Aerial archaeology: a full digital workflow for aerial photography

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  • Archaeological ProspectionArchaeol. Prospect. 12, 235244 (2005)Published online 17 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.260

    AerialArchaeology: aFullDigitalWorkflow forAerialPhotography

    JRGLECKEBUSCH*,y

    terravermessungenag,Obstgartenstrasse 7, CH-8006 Zurich, Switzerland

    ABSTRACT Aerialarchaeology isanestablishedprospectionmethod, but its systemsand techniqueshave chan-ged very little since its first applications.The advent of modern high-resolution digital cameras, withbetterimageresolutionandimagedepththancomparableanaloguephotography,offersthepossibilityofa fulldigitalworkflow.Anewsystemisbeingdeveloped to fulfilall therequirementsofaerialarchae-ology. The images are stored on a PC, together with associated geographical data, including thelocation and orientation of the camera stations. A GIS displays the camera stations together withthe flight path.Once thephotographic record ofa site is finished, thegeographical co-ordinatesof thetarget are calculated automatically, and combined with other data from a simple user interface toprovide a complete primary data set during the flight. This allows the investigator to spend moretime searching for sites during the flight, and simplifies the interpretation and analysis of the imagesin the office.This paper presents several new approaches to full integration of the systems involved.Copyright2005 JohnWiley&Sons,Ltd.

    Keywords: aerialphotography;GPS;GIS; database; digitalphotography; digitalworkflow

    Introduction

    Aerial photography is now a well-establishedmethod worldwide. The elevated position givesthe ight crew an overview of the landscape,allowing small single features to be observed aspart of much larger structures and sites. Afterpioneer work between the two World Wars,military reconnaissance during the SecondWorld War caused the techniques to developrapidly. The advent of good quality single lensreex cameras in the 1960s made the techniqueavailable to a wide range of users. High-wingaeroplanes, notably those manufactured byCessna, and 35mm or larger format camerasare now used to y across country and record

    the fugitive marks on the ground. During pre-ight planning, account is often taken of simplecurves of the cumulative water balance, to decidewhether there are good conditions for soil andcrop marks, the types of anomalies most fre-quently photographed. The ight navigation isusually directed with the aid of paper maps, onwhich the archaeological remains are recordedby a point, or by an extended area whenappropriate.Although there have been great developments

    in post-ight interpretation, the situation duringthe ight has seen minimal change sincethe beginning of the last century (Deuel, 1969;Dassie, 1978). Standard practice still involvesanalogue cameras with a paper map and a pencil,even though the lenses, the photographicmaterials and the maps have all improved inquality. Since GPS systems became available ata reasonable price, some aerial archaeologists usethem to record the complete ight path (Figure 1)or the position of the site by ying directly over it

    Copyright # 2005 John Wiley & Sons, Ltd. Received 5 August 2003Accepted 7 December 2004

    * Correspondence to: J. Leckebusch, terra vermessungen ag,Obstgartenstrasse 7, CH-8006 Zurich, Switzerland.E-mail: terra@terr.chyFormerly Kantonsarchaologie Zurich, Walchestr. 15, CH-8090 Zurich, Switzerland.

  • (Braasch, 2002), and sometimes also in ightplanning (Crawshaw, 2001). Several institutionsare already aware of the signicance of combin-ing GPS, digital imagery and GIS (Heller, 2000),but no fully integrated system of digital work-ow has yet been developed and tested.

    Digital photography

    Digital workow makes sense only if the cameraproduces at least the same quality images as

    those recorded with modern analogue equip-ment. Experience with scanning of conventionalphotographs suggests that a resolution of at least2500 3750pixel (10Mpixel) isnecessary to repro-duce comparable information to an analogue35mm lm. Tests with modern digital camerashave conrmed this (Table 1 and Figure 2). Toprovide the same conditions for all pictures,different cameras with the similar lenses andparameter settings were mounted on a tripodon top of a hill, equivalent to a ight at 270mabove ground level. Larger cameras, such as the

    Figure1. Flight route recordedwith a GPS.White circles indicate the position of images taken andgrey squares the documentedarchaeological features.Scale1:100 000.Reproducedbypermissionof swisstopo (BA035089).

    236 J. Leckebusch

    Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 235244 (2005)

  • 6 6 cm systems used by several groups, are notincluded in this comparison, because they requiredifferent lenses. The tests with 35mm systemsshow that the Kodak DCS Pro 14n camera, withan excellent lens and the latest 14Mpixel sensor,gives resolution and information content superiorto any black and white lm and much better thanconventional colour slides. Even a Fujilm S2 Procamera with a 6Mpixel sensor matches the bestanalogue results. Larger digital cameras should

    be capable of producing even better results, pro-vided that they are tted with a sensor with aresolution that is high enough. The conclusion isthat a modern digital camera can record moredetails of the ground than conventional lm. Incombination with the advantage that only onedigital camera is needed for all forms of photo-graphy, so that handling in the aircraft is easierand safer, this demonstrates that digital systemsare ideally suited for aerial archaeology.

    Table1. Detailed specificationsof systemsandparametersused for the test comparison in Figure 2

    Camera Recording Sensitivity Focal length Shutter speed Aperture

    a Nikon F601 KodakTechnical Pan 80ASA 85mm 1/500 s 5.6b Nikon F601 Kodak Ektachrome E100 GX 100ASA 85mm 1/500 s 5.6c Fujifilm S2 Pro digital, 6Mpixel 100ASA 50mm1 1/500 s 5.6d Kodak DCSPro14n digital,13.5Mpixel 100ASA 85mm 1/350 s 5.6

    1A focal lengthmultiplierof1.5 for the Fujifilm camera required another high quality lens from Nikon (same series) to produce comparableresults.

    Figure 2. Comparison of aerial photographs on a detail of the complete image, recorded on analogue film andwith digital cam-eras, for details seeTable 1.The Fujifilm camera (c) has a similar resolution and image depth as a black and white negative (a)whereas the Kodak DCSPro14n (d) is superior to any conventional recording.

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    Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 235244 (2005)

  • A full digital workow

    With the last conclusion in mind, a completesystem can be built around digital images.Almost all meteorological data are now availableonline, eliminating the need to enter them manu-ally. Cumulative water balance curves can bedrawn automatically, allowing much faster deci-sions. Using the modelling capabilities of a GISwith meteorological data and digital elevationmodels, actual suitability maps with forecasts fordifferent types of vegetation and anomalies aregenerated when necessary, providing an up-to-date basis for ight planning (Figure 3).The paper map is replaced in ight with a GIS

    and a GPS provides the actual three-dimensionalposition (Figure 4). The system is designed as asingle application, combining all digital data,including the aerial images, in a very large imagedatabase. Because it is important that the digitalow is never interrupted, all information isentered digitally at its inception. Images cannotcurrently be transferred direct to the PC, as boththe Firewire (IEEE 1394) and USB 2.0 interfacesare too slow to transfer a typical image le ofaround 12MB in a reasonable amount of time.On the other hand an online connection is notstrictly necessary. The digital system describedhere records the actual position of the cameraeach time the shutter button is pressed, allowingthe time stamps of the images, stored inside thecamera, to be correlated with those of the GPSdata. Once the images have been downloaded tothe PC, the geographical locations of the cameracan be assigned to them perfectly. This system isvery exible, as no special software or hardwareinterface has to be developed for any specicmake of camera.The camera position of each image and the

    complete ight path, recorded on a small PC,need to be visualized online during the ight ona map, sometimes called a moving map. A GIS isthe best solution, because it is capable of reading,displaying and generating the data in the formatused in the ofce, without cumbersome dataconversion. Part of the existing database can betaken into the aircraft to provide maximuminformation during the ight. Data such asarchaeological zones, areas of interest, geologyand soil type can be loaded and displayed. It is

    possible to make a ight plan with an unlimitednumber of waypoints, which can handle requeststo document features, such as excavations,medieval villages, castles and landscapes.Airspace restrictions are also loaded, and thesoftware can generate a warning when they areapproached.All descriptions of an image must be stored in

    a relational database. To release the photogra-pher from tedious data entry tasks, predenedchoices should be available and selectable by afew clicks. User interaction for data entry is mosteasily accomplished by a touchscreen with a pen,tied with a string so it is always available andcannot get lost (Figure 4). A small keyboard withBluetooth or cable connection is an alternative.Data storage is not a problem. Flash disks of upto 4GB, or robust notebook hard-disks, havesufcient capacity and can withstand the roughconditions inside the aeroplane. The screen is acritical part of the computer. Limited spaceinside the cockpit necessitates a lightweightLCD that can be mounted or xed with suctioncups, either on an unused part of the instrumentpanel or above the head of the photographer. Itmust be readable in both strong sunlight andshadow, requiring a transective type of LCD.A display of 800 600 pixels provides sufcientinformation, but the higher the resolution themore data can be shown. A rugged tablet PC,or even a pocket PC, is ideally suited for thisapplication, as its low power consumption allowsit to be operated independently of the aircraft.There is often a problem in locating a photo-

    graphed site when there are insufcient visiblereference points. When the position and orienta-tion of the camera are recorded with every imagetaken, the geographical co-ordinates of thearchaeological target can be reconstructed(Figures 5 and 6). In order to measure the orien-tation, a small sensor, very light in weight andwith low power consumption, is mounted on topof the camera. An example of such a sensor is the3DM-G from MicroStrain, which delivers arobust signal under magnetic elds and accel-erations (Churchill et al., 2002; MicroStrain, 2002).When a target appears in several images, thesignals from the sensor can be matched withthe appropriate GPS positions to allow the targetposition to be calculated online without a digital

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    Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 235244 (2005)

  • elevation model, assuming the archaeologicalfeatures are in the middle of the images. Theviewing cone of each image is assigned adecreasing probability from the centre to theborder in all four directions. The combination

    of all these probability distributions provides themost probable location of the target. Assumingan elevation of 300m above ground, a GPSaccuracy of 1m, a viewing angle to the side of45 and a typical angular accuracy of 5, the

    Figure 3. Usingavegetationgrowthmodel, cumulativewaterbalancevaluesandadigitalelevationmodel, apotentialmapcanbeproducedrepresenting thepossibilityofarchaeologicalstructuresshowingup fora specifieddateduring theyear.Suchacontinu-ously updated suitabilitymapwill provide an excellent basis for flight planning.Potential for cropmarkson the 21June1998 in theCantonof Zurich.Forest areas are excluded for clarity.DHM25# 2003 swisstopo (BA035089).

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    Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 235244 (2005)

  • position of the target can be estimated from asingle image to about 100m. With a combina-tion of nine pictures, the error diminishesto 30m. In the real example of Figure 5 (seealso Figure 1 for different geometries), the dif-ference between the calculated and manuallyentered target point is less than 20m. Thisdemonstrates that with minimal additionaleffort, even a small eld covering the archaeolo-gical features can be identied. An approximateestimate of the target can be made without anorientation sensor position, provided that severalconstraints are taken into account. These includethe side of the aircraft used by the photographer,the approximate inclination of the aircraft esti-mated from the ight direction, and the possiblerange of horizontal and vertical angles. A time-consuming ight directly over the target in orderto make a GPS reading is unnecessary.On return to the ofce, the PC is connected

    with the geographical and image database. Alldata recorded during the ight are automaticallytransferred to the main system and the databaseis updated, including the generation of thumb-nails for the images. Within a few minutes, allinformation is available to the public archive andon the Internet.The features are interpreted in three phases of

    increasing detail. The rst very rough descrip-tion is made during the ight and is availabledirectly after connecting to the main system inthe ofce. Afterwards the information for eachsite is intensied with data on structural type,

    form, distribution, vegetation, type of anomalyand archaeological interpretation, all in textualformat, providing a general basis for all users ofthe archive. If necessary, images are rectied andaccurate plans of the features from the availableights are drawn, each structure coded anddescribed in appropriate detail. All information,including text and images, can be retrieved eitherby geographical co-ordinates, by location, or bythe name of a site.The complete digital ight system described

    above is more portable than the combination ofconventional materials that are currentlyrequired. It is essential that all subsystemsGPS, GIS, digital camera and image databasebe fully integrated, because this is an absoluteprerequisite to generate the maximum benet foraerial archaeology.

    Digital reality

    With the digital workow described above, aight must start with planning. Targets aredened in the ofce and additional informationentered for display in the air. Consultation of theautomatically updated suitability map helps todecide when the best moment has come to detecta specic type of anomaly.In the airport, the system is easily mounted in

    the aeroplane and general information isrecorded. The system is then ready to recordthe ight path. The software ArcPad from the

    Figure 4. Schematic sketchof theproposedsystem, enablinga fulldigitalworkflow.The touch-screen LCDisoperated...

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