.

3. The Blue J community

Located in northwestern New Mexico (Fig. 1), Blue J is an

Ancestral Puebloan community that was first identified by ar-

chaeologists in the 1970s, although official records do not appear

until the 1980s and 1990s through cultural resource management

reconnaissance efforts (Kantner, 2010). These projects, however,

only examined a few of the households located on the edges of the

larger prehistoric community. It was not until Kantner’s in-

vestigations in the 2000s under the auspices of Georgia State

University and the School for Advanced Research that the full

extent of the visible surface occupation was identified and the

community fully recorded (Kantner, 2013).

The Blue J Community consists of nearly 60 Ancestral Puebloan

households loosely clustered over approximately two square kilo-

meters around what was once a large spring (Fig. 2A). The majority

of the households were built along a sandstone escarpment, lead-

ing to considerable alluvial, colluvial, and aeolean deposits over the

community (Fig. 2B). Each household, which is recorded as a

separate site, typically includes a masonry roomblock of 3e15

rooms that faces a flat plaza area to the east or southeast. Beyond

this, a shallow and usually expansive midden area is typically

found. This layout is known in the American Southwest as a

“Prudden unit,” named after the archaeologist who first discussed

the ubiquity of this layout across the early Puebloan world

(Prudden, 1903). In principle, Prudden unit households should also

feature subterranean structures in the plaza that are usually

interpreted as having served a combination of ceremonial and

domestic functions. In the Blue J Community, however, households

were buried by up to a meter of eroded sandstone and wind-blown

silt, making identification of subsurface deposits difficult without

excavation (Kantner, 2013). In fact, the only two subterranean

structures identified by Kantner were found through a program of

subsurface test excavations. Similarly, although the roomblocks

could usually be identified on the surface, their full extent was only

revealed through excavation; testing of several roomblocks found

that approximately 30% of each roomblockwas too deeply buried to

be identified on the surface.

The issue of extensive deposition over the Blue J Community is

relevantbecauseof the sites’potential importance forunderstanding

Fig. 2. (A) Oblique aerial photograph showing part of the Blue J Community and adjacent sandstone cliffs; (B) Site LA 170607 as it appears on the surface today.

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219 209

Page 4: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

regional dynamics of the Ancestral Puebloan occupation. In-

vestigations in the community show that it was occupied from at

least A.D. 600 to as late as A.D.1150, but the greatest intensity of use

was in the eleventh century (Kantner, 2013). This is during the

fluorescence of an influential ceremonial center in Chaco Canyon,

during which the vast majority of Puebloan communities across the

northern Southwest constructed two forms of monumental archi-

tecture known as “great kivas” and “great houses.” Great kivas are

large, circular subterranean structures with diameters of 10 m or

more that have a deep history in the Puebloan Southwest long pre-

dating the Chaco period, but which became more formalized in

Chaco Canyon and surrounding areas during the eleventh century.

Great houses, in contrast, are monumental surface structures that

seem to have been inspired by archetypes constructed in Chaco

Canyonbeginningperhaps in theninth century butfluorescing in the

late AD 1000s. The presence of both great houses and great kivas in

communities across the Puebloan Southwest are thought to reflect

the degree of adherence to the ceremonial tradition based in Chaco

Canyon (Kantner and Kintigh, 2006; Van Dyke, 2003, 2007).

Kantner’s investigations in the Blue J Community have located

neither a great house nor a great kiva, making it unusual among its

neighbors. In fact, these architectural forms have been identified in

every other Chaco-era community in the immediate region, even

though few have been the subject of intensive archaeological

investigation (Kantner, 2010). Whether the absence of these fea-

tures is the result of deposition over the community, or whether

these structures were never constructed there, is important for

understanding when, how, and why communities like Blue J

aligned themselves with the ceremonial center of Chaco Canyon

(e.g., Kantner and Vaughn, 2012; Lekson, 2009; Reed, 2011).

Although some targeted magnetometry and resistivity work has

been conducted in the Blue J Community, with mixed results, its

considerable size calls for a remote sensing approach that can

effectively and efficiently cover a large area.

4. Instrumentation and methods

4.1. Thermal camera

The thermal camera we use is built around FLIR’s Tau II LWIR

uncooled camera core. The camera records light in the 7.5e13 mm

range, with a sensitivity of 0.05 K at f/1.0, effective at ambient

temperatures from �40 to 160 �C. The camera has a resolution of

640 � 512 pixels, with a pixel pitch of 17 mm, and records 8-bit

video images at a frame rate of 30 Hz. While the resolution of the

camera is very low by comparison to color light cameras, it is five

times better than the best cameras on the market only a few years

ago and among the highest resolution thermal cameras available

commercially. To maximize the camera’s field of view, and thereby

reduce the altitude at which the camera would need to be

positioned during aerial surveys, we use a relatively short focal

length, the 19 mm lens option. The Tau II camera core, designed

more for laboratory than field applications, was attached to a power

source and SD card recorder, and fitted inside an aluminum case for

aerial deployment (Fig. 3).

4.2. Unmanned aerial vehicle

UAVs offer key advantages over traditional forms of archaeo-

logical aerial imaging (cf. Verhoeven, 2009), particularly in their

ability to cover large areas at a fixed altitude and speed under a

wide range of wind and weather conditions. For aerial thermog-

raphy, these capabilities are essential, because imagery acquisition

is highly time-sensitive. To maximize archaeological visibility,

thermal imagery has to be collected at a time of day when the

contrast in thermal inertia is highest between the archaeological

target and the background soil and ground cover. Moreover, the

entirety of the survey area must be covered with as little temporal

variability as possible in order to reduce drift in thermal values

across a single mosaicked scene. Additionally, the relatively stable

and slow moving platform of a copter-type UAV is advantageous

because the pixels of the Tau II camera heat and cool continuously

as the IR signal is integrated. With an unstable platform, IR frames

are comparatively more difficult to mosaic as the imagery is

subject to artifacts related to the camera’s shutter type (FLIR,

2013).

In this project we utilize a CineStar 8 (Fig. 3), one of the

preferred UAVs for aerial cinematography and other similar appli-

cations. The eight-rotor “octocopter” can lift around 2 kg (4.4 lbs) of

payload, with cameras mounted on an independently operated

gimbal suspended below the UAV. The gimbal is capable of a full

360� of motion, enabling cameras to be pointed in a pre-

determined direction or at a specific point regardless of the mo-

tion of the UAV (Freefly Systems, 2014). It also has vibration control

and stabilization devices that greatly reduce image blur. The flight

and navigation controls for the CineStar are produced by industry

leader MikroKopter, and are well suited to the needs of surveying.

The custom software enables users to download a Google Maps

image of the survey area, and then to plan flight paths with up to 35

GPS-guided waypoints (Fig. 4). Users can therefore specify the

precise location and speed of the UAV, as well as the camera

orientation throughout the survey. A telemetry device enables the

craft to be monitored while in flight on a laptop ground control

station, and all flight data is recorded on an SD card aboard the

craft. Finally, the fact that the craft is sold as a kit means it can be

easily customized to suit users’ needs, and can also be readily

repaired in the event of a crash or other equipment failure.

With these advantages, there are still many issues with the

system, particularly for novice operators, as a perusal of the dis-

cussion boards dedicated to UAVs will quickly demonstrate. While

Fig. 3. The CineStar8 in flight with thermal camera aimed downward (left) and a closeup of the FLIR Tau II camera core inside our custom-built case.

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219210

Page 5: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

the CineStar 8 can have a flight time of up to 24min if outfittedwith

a GoPro camera and flown in windless conditions, using a larger

camera in real world conditions flight times are considerably less,

probably around 15 min (although one would not want to find out

the limit during a survey, so some additional buffer time is always

required). Thus, flight time becomes the major limiting factor in the

size of survey areas. The engineering of many components is also

far from robust, and in our experiments with the UAV, we found it

prone to frequent failures for a multiplicity of reasons, including

faulty electronics, wires of inadequate gage, vibration loosening of

propellers, and many other issues. Nonetheless, considering the

low cost and flexibility of the CineStar 8, it suits the needs of

archaeological aerial thermography reasonably well.

4.3. Imagery acquisition

As discussed above, one of the main strengths of the UAV over

other aerial photographic platforms such as kites and balloons is

the control it gives users in planning surveys precisely. While the

relatively short focal length of our thermal camera (19 mm) in-

creases the area of the ground that can be covered in one image, its

small sensor size (8.70 � 10.88 mm) restricts the camera’s field of

Fig. 4. (A) The MikroKopter-Tool software used to program and pilot the CineStar8, with waypoints, flight path, and telemetry display as used at the Blue J Community, and (B)

actual flight path of 7:18am flight as recorded by the onboard GPS, overlaid on a Google Earth image.

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219 211

Page 6: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

view to only 26 � 32�. Combined with its rather low resolution of

only 512 � 640 pixels, we are faced with a trade-off between the

ground area of an image and the resolution of the resultant data,

determined by the altitude of the camera. While it would be

theoretically possible to simply collect a larger number of high

resolution images from a lower altitude, the short flight time of the

CineStar 8 at around 15 min, makes this impractical. Thus to

maximize the areal coverage of a survey, which is typically the goal,

one can either increase the altitude of the UAV, at the expense of

image resolution, or increase the speed of transects, at the risk of

blurring images during flight. For the surveys at the Blue J site, we

selected a compromise of 70 m elevation for the UAV, yielding 6e

7 cm ground sampling distance for thermal images (comparable to

high-resolution geophysical datasets) and an image footprint of

approximately 32 � 40 m. Survey speed was set at 5 m per second

(11.2 mph) as our past experiments suggest that at 70 m, flying

faster than this will result in an excessive number of blurred

images.

In order to ensure that images can be effectively processed in

PhotoScan or other photogrammetric software packages (see Sec-

tion 4.4) a high percentage of image overlap is required, generally

70e80%. In order to reduce the number of transects in a survey, we

orient the camera perpendicular to the flight path. The relatively

high frame rate of the video feed (30 Hz) means that wewill collect

many images per meter along each transect, enabling us to extract

images with 80% or greater overlap. To align transects with one

another, we plan on approximately 40% cross lapping images. At the

altitude and orientation described above, this requires transects to

be spaced approximately 20 m apart.

At the Blue J community, we conducted surveys with eight east-

west oriented transects of 300 m length, spaced at 20 m, enabling

us to image an area of approximately 220 � 360 m in thermal

infrared. Color imagery was also collected using a Nikon D6000

camera, with its timer set to take photos at 1 s intervals. The color

image helps to interpret results of the thermal data, producing

images of 2e3 cm ground sampling distance, and is also used to

generate a high resolution digital surface model. The wider field of

view of the camera, with focal length of its variable zoom lens set to

18 mm, enables a larger area of the ground (280 � 420 m) to be

imaged using the same flight parameters.

To aid in georeferencing of image mosaics, we placed eight

ground control targets across the survey area, and using a total sta-

tion, recorded their locations within the coordinate system used in

previous investigations at the site. Targets were made of aluminum

sheeting because metals have very low thermal emissivity, and thus

tend to appear very dark on thermal imagery. Our targets measured

60 � 15 cm, with the expectation that they would appear as 2 � 10

pixel targets in thermal data.While all of the targets are visible in the

thermal data, most tend to blur in the imagery, appearing more as

dots than Xs, and so in future surveys we plan to increase their size,

perhaps to 1.2m in length. A larger number of targets could improve

theaccuracyof thefinal images, althoughour results usingonlyeight

targets were still very good (see Section 4.4).

4.4. Image processing

We rely primarily on Agisoft PhotoScan to produce both color

and thermal ortho-imagery. For each thermal flight, we first extract

still images from the video feeds recorded by the Tau II camera.

Although the camera records at a high frame rate (30 Hz), we found

that only one frame per second is necessary for sufficient overlap

along flight transects. For color photographs, we selected approx-

imately every fourth image between the first and final waypoints,

avoiding inclusion of either blurred or excessively oblique images.

PhotoScan is one of many modern photogrammetric applica-

tions which integrate relatively new feature extraction and

matching capabilities with traditional aero-triangulation based on

the self-calibrating bundle adjustment (Mikolajczyk and Schmid,

2004; Triggs et al., 2000). As such it is capable of creating accu-

rate orthoimagery and digital surface models from overlapping

two-dimensional imagery with manual point measurement only

required for ground control identification (Verhoeven, 2011;

Verhoeven et al., 2012a; De Reu et al., 2013). Briefly put, after

adding all photographs from a flight to PhotoScan, the software

automatically aligns the images by identifying corresponding fea-

tures shared among photographs (Agisoft, 2013). The software es-

timates the locations of these points, from which a sparse point

cloud is derived, and more importantly, camera locations are

approximated and internal camera parameters are estimated

(Fig. 5). Using the estimated camera positions and the actual pho-

tographs, PhotoScan then calculates depth maps from which a

dense point cloud and meshed model are generated. We used the

highest accuracy setting to align photographs and the height field-

smoothmethod to build geometry. Initially, we produced amedium

qualitymodel, enabling us to use a guided approach by then placing

our eight ground control points. After refining marker placement in

each photograph and adding ground control point coordinates,

PhotoScan optimizes the alignment to build a more accurate and

georeferenced mesh. We used the height field-smooth method to

reconstruct geometry to produce an ultrahigh quality mesh from

Fig. 5. Thermal image mosaic from the 7:18am flight illustrating the estimated camera locations along the flight path as calculated in Photoscan.

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219212

Page 7: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

which we generate color and thermal orthophoto mosaics, as well

as a digital surface model.

Microsoft ICE, a freely available panoramic image stitching tool,

offers an alternative to PhotoScan, and a particularly good method

for processing thermal data because ICE is capable of producing a

seamless high-resolution panorama directly from a video feed.

While not offering the flexibility to change settings and parameters

like Photoscan, ICE requires dramatically less processing time and

memory demands, meaning one does not need a high-end com-

puter to process imagery. The software also seems to select images

from the video feed that have good contrast and clarity, thereby

producing image mosaics on which it can be somewhat easier to

recognize archaeological features. After producing image mosaics

in ICE, we then simply used our ground control points to geore-

ference the mosaic in ArcGIS. To be clear, ICE does not produce an

orthophoto and in areas of high topographic relief, the image

stitching applicationwill produce excessive errors. For small scenes

over relatively flat terrain, we found results to be visually pleasing

and somewhat easier to interpret than the resampled orthophoto

produced by PhotoScan.

5. Results

We flew five surveys at the Blue J community, collecting four

thermal datasets and one color photographic series. Each flight

took approximately 11 min from takeoff to touchdown. The four

thermal flights were begun respectively at 9:58pm, 5:18am,

6:18am and 7:18am, while the color photographic flight was begun

at 5:45am, just prior to sunup (Table 1). We had intended to collect

imagery at peak afternoon heat as well, but on the days of the

surveys there were frequent wind gusts of up to 30 mph during the

afternoon and early evening, making flights impossible until

around 9pm. While the CineStar 8 can fly in far more diverse wind

conditions than other aerial photographic platforms, strong, gusty

winds make flying challenging even for experienced pilots. On the

days in question (June 20e21, 2013), temperature variability was

high, as is common in deserts, reaching 32 �C (90 �F) on the af-

ternoon of June 20 and a low of 8 �C (46 �F) in the early morning

hours of June 21. Such temperature oscillations may improve the

likelihood that archaeological features will appear because the

ground will likewise experience more extreme heating and cooling

over a diurnal cycle.

As outlined in Section 4.4, color and thermal imagery was

orthorectified, and georeferenced mosaics of each flight were

produced. Each thermal orthophoto mosaic was composed of

approximately 350 still frames while 118 color images were used to

create a color orthophoto mosaic (Fig. 6) and digital surface model

of the site (Fig. 7). Alignment of the thermal images was based on at

least 200,000 matched features across the scene. Interior orienta-

tion of the camera followed Brown’s model (Triggs et al., 2000)

with only the focal length, principal point, pixel aspect ratio and

three radial distortion parameters treated as unknowns. In all cases,

the resulting estimated parameters (19.80 mm calibrated focal

length, 3% longer in the camera x direction) did not deviate

significantly from our initial estimates. Four of the surveyed ground

points were used in the alignment while the other four were

withheld and used as check points (Fig. 7). The alignment (or aero-

triangulation) resulted in control point RMSE errors of approxi-

mately 6 cm horizontal and 3 cm vertical. With so few control

points this is expected and so we rely on the check point errors to

indicate the quality of the alignment. Check point errors at each of

the four points for the 6:18am flight are shown in (Table 2). Hori-

zontal and vertical errors at the check points are larger than at the

control points e again, expected e but still only about the size of a

pixel in the final orthophotos.

In all thermal images, the modern road, an unimproved dirt

track, appears very clearly at the top of the image, as does the tarp

used to launch the UAV (in order to minimize dust particles from

getting inside the motors) (Fig. 6). Our two vehicles and team

members also appear at the upper left of the images. Because the

UAV tends to swing as it adjusts to new waypoints, we collected

more oblique images at transect ends than we had anticipated. The

best image for archaeological visibility is the 5:18am flight (Figs. 6

and 8A), which reveals many surface and subsurface features. The

ambient warming of the vegetation at the site by 6:18am results in

increased noise (Fig. 8D), while by the time the 7:18am image,

intense raking sunlight covered the entire site, producing an image

that primarily records how surface features differentially reflect

thermal radiation (Figs. 7 and 8E). While subsurface features are far

less apparent in the 7:18am image, the fact that the relief of the

ground surface is so clearly visible makes archaeological features

with topographic expression quite evident. Experimental data from

the 1970s show that in some cases the ability of early morning

thermal imagery to reveal micro-relief can be used to extensively

map archaeological remains (Scollar et al., 1990: 623e8).

In all nighttime images, modern vegetation creates a very strong

signature, as thewater in trees and plants tends to retain heat much

more readily than the dry desert soils. In the 9:58pm image, there is

more vegetation noise than in the pre-dawn images, as even grasses

and small shrubs produce high thermal infrared values (Fig. 8F). By

the following morning, the larger trees and woody shrubs still

appear as high value spots, while most of the grasses and smaller

plants had cooled, reducing noise and increasing the likelihood that

archaeological features will appear (Fig. 8A). Similarly, the complex

geology of the cliffs and talus slopes at the northern edge of the

survey produce very strong thermal noise (Fig. 6), with extreme

high and low values, and thus make detection of potential archae-

ological remains unlikely. In both evening and pre-dawn images

there are numerous low value, round features measuring 1e3 m in

diameter, some of which could be ancient storage pits, which are

commonly found on Puebloan sites in the region (Kantner, 2004).

However, comparison with the color imagery shows that most of

these features correspond with patches clear of vegetation, and

ground truthing reveals that most of these features are antmounds.

In the 7:18am image, many of these features have a small mound at

their center, confirming this interpretation (Fig. 8E).

After images were georeferenced, we plotted the locations of all

the archaeological sites and features that were previously identified

through surface survey at Blue J in order to evaluate whether and

how these features appear on the thermal imagery. Careful analysis

of the thermal image datasets in comparison with the color image

enables us to quickly eliminate those features in the imagery that

are caused by modern, surface landscape features from those that

likely represent subsurface archaeological remains. Section 5 out-

lines key findings and their interpretation.

6. Discussion

In the 5:18am image, most of the previously identified sites at

Blue J can be recognized, and in many cases it provides a detailed

Table 1

Summary of flight times, temperature, and weather conditions during aerial surveys

at Blue J on June 20e21, 2013.

Time Camera �C/�F Conditions

9:58pm FLIR Tau II 22/72 Dark, occasional wind gusts

5:18am FLIR Tau II 10/50 Dark, no wind

5:45am Nikon D6000 11/52 Light on horizon, no wind

6:18am FLIR Tau II 13/55 First sunlight on site, light wind

7:18am FLIR Tau II 16/61 Strong raking sunlight, light wind

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219 213

Page 8: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

view of subsurface remains, including the size and orientation of

roomblocks, individual walls, and potential subterranean features

(Fig. 6). Field investigations show that many of these sites are re-

mains of stone-built architectural roomblocks, often visible at the

surface todayas piles of rubble and artifact scatters (Fig. 2B; Kantner,

2013). These features generally appear on the early morning ther-

mal imagery as high value clusters, presumably because the stone

walls and collapsed masonry retain heat longer than surrounding

desert soils. The deepest middens, such as the one just east of LA

170609, also appear as high value patches, probably because they

similarly contain dense concentrations of stone and artifacts.

Significantly, atmanyof these sitesweare also able to distinguish

subsurface architectural remains. One of the best examples comes

from LA 170609 (Fig. 8). First identified during survey operations in

2004, this domestic roomblock and its associated plaza andmidden

areas were tested and completely backfilled in 2006 (Fig. 8B).

Excavation of the roomblock was limited to uncovering the tops of

the buried walls in order to determine masonry type, room sizes,

and structure orientation. While the survey estimated roomblock

size at five rooms based on the surface rubble, excavations revealed

at least six and probably eight buried rooms and, most notably, a

lengthymasonrywall surrounding the plaza area to the southeast of

the roomblock. This feature was not visible on the surface, but can

readily be seen as a high-value arc in the thermal data that may, in

fact, extend farther north than the test excavations identified

(Fig. 8A). A dark circular area indicating lower thermal values just

Fig. 6. Thermal imagery of Blue J from the 5:18am flight (top) compared to color imagery (bottom). Locations of features discussed in the text are indicated. (For interpretation of

the references to color in this figure legend, the reader is referred to the web version of this article.)

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219214

Page 9: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

inside the plazawall might indicate a subsurface structure such as a

kiva, but this area was not tested during the 2006 operations. The

subtle topographic expression of architectural features at LA 170609

is also evident in the 7:18am thermal image, which reveals the

micro-relief of the site quite well (Fig. 8E). The previous partial

excavation of LA 170609 may somewhat improve the clarity with

which it appears on thermal imagery, although architectural details

of numerous other roomblocks that have never been excavated can

also be resolved quite clearly, as described below.

At nearby LA 170607, just 30 m to the southwest, surface survey

recorded two possible clusters of buildings, based on the presence

of stones and artifacts, but the paucity of material prevented cer-

tainty. On the thermal data, these two building clusters appear

clearly, and remains of subsurface rectilinear walls can also be

traced (Fig. 9A). The clearest of these room blocks are the same size

and shape as the architecture excavated at LA 170609, providing

some confidence in our interpretation.

At LA 170608 just south of the modern road, surface manifes-

tations were especially difficult to discern during survey operations

due to dense vegetation growing over the site (Fig. 9B). Thermal

data, in contrast, provide a clearer indication of the roomblock’s

layout because by comparing thermal and color imagery we can

Fig. 7. Thermal imagery from the 7:18am flight revealing micro-relief (top) compared to a digital surface model created in Photoscan using color photographs (bottom). Locations of

ground control points (dots) and check points (crosses) are indicated. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of

this article.)

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219 215

Page 10: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

easily eliminate high value features caused by vegetation. Analysis

reveals what is likely a long arc of rectilinear rooms flanking a

central plaza area and a smaller architectural feature, perhaps an

enclosure wall like the one that identified at LA 170609 (Fig. 8), to

the east. Similarly, at LA 170613 (Fig. 10A), thermal data reveal a

rectilinear roomblock facing east towards a small plaza or courtyard

area and a smaller arc of rubble to the east that may also be an

enclosing wall. Several other similar clusters of architectural

Fig. 8. Roomblock LA 170609 as it appears in the (A) 5:18am thermal image and (B) architectural plan produced by test excavations. Compare with the (C) color image, and thermal

images from (D) 6:18am, (E) 7:18am, and (F) 9:58pm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2

Error summaries at four check points used to indicate the quality of alignment for the 5:18am thermal images.

Label X (cm) Y (cm) Z (cm) Total (cm) # images on which

this point appears

Reprojection error

(pixels)

Point 1 �12.00 6.26 4.54 14.31 10 0.15

Point 2 �7.88 0.21 29.48 30.51 15 0.11

Point 7 �4.14 6.39 11.41 13.71 13 0.28

Point 8 11.53 �1.63 13.00 17.46 13 0.12

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219216

Page 11: Journal of Archaeological Science...2. Archaeological thermography The principles behind archaeological thermographic imaging are relatively simple: due to their composition, density

remains are clearly visible in the thermal data, but none of them are

evident in color imagery, highlighting the potential of our approach

for identification and mapping of such features at regional scales.

As mentioned previously, identification of subsurface features

such as domestic pithouses and kivas has been difficult in the Blue J

Community. Thermal imagery, however, reveals several features

that may prove to be such structures. One such case discussed

earlier is the possible pit structure in the plaza area of LA 170609

(Fig. 8). Another example is at LA 170612, where surface survey

discovered a low-density scatter of artifacts but no indication of

stone architecture (Fig. 10B). Unlike the high-value clusters that

typify the buried masonry roomblocks, a 10 m diameter, circular

low-value feature with a large number of plants growing inside it is

visible at LA 170612. Its size and configuration indicates it could be

a buried great kiva, a feature expected but not yet identified in the

Blue J Community. It could also be some other type of subsurface

feature like a large pit house. Recent ground-truthing at LA 170612

found nothing on the surface to conclusively indicate the presence

of a subsurface feature, nor did it identify another reason for the

low-value circular signal. Certainly, if a deep kiva structure or large

pit house had been filled by aeolian and slope wash sediments, the

differences in composition and porosity could reasonably be ex-

pected to create a low-value (i.e., cool) thermal anomaly. Of course,

subsurface testing will be needed to determine what the feature is,

but it offers an example of themany features that might be revealed

by future thermal imaging.

7. Conclusions

The aerial thermographic imaging presented here produced

valuable insights regarding our study site, the Blue J Community,

and show both the potential of the methodology for archaeological

investigations at sites in similar environments as well as for rapid

archaeo-geophysical data collection over very large areas. At Blue J

itself, the thermal imagery we collected reveals nearly all archae-

ological features that had been previously recorded through

pedestrian survey and subsurface testing, suggesting that in the

future, thermography could be used as a means to identify sites

ahead of field survey. The data also reveal many features that were

not evident on the ground surface and which merit additional

investigation. This includes the possible northern extension of the

enclosing wall at LA 170609 that testing may have overlooked

(Fig. 8), the roomblock layout at LA 170613 that dense vegetation

otherwise obscured (Fig. 9B), and the low-value circular features at

LA 170609 and LA 170612 that may be buried pit structures or kivas

(Figs. 8 and 10B). Future fieldwork at the Blue J Community will be

able to target these areas for additional investigation.

Previous studies have already demonstrated thermography can

be a powerful complement to conventional archaeological

geophysics, by revealing a distinct set of physical phenomena (e.g.,

Kvamme, 2006, 2008). Moreover, investigators working at sites in

the American Southwest that are similar to Blue J, where soils are

very dry and architecture is built of stones with little or no

Fig. 9. Sites LA 170607 (A) and LA 170608 (B) in 5:18am thermal image (left), interpretive plan (center), and color image (right). (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of this article.)

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219 217

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magnetic signature, have often found that both electrical resistivity

and magnetic gradiometry yield poor results (e.g., Ernenwein,

2008). The extremely rocky, topographically complex surface at

Blue J, combined with the dense, thorny vegetation would likewise

make ground penetrating radar survey extremely challenging. Our

aerial thermographic method thus offers a key tool to complement

other more conventional archaeological geophysics, and an

approach that is of particular value to investigations in the Amer-

ican Southwest as well as at sites in other similar environments

worldwide.

Finally, the method for UAV-based thermography outlined in

this paper offers a means to collect and process thermal imagery

over very large areas extremely rapidly, which is perhaps its

greatest advantage. For example, the technique could be a powerful

method for investigations of large sites where conventional

geophysics might require many months of fieldwork, as well as in

situations where more ephemeral remains are scattered over

extensive areas. It could also provide a valuable complement to

regional archaeological surveys, by revealing the likely location of

archaeological features across large areas. While it would likely be

difficult to acquire archaeologically useful thermal data in situa-

tionswhere cultural remains are covered by dense vegetation or are

deeply buried, the methods we have described herein have broad

applications in many contexts worldwide. Moreover, the ease with

which imagery can nowbe acquired opens upmany newavenues of

future research. Investigators could, for example, exploit diurnal

and seasonal differences in the appearance of archaeological fea-

tures to enhance their visibility or determine their depth. Similarly,

the differential thermal properties of vegetation could be used to

indirectly detect archaeological features, as could the subtle topo-

graphic relief that is recorded in early morning thermal imagery

(Fig. 7). Likely improvements in the reliability and simplicity of

UAVs will make acquisition of thermal imagery increasingly easy

and inexpensive, facilitating continued research into its many

possibilities and ultimately making aerial thermography a standard

stage in archaeological investigations.

Acknowledgements

Research presented herein was supported by a grant from the

National Endowment for the Humanities through the Digital Hu-

manities Start-Up program (HD-5159012). Previous archaeological

investigations at Blue J, including excavations at LA 170609, were

supported by grants from the National Science Foundation

(#0237579 and #0715996). Instruments, hardware and software

used in this project were made available through the University of

Arkansas’ Center for Advanced Spatial Technologies (CAST). Special

thanks goes to CAST staff members who provided invaluable

consultation and assistance with various aspects of this project,

especially Bill Johnson, Adam Barnes, and Katie Simon. Ken

Fig. 10. Sites LA 170613 (A) and LA 170612 (B) in 5:18am thermal image (left), interpretive plan (center), and color image (right). The low-value circular anomaly evident at LA

170612 may indicate the location of subterranean pit house or kiva. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this

article.)

J. Casana et al. / Journal of Archaeological Science 45 (2014) 207e219218

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Kvamme also offered useful suggestions regarding data acquisition

and processing, and the development of our methodologies

benefited from collaboration with Kevin Fisher and Andrew Clark.

Finally, we owe enormous thanks to the student volunteers who

piloted the UAV during all flights undertaken as part of this project.

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