Aerial archaeology: a full digital workflow for aerial photography

Archaeological ProspectionArchaeol. Prospect. 12, 235–244 (2005)Published online 17 June 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.260

AerialArchaeology: aFullDigitalWorkflow forAerialPhotography

J�RGLECKEBUSCH*,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.Copyright�2005 JohnWiley&Sons,Ltd.

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

Introduction

Aerial photography is now a well-establishedmethod worldwide. The elevated position givesthe flight 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 lensreflex 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 fly across country and record

the fugitive marks on the ground. During pre-flight 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 flight 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 developmentsin post-flight interpretation, the situation duringthe flight 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 flight path (Figure 1)or the position of the site by flying 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: [email protected] Kantonsarchaologie Zurich, Walchestr. 15, CH-8090 Zurich, Switzerland.

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

Digital photography

Digital workflow 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 film. Tests with modern digital camerashave confirmed 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 flight 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, 235–244 (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 film and much better thanconventional colour slides. Even a Fujifilm 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 fitted with a sensor with aresolution that is high enough. The conclusion isthat a modern digital camera can record moredetails of the ground than conventional film. 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.

Aerial Photography Digital Workflow 237


A full digital workflow

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 flight planning (Figure 3).

The paper map is replaced in flight with a GISand 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 digitalflow 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 file 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 flexible, as no special software or hardwareinterface has to be developed for any specificmake of camera.

The camera position of each image and thecomplete flight path, recorded on a small PC,need to be visualized online during the flight 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 office, without cumbersome dataconversion. Part of the existing database can betaken into the aircraft to provide maximuminformation during the flight. Data such asarchaeological zones, areas of interest, geologyand soil type can be loaded and displayed. It is

possible to make a flight 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 ina relational database. To release the photogra-pher from tedious data entry tasks, predefinedchoices 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, havesufficient 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 fixed 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 transflective type of LCD.A display of 800� 600 pixels provides sufficientinformation, 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 insufficient 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 fields 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

238 J. Leckebusch


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).



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 field covering the archaeolo-gical features can be identified. 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 flight direction, and the possiblerange of horizontal and vertical angles. A time-consuming flight directly over the target in orderto make a GPS reading is unnecessary.

On return to the office, the PC is connectedwith the geographical and image database. Alldata recorded during the flight 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 ofincreasing detail. The first very rough descrip-tion is made during the flight and is availabledirectly after connecting to the main system inthe office. Afterwards the information for eachsite is intensified 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 rectified andaccurate plans of the features from the availableflights 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 flight system describedabove is more portable than the combination ofconventional materials that are currentlyrequired. It is essential that all subsystems—GPS, GIS, digital camera and image database—be fully integrated, because this is an absoluteprerequisite to generate the maximum benefit foraerial archaeology.

Digital reality

With the digital workflow described above, aflight must start with planning. Targets aredefined in the office and additional informationentered for display in the air. Consultation of theautomatically updated suitability map helps todecide when the best moment has come to detecta specific type of anomaly.

In the airport, the system is easily mounted inthe aeroplane and general information isrecorded. The system is then ready to recordthe flight path. The software ArcPad from the

Figure 4. Schematic sketchof theproposedsystem, enablinga fulldigitalworkflow.The touch-screen LCDisoperatedwithapen.Transflective LCD,GPSinthemiddleandorientationsensorontopofthedigitalcameraare connectedwithacable to thePC.Alter-nativelyawireless communication ispossible.Smart displayshave this feature alreadybuilt-in.

240 J. Leckebusch


product family ArcGIS from ESRI has all thenecessary features built-in, without requiringthe latest processor technology (Bell, 2002;ESRI, 2002). Because it uses the same interfaceand logic as ArcGIS in the office, no additional

training is necessary, making it the obviouschoice for the flight system. When some interest-ing features are seen from the air, a series ofpictures is taken with a single digital camera.When the recording is completed and the flight is

Figure 5. The conceptused to calculate the target during the flight.Theimageplanesof thephotographsareprojectedastrapeziaon the ground and the target position estimated by the mass centre of a polygon of the unified area. Example taken from realdata. The difference between the manually determined target displayed as the filled smaller circle and the calculated markedwith a larger circle is only16m.Scale1:7500.Reproducedbypermissionof ARVKanton Zˇrich (2003-008).



continued, the system displays the estimatedtarget position, which can be modified interac-tively if necessary. The primary description ofthe target is made by selecting the appropriateitems from a simple dropdown list, and takesonly a few seconds. If the aircraft approaches arestricted flight zone, an optical and acousticsignal is generated automatically to warn theflight crew. Because the pilot and the photogra-pher, if not the same person, have the sameinformation, it is easy to decide in which direc-tion to go. If a predefined target is selected fromthe list, the GIS system will display the directionand distance towards it, as guidance for the pilotand the photographer. A Panasonic Toughbookwith a wireless display is ideally suited for thisapplication.

On return to the office the image files aredownloaded from the camera to the PC, makingall primary information available and allowingany further interpretation and description of the

archaeological structures to start immediately.Access from the Internet is established throughArcIMS, another product from ESRI.

Potentials and disadvantages

The potentials and advantages of the digitalworkflow are enormous. First more time can bespent in searching for structures out of the win-dow during the flight, because only a few clicksare necessary to complete the documentation,compared with time-consuming location on themap and hand written text. The display providespilot and photographer with the same informa-tion, and the moving map removes any need tosearch for the next paper map. The technology ofa GIS permits a wide variety of information to bedisplayed online, giving a better and moredetailed basis for any decision in the air thanpreviously.

Figure 6. Three-dimensionalvisualizationoftheestimationofthephotographed target, samedataset as Figure 5.DHM25#2003swisstopo (BA035089).

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With minimal time and effort, more informa-tion is stored than with conventional paperrecording, including the three-dimensional loca-tion and orientation of the camera. Therefore it isno longer necessary to search to identifythe target after the flight, and there should neverbe an image that cannot be located in the future.Flight crew and ground personnel will bereleased from tedious data entry and mapping.The digital data of the camera position andorientation together with the focal length (storedin the EXIF format) and a digital elevation modelpermits the area covered by the photograph to beestimated without any user interaction, evenduring the flight if desired.

All primary information is available indigital format on returning to the office, allow-ing the existing database to be updated imme-diately. As there is no need to wait for anyfilms to be developed, all the information canbe accessed very quickly, especially importantin urgent cases where treasure hunting ordestruction by an ongoing building project hasbeen reported. If a plan of the site is necessary,images do not have to be scanned before theycan be rectified by special software such asAirPhoto (Scollar, 2002) or Aerial (Haigh,2000). The risk of information loss during scan-ning is eliminated. The main disadvantage ofdigital workflow is its complete dependence onelectronic systems. All the systems discussed inthis paper are sufficiently robust to withstandthe vibrations of the aircraft. If necessary, asecond system can be taken on board as abackup. Another problem is the long-term sta-bility and accessibility of the digital information,when more than 10GB of images are normallyrecorded each year. It is essential to establish acareful maintenance programme for long-termarchiving, including transformation to a definedfile format, regular copying of data, andexchange of backup media. These are standardoperations in all major computing facilities.

Conclusions

The system of full digital workflow representsthe first fundamental change in aerial archaeol-ogy since its inception. When all the components

are fully integrated, including a digital camerawith an orientation sensor, a GPS, a GIS, adatabase and an image database, the systemreleases aerial archaeologists from basic docu-mentation tasks and allows them to focus ontheir main interests. The data is recorded veryefficiently, leaving much more time for interpre-tation and analysis. This system is the first one tobe fully integrated for archaeological purposes,and surpasses all existing systems in quality,speed and availability of the data.

Acknowledgements

The author is grateful to Fujifilm Switzerland,who supported the tests by lending their latestdigital camera with the extraordinary sensor,and to Patrick Nagy, who provided the real flightpath data. He would like to thank John Haighvery much for his fruitful suggestions on impor-tant practical issues of the system.

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Aerial archaeology: a full digital workflow for aerial photography