Helikite aerial photography – a versatile means of unmanned, radio controlled, low-altitude aerial archaeology

Archaeological ProspectionArchaeol. Prospect. 16, 125–138 (2009)Published online 23 April 2009 in Wiley InterScience

353
(www.interscience.wiley.com) DOI: 10.1002/arp.
* Correspondence to: G. JDepartment of ArchaeologBlandijnberg 2, B-9000 GhE-mail: Geert.Verhoeven@

Copyright # 2009 John

HelikiteAerial Photography^aVersatileMeansof Unmanned,Radio Controlled,Low-Altitude Aerial Archaeology

GEERTJ. J.VERHOEVEN*, JOLOENDERS,FRANK VERMEULENANDROALDDOCTER

Ghent University, Department ofArchaeology andAncient History of Europe,Blandijnberg 2, B-9000 Ghent, Belgium

ABSTRACT During the past 100 years, various devices have been developed and applied in order to acquirearchaeologicallyusefulaerialimagery fromlowaltitudes (e.g. balloons, kites, poles).Thispaperintro-ducesHelikiteaerialphotography (HAP), anew formofcloserangeaerialphotographysuitableforsiteor defined area photography, based on a camera suspended from a Helikite: a combination of botha helium balloon and kite wings.By largely overcoming the drawbacks of conventional kite- and bal-loon-basedphotography,HAPallowsforavery versatile, remotelycontrolledapproach tolow-altitudeaerial photography (LAAP). In addition to a detailed outline of thewhole HAPsystem, itsworking pro-cedure andpossible improvements, some of the resulting imagery is shown to demonstrate the use-fulness of HAP for several archaeological applications.Copyright# 2009 JohnWiley & Sons, Ltd.

Keywords: Helikiteaerialphotography; Helikite; low-levelaerialphotography;mapping; aerialarchaeology; remote sensing

Introduction

In 1908 L.P. Bonvillian took the first photographfrom an aeroplane near Le Mans (France),although it was actually one single frame of amotion picture (Newhall, 1969; Doty, 1983). Afew years later and largely due to the techno-logical catalyst of World War I, aerial photogra-phy from an aeroplane became a standardpractice (Deuel, 1973). To date, active aerialphotography still largely depends on thesemanned, heavier-than-air motor-driven aircrafts.In practice, individual archaeologists often

. J. Verhoeven, Ghent University,y and Ancient History of Europe,ent, Belgium.UGent.be

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acquire their own data from the cabin of a small,relatively low-flying, conventional fixed-wingaircraft, utilizing 135mm format (or slightlylarger or smaller) hand-held still cameras toacquire imagery that is mostly oblique in nature.On some occasions, however, this conventionalway of image acquisition is impossible (e.g.forbidden by the military), inconvenient orunsuited to reach particular goals. As an exampleof the latter, one might think of beyond visibleimaging (e.g. ultraviolet photography) or large-scale photography (e.g. 1/250), in which case theforward movement of the aircraft is far too fastto compensate for the situation-specific shutterspeed needed.To deal with these issues and still be able to

obtain qualitative imagery, archaeologists oftenresort to unmanned devices such as balloons,

Received 16 October 2008Accepted 20 January 2009

126 G. J. J. Verhoeven et al.

kites, model aircraft, blimps and poles to acquireimagery from the air. Generally, these devicesallow the (digital) still or video camera to bemoreor less stationary over specific spots of interestat a particular altitude, which is difficult orimpossible to achieve through all kinds ofmanned aerial platforms such as aeroplanes,powered parachutes, helicopters, balloons,ULMs (ultra legers motorises)/ultralight air-crafts, gliders and paramotors. As most of thesedevices have a restricted operation height (e.g.100m), they are ideal to perform low-altitudeaerial photography (LAAP), also called close-range aerial photography.The system presented here is a new approach

to such low-altitude aerial archaeology, devel-oped in order to acquire highly detailed (digital)aerial imagery on most occasions by means of aHelikite. Before outlining the system, the ground-based approaches mostly used nowadays willbe shortly reviewed, as their characteristics willprove essential in showing the advantages ofusing Helikites.

Low-altitude aerial photographyplatforms

There are several means used in archaeology andother scientific fields to lift radio controlled (RC)or action-delayed photographic (or video) cam-eras and acquire large-scale imagery. In general,the following low-altitude unmanned cameraplatforms are in use to capture low-altitude aerialimagery in archaeology, each with its distinctadvantages and drawbacks.

(i) M

Copy

asts, poles or booms – although theseplatforms are cost efficient, very portableand stable, they are limited by a moderatemaximum operation height of 20m.

(ii) U
nmanned aerial vehicles (UAVs) – encom-passing mostly RC model aeroplanes andhelicopters, this category is generally charac-terized by superior navigation possibilities,but problems with induced vibrations, costand less straightforward operation stillallow kites and balloons to be the mostwidespread LAAP platforms.
right # 2009 John Wiley & Sons, Ltd.

(iii) K
ites – since the 1970s, kite aerial photogra-phy (or KAP) is practised by many individ-uals and archaeological teams, as thesehighly inexpensive and portable platformscan accommodate a few kilograms of pay-load. Moreover, only wind is needed tomake it work. This dependency is also itslargest drawback, as irregular winds are notsuited for KAP and the size of the kite isdependent upon the wind speed. It goeswithout saying that ‘KAPing’ is not possiblein windless situations.
(iv) B
alloons and blimps – these lighter-than-airdevices fill in the gap characteristic for KAP,as they can be used in windless and verylight wind conditions. Moreover balloonphotography is extremely flexible in itssetup and operation is easy. However,balloons and blimps become difficult toposition and hold steady if the wind speedexceeds approximately 15 kmh�1.
Thus it is clear that a combination of a balloonand a variety of kites is often essential tomaximize the conditions for low level aerialarchaeology. However, rather than using two ormore separate lifting platforms (e.g. Whittlesey,1968, 1974; Aber, 2004; Ahmet, 2004; Baumanet al., 2005; Wolf, 2006), a Helikite can be used.

Helikite

The Helikite is a unique design, patented bySandy Allsopp in 1993 and currently manufac-tured by Allsopp Helikites1 Ltd, combining thetwo aforementioned constructions. By joining ahelium balloon with kite wings (Figure 1), thislighter-than-air device combines the best proper-ties of both platforms. The helium filled balloonallows it to take off in windless weatherconditions, whereas the kite components becomeimportant in case there is wind: first of all, theylift the construction up in the air to altitudeshigher than the pure helium lift. Moreover, theHelikite’s lift becomes stronger with increasingwind speed (with an upper lift limit dependingon the Helikite’s size). Second, the wingscounteract any unstable behaviour that is charac-teristic of balloons and blimps flown in windy

Archaeol. Prospect. 16, 125–138 (2009)

DOI: 10.1002/arp

Figure 1. Inflating the Helikite (photographby G.Verhoeven).

Helikite Aerial Photography 127

conditions, hence stabilizing the Helikite (AllsoppHelikites Ltd., 2007).

The Helikite’s distinct excellent all-roundbehaviour has also been reported by researchersof the Centre for Transportation Research andEducation (CTRE) at the Iowa State University,who compared the photographic conditionsyielded by a kite, blimp, Helikite and balloonin several wind conditions (Figure 2). In thisrespect, the Allsopp Helikite is not only moreversatile than comparable devices, but it sup-ports more payload for its size when comparedwith ordinary aerostats, and operates in strongerwinds than traditional blimps or balloons(Allsopp Helikites Ltd., 2007). Due to the factthat it can be used in adverse weather (e.g. rain,fog, freezing conditions) it is also more flexible inoperation than most UAVs.

Based on these properties, a complete photo-graphic system using a 7m3 Skyhook Helikite

Figure 2. Wind speed versusphotographic characteristics forcompetingalternatives (adapted from CTRE, 2004).

Copyright # 2009 John Wiley & Sons, Ltd.

(Figure 1) was designed in 2005–2006 by theClassical Archaeology section of the Departmentof Archaeology and Ancient History of Europe(Ghent University, Belgium), in collaborationwith the Department of Industrial Engineering,KaHo Sint-Lieven, Ghent. For obvious reasons,this type of unmanned photography was termedHelikite aerial photography (HAP).

Helikite aerial photography

The system

The requirement was to allow the acquisition ofgeneral overviews as well as highly detailedimages of specific locations, both in the visibleand invisible range of the electromagnetic (EM)spectrum, so this complete system had to bestable, easily maintained and remotely control-lable. In this section, the nine major componentsthat are part of the finally assembledHAP systemare described separately (and indicated onFigure 3).The camera-lifting device is a 7m3 Skyhook

Helikite that can lift a mass of about 3.5 kg inwindless conditions at ordinary air pressure and

Figure 3. The complete Helikite aerial photography system(illustration by G.Verhoeven).


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temperature. Due to its wings, a 25 kmh�1 windallows for a buoyancy of 100N: i.e. 10 kg ofpayload. This value largely surpasses the grosslift of pure helium (He). With a gaseous density dof 0.18 g L�1 (or 0.18 kgm�3) at 08C and 1 atmos-phere (i.e. standard temperature and pressure:STP) – compared with 1.29 kgm�3 at STP forair (Zumdahl, 1997; Silberberg, 2006; MesserGroup GmbH, undated) – 1m3 helium lifts atSTP slightly more than 1.1 kg (¼ 1.29 kgm�3–0.18 kgm�3) of payload, a value that will alterwith varying temperature and atmospheric pres-sure (Cuneo, 2000). Consequently, a 7m3 airshipcan have a maximum buoyancy of about 77N (ifits own mass was zero). However, the design ofthe Helikite overcomes the physical restraints ofits lifting gas. By way of comparison, Aber (2003)mentions a helium blimp of 7m3 with a net lift of2.3 kg, whereas Summers (1993) utilized a 20m3

helium blimp, yielding 95N net buoyancy.Table 1 gives an overview of the current (April2008) Allsop elikite product line, with the modelspecific (lifting) capabilities indicated.To securely fly the Helikite, an appropriate

tether must be used, taking several consider-ations into account. First of all, higher flying (i.e.above 100m) causes sag of the line due togravity. Therefore, the tether’s mass is best keptto a minimum. Additionally, the line is also notallowed to stretch too much (certainly not when300m of line is used to reach maximumoperation height), as the combination of bothsag and stretch will make it very hard to keep

Table 1. Helikitemodels andperformance (adapted from Allso

Helikitetype

Heliumcapacity (m3)

Materialthickness

(inch�10�3)

Lift innowind (kg)

Vigilante 0.15 1 0.03Lightweight 0.15 1 0.06Skyshot 1.6 2 DSCSkyhook 1.0 2 0.3Skyhook 1.6 2 0.5Skyhook 2.0 2 1.0Skyhook 3.3 3 1.2Skyhook 6 3 3.0Skyhook 11 3 5.5Skyhook 16 3 8.0Skyhook 25 6.0 9.0Skyhook 35 6.0 14.0Skyhook 64 6.0 30.0


tight control over the Helikite. Therefore,Dyneema1, an extremely strong polyethylenefibre, is advisable (Bults, 1998), as it has only 5%stretch and is more than ten times stronger thansteel per unit of weight (DSM, 2008). With a highbreaking strain of 270 kg and a diameter of only2.2mm, the currently employed Dyneema1

tether allows the Helikite to fly securely andmakes steering easy, as the operator can readilyfeel the connection with the Helikite. in order toattach the Dyneema1 line to the Helikite knotsare inevitable. However, together with kinksor angles, knots stress the fibres of the tetherunevenly, weakening the strength of the line.The degree of strength loss is largely knotdependent (Leffler, 1999), with particular knotsweakening the line to about 50% of its ratedstrength. It is therefore safe to assume that thetether will never perform at more than half itsclaimed breaking strain (Richards, 2005; GrogLLC, 2007). Hence, 270 kg Dyneema1 is still safeas a tether. Ideally, 500 kg Dyneema1 lineshould be used, as resistance to abrasion alsoneeds to be taken into account. However, itsmass and diameter would compromise the pay-load capacity too much in windless conditionsand create problems with the amount of flyingline that can be held by the reel (Figure 3(8)).

The camera rig is attached to the tether some20m below the Helikite (to avoid vibrationeffects and sudden movements of the Helikite),and consists of a camera-supporting frame orcradle and a suspension system. The Picavet

pp Helikites Ltd., 2007)

Lift in15mphwindspeed (kg)

Approximatemaximum

windspeed (mph)

Approximatemaximumunloadedaltitude (ft)

Helikitelength (ft)

0.15 25 1000 30.18 25 1300 3DSC 30 2500 61.5 28 2000 52.5 30 2500 64.8 32 5500 76.5 35 6000 99.0 40 6500 1112.0 45 9000 1216.0 46 9000 1320.0 50 9000 1630.0 60 11000 2270.0 70 15 000 26


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suspension applied in this system allows for self-levelling and securing the cradle. Named after itsFrench inventor, the Picavet suspension system(Picavet, 1912) can be applied in several variants(Beutnagel et al., 1995; Hunt, 2002). The one usedin this project is a large rigid cross (known to bemore twist resistant than small crosses – Gentles,2007) with each of the four ends (Figure 4 (1–4))connected at two anchor points (Figure 4 (A andB)) to the flying line. The latter is accomplishedby means of a double pulley block, combinedwith a dedicated fishing hook that is attached tothe line by means of a Brooxes HangupTM. Thesmall Ron Thompson DynaCable line (a micro-filament fishing line of 0.25mm diameter and a20.2 kg breaking strength) that provides thisconnection is one looping string. Lastly, a ringconstrains the two innermost crossing cables. Theresult is a simple, very lightweight dampenedsuspension for the cradle, superior to the oftenused pendulum (Beutnagel et al., 1995), and

Figure 4. The Picavet suspension (illustrationbyG.Verhoeven).


capable of minimizing camera swinging whenmanoeuvring (which changes the angle of thetether) as well as absorbing all kinds of vibrations(e.g. those induced by the wind), the latter beingone of the prime requisites for convenient LAAP(Ebert et al., 1997).The sturdy aluminium and carbon cradle, the

camera supporting part of the rig, was specifi-cally designed and built by J. Loenders within theframework of his master’s thesis. Except for thefour carbon legs and the carbon Picavet cross –allowing the construction to stand and take-offindependently and protect the still camera in caseof a rough landing (Figure 5A) – the cradle iscompletely made of aluminium: a cheap, light,but sturdy, bendable and easily drilled materialthat is often used to construct cradles (Eisen-hauer, 1998). Due to its design aswell as the solid,precisely lasered and aluminium frame profilesused, the cradle experiences a very low staticnodal stress when loaded (Loenders, 2006),allowing for extremely fluid rotations, the lattercontrolled by three small servo motors (typeGraupner C577). Although existing cradles comein all kinds of designs (Eisenhauer, 1996; Hanson,2001), many of them, certainly the older ones,only allow the camera’s orientation to be setbefore taking it aloft. The more advanced onesenable remote control of the attitude of thecamera, generally allowing for rotation (08–3608)and tilt (08–908). However, altering the camera’sorientation with only two degrees of freedom(DF) will always exclude certain combinations ofview. Hence, this cradle was designed as to allowfor three remote controlled DF of the camera (seeFigure 5B): v¼�458 to þ458 around X (tilt orroll), w¼�458 toþ458 around Y (tip or pitch) andk¼ 08 to 3608 around the Zaxis (swing or yaw, butalso called pan in this context). By implementingthese three functions both vertical and obliquepictures can be taken, the latter with everypossible orientation in relation to the object/siteunder investigation and/or position of the Sun.Even though still a prototype, the current cradlelargely fulfils the initial design goals, as it isdurable, easily operated, and steadywith smoothrotations at the joints.The cradle was designed to allow a variety of

cameras to be mounted, but not more than onesimultaneously. Although the initial testing was


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Figure 6. Steering the digital still camera by assistance of thelivevideo link (photographbyW.Gheyle).

Figure 5. (A) Thecradle (photographbyW.Gheyle), whichallows (B) the camera to tilt, tipandswing (illustrationbyG.Verhoeven).


performed using a film-based Nikon F70 single-lens reflex (SLR) camera, all currently cradle-mounted cameras are digital still cameras (DSCs)of the SLR-type: the Nikon D50NIR, D70s andD80FS.Whereas the first SLR is a convertedNikonD50 that allows only true near-infrared (NIR)photographs to be taken (Verhoeven, 2007, 2008b;Verhoeven et al., 2009), the full spectrum (FS)modified Nikon D80 enables NIR, visible andnear-ultraviolet (NUV) photography (Verhoe-ven, 2008a). The Nikon D70s is a conventionalDSC, which is used to acquire ‘normal’ visiblephotographs. To trigger the shutter of these SLRs,a gentLED is used. When connected to a RCreceiver, these tiny devices can be triggered bythe latter to emit infrared (IR) signals that canoperate digital still and video cameras with IRreceivers (Gentles Ltd, 2007). As all aforemen-tioned digital SLRs (D-SLRs) contain such awireless receiver, they can be operated remotelyby a gentLED SHUTTER (only one of the severalgentLED options) to enable focusing and releas-ing the shutter. Moreover, D-SLRs suspendedfrom the Helikite have many advantages: theyare lightweight (620 g, 679 g and 668 g respect-ively, batteries included), there is no restrictionto 36 frames, the exposure can be calculatedautomatically (with aperture or shutter speedpriority), and a wide choice of lenses withdifferent focal lengths is available (at a rangeof qualities and prices); all four are essentialfeatures for convenient RC photography (Fosset,1994).To control the shutter and enable the steering

of the camera, a six-channel proportional RChand-held transmitter (type Graupner X-412


UNIT 35MHz – Figure 6) uses radio signals of35MHz to wirelessly send the commands givenby the operator to a proportional six-channelreceiver (type Graupner R700 miniature SUPER-HET) mounted on the cradle. This receiver, for itspart, controls the three small Graupner servomotors to make the camera rotate in all possibledirections, while a fourth receiver channel is usedto trigger the gentLED and allows the DSC tofocus and take a photograph.

Being unable to directly observe the areaphotographed is one of the greatest disadvan-tages of certain close range aerial photographicsolutions (Harding, 1989), as it is very hard toestimate what the still camera will exactlyphotograph (Heafitz, 1992). To counteract this,a direct video link was established using the ProX2. This very handy plug and play video system


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Figure 7. Fishingrodandreelattached to thenavigator’sbody(photographsbyW.GheyleandD.Van Limbergenrespectively).


consists of: a tiny 4.8V Hi Cam EO5-380 CCDcamera(W�H�D¼ 30mm� 25mm� 28.6mm)attached to look directly through the camera’seyepiece; a connected micro FM (frequencymodulation) transmitter to send the video signalwirelessly to the ground; a ground-based audio/video receiver with a patch antenna (8 dBi) topick up the signal and feed it to a small monitor(1440 pixels� 234 pixels). In this way, the TFT(thin film transistor) screen, which runs on a 12Vbattery, instantaneously displays the area seen bythe camera-lens combination (Figure 6), allowingthe camera operator(s) to correctly orientate theD-SLR, compose the shot and decide whether ornot to take the image. Moreover, as the view-finder display also shows useful information onthe focus, the number of exposures remaining,the battery status, aperture and shutter speed, thecamera operators can check the DSC’s normaloperation and verify when the memory card isfull. Thanks to its compactness and low mass(about 65 g, excluding the 4.8V battery needed tofeed both the video camera and transmitter), thePro X2 system is ideal for RC aerial photography.Moreover, the 2.4GHz transmitter’s outputpower of 200mW allows the video signal to besent over about 300m line-of-sight (Hi Cam,2005).

One can imagine the tractive force of theHelikite when flying even in moderate winds.Using a big-game fishing lever drag reel (typeShimano TiagraTM 80W) and accompanyingcarbon sea-fishing rod (type Shimano TiagraTM

Trolling 80AX), these forces can be managedreasonably well and allow the Helikite’s operator(i.e. the ‘navigator’) to freely walk around whileletting out tether smoothly and quickly, using therod to guide the line. Fixing a large and solidwinch to the ground could be more convenientto pull down the Helikite, but severely restrictssteering and risks the penetration of archaeolo-gical layers. With a mass of 3.2 kg, a large winderhandle and a two-speed gear ratio (1/2.5 totake in line fast and 1/1.3 for high powerretrieves, a feature that is essential for flyingkites/Helikites – Eisenhauer, 1995), this reel iscapable of holding at least 300m of the specifiedDyneema1 line. When the spool is completelyfilled, the reel’s maximum drag is about 18 kgand therefore sufficient to operate in winds of at


least 30 kmh�1. The reel’s construction alsocounteracts the faster winds that can be expectedat higher altitudes, as its drag increases withreduced line level. In practical terms, the reel’sdrag force will be doubled to some 36 kg whenapproximately 225m Dyneema1 line (about 75%of the full spool) is off the reel, although thesefigures should not be followed too strictly, asmany external factors can affect drag perform-ance (Shimano1, undated). This means the reelcan always be slowed down and even stoppedcompletely by the Helikite’s navigator, as35 kmh�1 is determined to be the maximumground wind speed to safely and convenientlyperform HAP – hence covering about the sameoperating range as a kite and blimp combination.The fishing rod and reel are attached to thenavigator’s body using a big-game fishingharness and a TsunamiTM TS-A-1 Gimbal UtilityBelt (Figure 7). Although this combination isprimarily designed to allow the body to delivermaximum pulling leverage when the rod isdrawn downwards – rather than upwards as isthe case in HAP – it is still very useful whenwalking around, pausing or reeling in theHelikite, as the rod will always rest in the gimbalbelt, while the shoulder harness makes sure thereel and rod stay securely attached to thenavigator’s body in every situation.As a fast running tether (and even Dyneema1

line under tension) can severely cut exposed skin,gloves are of the utmost importance. Therefore,both the navigator and the persons assisting inattaching the cradle to the tether (generally the


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camera operators), always wear Marigold Indus-trial Kevlar handgloves (FB20PD).The combination of all these nine elements

forms the complete HAP system.

Cost, maintenance and operation

Apart from the initial purchase cost ofthe Skyhook Helikite with accompanyingDyneema1 tether (circa s 4000), the fishing gear(s 1350) and the Pro X2 (about s 550), the pricetag of other necessary parts (such as TFT screen,servos, RC transmitter, batteries, flight case,gloves, etc.) was very moderate. In the end, thebuilding of the complete HAP system – with itsprototype aluminium cradle – was about s 8000.Besides being affordable, HAP’s running costsare low, because all equipment is driven byrechargeable batteries and the construction isvery cheap to maintain. As the buoyancy comesfrom helium (He), the only additional cost whenapplying HAP is the need for this completelyinert, non-toxic, colour-, taste- and odourlessnoble gas. The non-flammable helium is trans-ported in a pressurised B50 cylinder containing10m3 of this gas, priced at about s 150(þ additional rent for the cylinder). Once inflated,the Helikite only needs to be topped up with aminimal amount of helium twice a week. Asa result, one B50 cylinder proved to be largelysufficient for one month of HAP over one area.When different locations have to be photo-graphed, a delivery van is rented to store theHelikite partly deflated and transport it to thearea of investigation, as it is not feasible torecover the helium from the balloon in the field,while completely refilling a 7m3 Helikite dailywould be too expensive. In practice, one week ofintensive HAP (on six different sites) wascompleted with only one 10m3 volume cylinder.To cut down costs further, the purchase of a largetrailer that enables the storage of the whole,inflated system is considered. Moreover, thetrailer could act as a protective hangar. As aHelikite crash is almost out of the question,additional expenses due to broken equipmentcan largely be prevented.To apply HAP in practice, the Helikite is first

inflated to its desired pressure using a tapered


foil filling outlet mounted onto the heliumcylinder and slid into a plastic plug, which isconnected to the Helikite’s non-return valve.Afterwards, the DSC and lens are mounted ontothe cradle and all mechanisms thoroughlychecked twice. Once the maximum wind speedis verified with a hand-held anemometer and theDyneema1 line attached to the Helikite using akarabiner, the Helikite operator attaches the reeland rod to his/her body by means of the harness.When the aerostat is sent skyward, the RCtransmitter and receiver as well as the Pro X2 areactivated and tested once more by the cameraoperator(s). As soon as the Helikite is about20m aloft, the rig is attached to the tether andfinally more line is let out to send the completesystem skyward. To avoid any punctures in theHelikite’s delicate surface, all aforementionedoperations take place on a large and thick canvas.

Once the system is completely and safelyairborne, the Helikite’s navigator walks aroundto establish the correct position and height forimage acquisition, while at least one cameraoperator (preferably two) determines the DSC’sangle and decides to ultimately shoot theaerial imagery. Constant communication andco-ordination between both teams is enabled bytwo-way radios and is deemed absolutely crucialfor accurate positioning of the camera, flightplanning and signifying the presence of powerlines and potential conflicts with occupied aircraft;an issue not to be taken too lightly (Benton,1998a,b) and demonstrating the crucial needfor this second camera operator. Finally, sometraining and experience are vital to yield above-average results and to make sure efficiencyand reliability in image acquisition remainsconstantly high.

Possible improvements and drawbacks

Even though HAP completely meets mostpoints identified by Walker and De Vore (1995)and Schlitz (2004) for effective LAAP (i.e. lowvelocity, small take-off and landing space,portability, low cost, minimal operational staff,low vibration, reliable power supply, fast to setup and employ, low risk and low impact) and itsadvantages over conventional kite and balloon


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photography are evident, the system is notperfect. Aerial imagery with a high spatial andtemporal resolution can be generated in severalwind conditions, but the camera cradle is not(yet) fully weather proof, making it impossible touse in case of rainfall (although one can doubt theusefulness of aerial photographs taken duringsuch wet conditions).

Second, as walking the Helikite around andallowing it to ascend or descend to variousaltitudes is the only way this aircraft can bemoved into position, it remains rather challengingto accurately establish a precise camera locationand/or take photographs along a previouslyoutlined track – a problem encountered with allforms of photography using kites and blimps(Myers and Myers, 1980; Karras et al., 1999;Tielkens, 2003; Skarlatos et al., 2004). Currently,the position of the D-SLR is largely determined bylooking both at the Helikite’s location and thetransmitted video image on the ground-basedmonitor; by passing the required instructions (i.e.higher, lower, left, right) on to the navigator, thelatter decides how to move with the Helikite –taking the direction of the wind and localtopography into account – in order to move theDSC to where it should be. This approach hasalready proven to be a very convenient way ofworking. However, in cases where groundconditions are very monotonous (e.g. a veryextensive corn field), the camera operators willstruggle to be properly oriented. In an attempt tocounteract such issues (and improve the position-ing in general), the signal of a very small, cradlemounted GPS receiver with WAAS/EGNOS(Wide Area Augmentation System/EuropeanGeostationary Navigation Overlay Service) capa-bilities will in the near future be transmitted to theground. Its accuracy of geolocation, about 3mat 2s RMSE (root mean square error), should bevery helpful in establishing the required geodeticposition of the DSC, whereas the complete flightpath (which is continually logged) will beavailable afterwards to geocode the photographsand visualize the camera’s three-dimensionalposition through time.

Third, the possible places of survey can belimited by objects that may conflict with thetether: trees, high-tension power lines, houses,scrub, etc. Furthermore, there must be a suitable


place for the operator to stand for the desiredshots given the specific direction of the wind,the length of the tether and general topographicsetting. Consequently, the positioning capabili-ties of UAVs still remain superior for RC LAAP.Finally, the helium dependency might in some

situations be the largest drawback: besides itscost, it might not always be possible to purchasehelium locally and obtain it afterwards on site; insome situations, this can be a reason not to optfor such helium-filled devices (e.g. Allen, 1980;Owen, 1993; Asseline et al., 1999). Some authors,however, point to the advantages that heliumallows for very silent operation and the vehicle tobe aloft for extended periods of time (Marks,1989), and Aber (2004) favoured helium blimpsover hot-air blimps due to reasons of fieldoperation and handling, dimensions and cost.

Archaeological applications

Notwithstanding some inevitable drawbacks,HAP photography has been rigorously testedsince 2006 and proved to be a very stable, easilymaintained and versatile radio controlled sys-tem. From 2007 onwards, a large amount oflow-altitude archaeological image data has beengenerated at several geographical locations invarying weather conditions. Although HAP wasinitially developed to allow for analogue NIR sitephotography (Verhoeven and Loenders, 2006),its application became much wider and nowallows for various types of aerial archaeologicalapplications.

General overviews and small areareconnaissance

With a maximum operating altitude limited – forthe moment – to about 200m, a verticallyoriented DSC can record approximately240m� 160m when fitted with a 20mm lens,yielding a scale of 1/10 000. If some obliquenessis allowed, the area captured can be much larger.Such oblique angles are suited to the overviewof a large site and/or its direct environment.If climatic and environmental conditions are


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Figure 8. DecumanusmaximusleavingthewesterngateoftheRoman town Potentia, discovered by small-area reconnais-sance based on Helikite aerial photography (HAP with NikonD70sþNikkor 20mm f/3.5 AI-S).

Figure 9. Near-infrared photograph of Potentia’s temple area(HAP with Nikon D50NIRþNikkor 20mm f/3.5 AI-S).


favourable, these overviews can also yield newarchaeological information, as was the case inFigure 8. When acquiring imagery to illustratethe direct relationship between the Romanmausoleum (1) and the western gate (2) of theRoman city Potentia (Adriatic Italy, RegioneMarche), the trajectory of the decumanus max-imus’s extra muros prolongation became largelyapparent as a very distinct negative crop mark.Given the fact that this feature had previouslybeen unnoticed in the grassland, two newconventional reconnaissance flights were initiated.Hence, HAP also gave clear indications about theinformation one could expect when steppinginto a small aeroplane. The latter method thusstill remains mandatory because – just like allunmanned systems but the very heavy, militarybased UAVs – HAP is impractical for surveyinggeographically extended areas.


Site overview for documentation andinterpretation

On a much larger scale (ca. 1/500 to ca. 1/5000),highly detailed overviews of archaeological sitescan be produced (Figure 9), imagery that can suitseveral purposes: documenting the progress ofongoing excavations and the several field phases(e.g. the different layers excavated), generatingimagery to use in presentations, folders andbooks, revealingminute aspects about individualfeatures that cannot be seen in plans or fromtraditional images as well as aiding in theinterpretation of a site. In case the site is tooextensive to be caught in one frame, a combi-nation of overlapping photographs can be usedto generate site-encompassing mosaics (e.g.Ahmet, 2004; Owen, 2006). Thus, HAP bridgesthe gap between the lowest conventional aerialphotography and the highest ground supportedpole photography. Although one could acquiresuch imagerywithout a live video link, the abilityto see what the D-SLR will capture is a verywelcome means in establishing a cost-efficientworkflow, because it allows the operators toframe and compose in a convenient way insteadof shooting dozens of frames approximating aworkable image of the subject under consider-ation.

Site mapping and photogrammetry

Even though such close range photographsmay not always result in plans, HAP is wellsuited to the acquisition of stereophotographs tosubsequently generate topographic surfaces and


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accurate planimetric information by means ofphotogrammetry – the latter process alreadybeing explored since the nineteenth century(Birdseye, 1940). In a similar way, stereoscopicphotography has already been performed severaltimes with kites, blimps and balloons (e.g.Whittlesey, 1968, 1970; Chagny, 2001). Althoughthere are people that fly stereo camera rigs (e.g.Schenken, 1997; Aber et al., 2002), the baselineprovided by these constructions is often too smallto yield stereo imagery for precise heightmeasurements (Becot, 1998). Therefore, themono-DSC of the Helikite has to be moved fromone point to another, so as to generate multiplestereo image pairswith a certain overlap. In orderto generate complete excavation plans or intrasitemaps as smoothly as possible, the cradle is set toacquire (near) nadir photographs (i.e. vertical).Because the appropriate ground control points(GCPs) – marked prior any excavation andmeasured by a total station survey – must beincorporated into the imagery (Figure 10), suchvery low-altitude mapping needs rather preciseframing (Bollinger, 1995), making the live videolink a necessity. However, the end products willallow the generation of maps much faster andmore consistently than most other, low-costtechniques (Horton, 1994), and the textural andcolour information provided by the ortho-imagesremains essential to complement expensivethree-dimensional laser scanning (Shaw and

Figure 10. Near vertical photograph of an excavation areawith ground-controlled positions indicated (HAP with NikonD70sþNikkor 20mm f/3.5 AI-S).


Corns, 2008). This way, HAP is consistent withthe view of Zurawski, who stated that all ongoingexcavations should have a ‘handy, reliable andinexpensive vehicle capable of shooting aerialpictures at a particular moment of the fieldwork’(Zurawski, 1993, p. 244). The fact that thisapproach is not restricted to conventional siteshas been illustrated by several scholars, whoused similar unmanned lighter-than-air con-structions to map underwater remains (e.g.Whittlesey, 1970, 1974; Jameson, 1976; Myersand Myers, 1985).

Monitoring

Due to its ability to generate imagery with anextremely high temporal resolution (i.e. theability of a system to record images at a certaintime interval – e.g. one day versus one month), afast response to events and detailed site-basedmonitoring at short time intervals is possiblewith HAP (Figure 11). As an example, one couldbase a flying strategy on the outcome of suchmulti-temporal, sequential data: as soon as thefirst crop marks start appearing in the HAPimagery over a known crop-mark-sensitive site,the urgency to begin conventional reconnais-sance flights can be registered.

Multispectral sensing

Being a stable system, HAP is suited for thosesituations where long shutter speeds are inevi-table: low light conditions, narrow-band andnon-visible remote sensing. As previously men-tioned, the initial aim in developing HAP was toperform close range, film-based NIR photogra-phy (Verhoeven and Loenders, 2006). However,it soon transpired that digital NIR photographyis far less cumbersome, with much shorterexposure times compared with the analoguetechnique (Verhoeven, 2008b). As an example,Figure 12B shows a NIR record of a towerand connected piece of wall belonging to thecentral Italian Roman city of Septempeda, andFigure 12A displays a conventional aerial photo-graph of the same location, taken one day laterbut with the anomalies imaged in a less distinctway.


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Figure 12. (A) Visible (photographby F.Vermeulen) and (B) purenear-infrared record (HAP with Nikon D50NIRþNikkor 20mm f/3.5 AI-S) ofa Roman townwalland connected tower.

Figure 11. Monitoring severalexcavation phases (HAP with Nikon D70sþAFNikkor 50mm f/1.8D).


Since the summer of 2008, the Helikite-basedsystem has also been applied as a researchinstrument for digital NUV photography bymeans of a modified Nikon D80 (Verhoeven,2008a). The same D-SLR will also be used inthe near future (i.e. summer of 2009) to explorenarrow-band photography, together with NUVimaging aimed at better revealing subsurfacestructures.

Conclusions

Fitting a (modified) DSC to a Helikite allowsfor low-level photography during conditions inwhich a tethered balloon or a kite would fail towork properly. By holding a 7m3 unmanned,helium-filled Helikite aloft with a tether, different


low-level aerial photographic methods can beused as the device is largely scale-independent.Although the solution is not perfect, HAP enablesphotography at particular times of the day, invarying weather conditions, is uncomplicated todeploy, has a large range of operation altitudesand is easy to maintain. Hence, it is often morecost effective and flexible than other individualapproaches in yielding high spatial and temporalresolution coverage.

Acknowledgements

This paper arises from the first author’s ongoingPhD, which studies the application of remotesensing in archaeological surveys. The researchis conducted with permission and financial sup-


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port of the Fund for Scientific Research - Flanders(FWO) and supervised by Professor FrankVermeulen (Department of Archaeology andAncient History of Europe - Ghent University).The development and construction of the HAPsystem benefited greatly from a generous grantafforded by the UTOPA Foundation (Voorhout,The Netherlands). The authors also owe much tothe collaboration of Devi Taelman and Dimitrivan Limbergen, who proved to be essential ascamera operators. Finally, Wouter Gheyle isacknowledged for taking pictures when testingthe Helikite.

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Helikite aerial photography – a versatile means of unmanned, radio controlled, low-altitude aerial archaeology