T he Taking of Helicopter Photographyfor Use in PhotogrammetricResearch and Training*

ROBERT N. COLWELL, School of Forestry,

Um'?'ers1"ty of Cal1fornia

ABSTRACT: For certain types of plwto.erammetric research and trainingit is desirable to obtain either still or motion picture photography fromlow altitudes and from precisely situated camera stations. When at­tempting to obtain such photography from fixed wing aircraft, theirrelatively high rate of travel and limited maneuverability pose seriousproblems in (1) framing the target properly, (2) photographing it fromthe correct camera stations, (3) minimizing image blur and (4) obtainingsu.fficient overlap to permit either stereoscopic or motion picture viewingof the photo images. Examples of both still and motion picture photog­raphy are presented in which these problems have been overcome throughemployment of a helicopter as the aerial camera platform. Means of in­stalling and operating aerial cameras in helicopters also are illustrated.

INTRODUCTION

M OST of the work reported upon in thispaper was performed in cooperation

with personnel of the United States avalAir Station at Oakland, California. Thevaluable assistance rendered by thatgroup is hereby gratefully acknowledged.All of the photography was obtained onan "incident-to-training" basis by per­sonnel, including the author, who had hadlittle or no experience in the taking ofhelicopter photography. Nevertheless, sat­isfactory results were obtained quite con­sistently. While this indicates a generalusability of the methods employed, it isquite probable that numerous improve­ments in these methods can be developedthrough additional experience in this rela­tively new field and through discussionson uses of the helicopter.

In attempting to illustrate briefly someof the important uses which can be madeof helicopter photography in photogram­metric research and training, and themeans of taking such photography, onlytwo specific examples will be discussed: aresearch project dealing with the aerialphoto assessment of cereal crop diseases;and a training project involving the prep­aration of a motion picture entitled "In-

troduction to Photo Interpretation." Em­phasis will be placed on the special typesof still and motion picture photographyrequired for these two projects and theextent to which the use of helicopterspermitted such photography to be ob­tained. Little will be said about the inter­pretation of the photography itself. Thisconstitutes the subject matter of otherpapers and does not fall within the pur­view of this discussion.

EQUIPMENT EMPLOYED

Two types of helicopters were employedin these tests: (1) the Hiller, Model HTE­2, having a single main rotor assembly asshown in Figure 1; and (2) the Piasecki,Model HUP-2, having two main rotorassemblies, as shown in Figure 2. Bothtypes proved to have satisfactory fligh tcharacteristics for obtaining the necessaryaerial photography, although excessiveaircraft vibration at times made it difficultto obtain sharp pictures. In obtainingphotography of the types here discussed,the excellent visibility afforded both thepilot and photographer from a helicopterand the relative freedom from aircraftobstructions when taking oblique andvertical photos proved to be important

* Paper presented at the Society's Semiannual Convention and Trade Show, Statler Hotel,Los Angeles, Calif., Sept. 9, 1955.

613

614 PHOTOGRAMMETRIC ENGINEEIUNG

FIG. 1.-A. Stereogram of Hiller helicopter, model HTE-2, and photographic equipment usedin taking still photography. Tail assembly of second helicopter is partially visible at right edge ofstereogram. B. Stereogram on K-17 12" camera showing electrical and vacuum connections to heli­copter. C. Attitude of photographer during approach to target. Tote that weight of camera issupported by a thin cable. D. Attitude of photographer while photographing the target. Entireweight of camera is now being supported by photographer. Note use of viewfinder and footrest.

HELICOPTEl{ PHOTOGRAPHY FOR USE I T RESEARCH 615

FIG. 2.-Stereograms of Piasecki helicopter, model HUP-2, and photographic equipment usedin taking motion picture photography. Note use of goggles, intercommunication equipment, and"pistol grip" handle on movie camera, as discussed in text.

factors favoring the use of helicoptersover conventional fixed-wing aircraft.

The K-17 aerial camera, with 12-inchfocal length, was used in taking still pho­tography; the Cine-Special 16 mm. motionpicture camera, with focal lengths of 25and 63 mm., was used for taking motionpicture photography. Installation of aspecial vacuum pump (Figure IB) provednecessary when taking still photography inorder to suck the film flat against the cam­era platen at the instant of exposure. Noproblems were encountered, however, infitting the electrical cables of the camerato the electrical system of the helicopter(Figure IB).

The weight of the loaded K-17 camerawas so great that a photographer experi­enced difficulty in hand-holding it for longperiods of time. Nevertheless, it was con­sidered desirable to obtain hand-heldphotography in order to minimize imageblur due to aircraft vibration. Therefore,a flexible supporting cable was used asshown in Figure le, so that the cameracould be suspended from the helicoptel­frame during each approach run on thetarget. This permitted the photographerto rest between runs and even during a

run except when actually taking pictures.At the instant of photography, however,none of the camera's weight was supportedby this cable; instead the photographersupported it entirely, as shown in FigureI D, thereby minimizing image blur due tovibration.

A rope sling was improvised to serve as afootrest for the photographer. Both thepilot and the photographer were equippedwith conventional safety belts and withintercommunication equipment. The lat­ter proved to be essential in implementingteamwork during a photographic run.

Goggles were consistently needed intaking motion picture photography, be­cause prolonged exposure of the photog­rapher to the airstream caused his eyes towater and impaired his vision. In some in­stances goggles also were required for thetaking of still photography.

The motion picture camera was hand­held by the photographer by means of awooden handle, (Figure 2) in order tominimize the amount of vibration trans­mitted from aircraft to camera. However,this did not completely eliminate therhythmic undulations caused by thehelicopter's rotary blades, which tended to

616 PHOTOGRAMMETRIC ENGINEERING

produce an unpleasant image movementwhen the motion pictures were projected.*

AERIAL PHOTO ASSESSMENT OF CEREAL

CROP DISEASES

This work was performed under spon­sorship of the National Research CouncilCommittee on Plant and Crop Ecology,Division of Biology and Agriculture.Among the many agencies which activelyparticipated in the project are: (1) theUniversity of California (Departments ofAgronomy, Forestry and Electrical En­gineering); (2) the National Bureau' ofStandards (under a U. S. Air Force,WADC contract for research in spectro­photometry); and (3) the Office of NavalResearch. Special acknowledgement for thegrowing of cereal crops and for the manip­ulation of diseases on them is given toMr. Coit A. Suneson, Research Agronomistof the Field Crops Research Branch, U. S.Department of Agriculture, assigned to theAgronomy Department of the Universityof California.

A primary objective of this research wasto determine the extent to which aerialphotography could be used in detecting thepresence, and determining the severity, ofcertain cereal crop diseases, such as BlackStem Rust and Yellow Dwarf Virus, onoats and wheat. While much of the aerialphotography obtained in support of thisproject was flown at high altitudes fromfixed wing aircraft, it was consideredprobable that certain kinds of informationrelative to the cereal crop diseases couldbest be obtained from low-altitude stereo­scopic photography, either vertical oroblique. When attempting to obtain suchphotography from fixed wing aircraft, itwas found that their relatively high rate oftra vel and limited maneuverability posedserious problems in (1) framing the targetproperly, (2) photographing it from thecorrect camera stations, (3) minimizingimage blur and (4) obtaining sufficienta verla p to permit stereoscopic or motionpicture viewing of the photo images. Inaddition, the restricted visibility from mostfixed wing aircraft made it difficult tonavigate accurately at low altitudes and to

* For the description of a special cameramOllnt designed to overcome this difficulty,see a U. S. Navy publication entitled "AerialMotion Picture Coverage by Helicopter"(NavAer 10-IRB-522).

obtain photography in which the appear­ance of wings or other aircraft parts didnot obscure important ground detail. Eachof these problems was satisfactorily over­come through employment of a helicopteras the aerial camera platform.

Since one important aspect of this studywas to determine the photographic film­filter combination best suited to the assess­ment of cereal crop diseases, it was quiteimportant that comparable photographyof the test area be taken with each com­bination. It was not feasible to makemultiple camera installations in the heli­copter; a separate photographic run wasrequired for each film-filter combination.Therefore such factors as flight altitude,overlap, photographic scale, and degree ofobliquity of the camera had to be carefullycontrolled on successive runs over thetarget if valid comparisons were to be madeamong the various film-filter combinationsbeing tested. Furthermore it was quiteimportant that, on successive dates duringthe development of each disease, com­parable photography be obtained as re­gards each of the above factors. The extentto which use of a helicopter permitted thephotographer to fulfill these many require­ments is indicated in Figure 3. I t should beemphasized that this illustration showsonly the photography obtained from threecamera stations; furthermore the photog­raphy shown is for only one date, for onlytwo of the film-filter combinations tested,and for only one disease, namely BlackStem Rust on oats and wheat. Howeverthe examples presented in Figure 3 ar~considered representative of the more than10,000 photos taken in support of thisproject.

Maneuverability of the helicopter was afactor of such importance in obtaining thisphotograpny that it can scarcely be over­emphasized. Specifically: (1) when takingoblique photos of a rectangular test areafrom all four sides (to determine the bestphotographic angle, with respect to thesun, for detecting diseased areas), the abil­ity to make right angle turns in the heli­copter permitted such photography to betaken in one continuous circuit rather thanin four separate passes; (2) the ability toreduce the forward rate of travel of thehelicopter resulted in its advancing only ashort distance during the period requiredfor camera recycling; this permitted thephotographer to obtain a sufficiently short

HELICOPTER PHOTOGRAPHY FOR USE IN RESEARCH 617

FIG. 3.-Matched sets of still photographs taken from the Hiller helicopter of a cereal cropnursery at Davis, California. All photos in left column were taken with panchromatic film and 25Afilter. All photos in right column were taken with infrared film and 89A filter. Figures pertain toseverity, in per cent, of black stem rust on oats and wheat. Camera stations were (from top tobottom) directly above the nursery, at south edge of nursery, and at north edge of nursery. respect­ively. Note that camera stations and angles of photography are sufficiently comparable in left andrigh columns to permit valid conclusions as to relative merits of the two film-filter combinationsfor disease assessment.

air base between successive exposures, sothat the resulting prints could be viewedstereoscopicall y, despi te the nearness ofthe camera stations to the target area; (3)the ability uf the helicupter to climb ordescend verlically, anu in small incre­ments, permitted accurale cuntrol of thealtitude of photography; and (4) t-heability of the helicopter to land and totake off from the road beside the testplots facilitated the changing of film

magazines-an important requirementsince, as previously stated, it was notfeasible to make multiple camera installa­tions in the helicopter. As a result of thesefactors, it was possible to obtain, in aVeriod of less than one hour, all the photocoverage required on une particular datefor a test plot such as that shown in Figure3. If an appreciably longer period of timehad been required, variations in sun alti­tude and light intensity would have been

618 PHOTOGRAMMETRIC ENGINEERING

sufficiently great to invalidate photoimage comparisons among the variousfilm-filter combinations.

PREPARATION OF A TRAINING FILM

ENTITLED "INTRODUCTION TO PHOTO

INTERPRETATION"

This film is being produced under spon­sorship of the Pan American Institute ofGeography and History of which theUnited States and other American Repub­lics are members. It is intended for use byall the American governments in teachingthe techniques of aerial photo interpreta­tion to selected personnel.

One of the basic problems which a stu­dent encounters when learning to inter­pret aerial photos results from the factthat objects present a much different as­pect or appearance to him in aerial viewsthan in the ground views with which he ismore familiar. Often those features whichare most conspicuous from the groundview, such as the steeple of a church orthe waterfall in a stream, are surprisinglyinconspicuous when viewed from the air.Conversely, certain features which arerarely noticed on the ground, such as roofpatterns and shadow details, may be sur­prisingly conspicious and of great diagnos­tic value in the aerial view. It seemed thatthis important point should be well illus­trated in an introductory training film onphoto interpretation. In order to accom­plish this effectively, there was need forsome means by which the motion picturecamera, while photographing a suitableobject such as the aforementioned churchor waterfall, (Figure 4) could be trans­ported through an arc extending from apoint on the ground near the object, to apoint vertically above the object. If thephotographer could then keep his motionpicture camera trained continuously onthe object while describing the arc, theresulting motion picture would illustratethe gradual- change in appearance of theobject during the transition from groundview, through aerial oblique views, to thetruly vertical view commonly encounteredwhen interpreting aerial photographs.

The potential value of this type of pre­sentation in a motion picture was firstdemonstrated in each of two ways: (1) byselecting several small items of ofticl'equipment, sticking them vertically into apiece of fiberboard and photographingthem with a motion picture camera while

it was being panned from horizontal tovertical, using two legs of the cameratripod for a fulcrum (three frames fromthis scene are shown in the left column ofFigure 4); and (2) by selecting a physio­graphic feature, sU'ch as the waterfallshown in Figure 4, which was so situatedwith respect to the surrounding topog­raphy that it could be photographedboth horizontally and obliquely from se­lected vantage points on the ground, andvertically from a conventional fixed wingaircraft. Except in such special cases,however, the desired photography couldnot be obtained when employing only con­ventional means of photography.

Although some difficulties were en­countered when attempting to describethe necessary arcuate flight path with ahelicopter, while taking motion picturesfrom it, generally satisfactory results wereobtained as indicated by photos in the tworight hand columns of Figure 4.

Since Figure 4 shows, for each of thetargets, only three frames clipped from themotion picture-a ground view, an obliqueview and a vertical view-the concept of agradual transition in object appearance,which can be so effectively ilIllstrated inmotion pictures, can only be deduced froma study of this figure.

Among the problems encountered, and.solutions devised, while taking this motionpicture photography are the following:

(1) When "panning" up and over a tar­get in a helicopter there is a strong tend­ency to rotate the camera about itsoptical axis, as this facilitates keeping thetarget centered in the field of view. How­ever, any such rotation obviously willdetract from the effectiveness of the photog­raphy when it is projected, and thereforemust be avoided. Usually the best resultswill be obtained by keeping one side ofthe camera in a vertical plane parallelto the main axis of the helicopter through­out the entire photographic run, rotatingit about an imaginary axis perpendicularto this plane to the extent required forkeeping the target centered in the field ofview throughout the entire run. This isquite feasible provided that all portions ofthe arcuate Hight path described by thehelicopter fall approximately in a verticalplane thai includes the target- to lx' photo­graphed.

(2) When attempting to obtain largescale motion picture photography of the

FiG. 4.-Matched sets of motion picture photographs of (left to right) standard items of office equipn'lent; a waterfall and lake; a church; and a stand oftimber which has a large hole in the crown canopy. Photos in the two right-hand columns were taken with a Piasecki helicopter as shown in Fig. 2. Such photosparticularly when shown in motion picture form, enable the beginning photo interpreter to visualize how objects which he is accu stomed to viewing on the ground(bottom row) should look in the aerial oblique (middle row) and vertical views (top row). For further explanation, see text.

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target from a helicopter several inter­related factors are involved: if flyingspeeds greater than about 40 miles perhour are employed while flying close to_thetarget, the motion picture tends to appeardiscontinuous or "jumpy" because ofinsufficient overlap between successive ex­posures; if the forward rate of tra vel of thehelicopter falls below about 30 miles perhour, increased aircraft vibration coupledwith a more erratic flight path, results inpoor image quality particularly when fly­ing close to the target; and if a lens of longfocal length is used (to permit the desiredscale of photography to be obtained at agreater distance from the target) vibra­tions and other sources of unsteadiness ofthe camera are greatly magnified in theprojected image. In the work here re­ported upon, by far the most satisfactoryremedy to this multifaceted problem re­sulted from employment of (a) high cam­era speeds (64 frames per second insteadof the conventional 16 to 24); (b) shortfocal lengths, to II:linimize unsteadiness ofthe image; and (c) flying speeds as highas 30 to 40 miles per hour, even when flyingwithin 200 to 300 feet of the target.

(3) When flying close to the groundover dry terrain, the strong downdraft ofair beneath the helicopter's rotary bladestended to create such a dust-storm that itwas virtually impossible to take goodphotography; for the same reason thephotographing of cereal crops from verylow altitudes sometimes resulted in imageswhich could not be fused stereoscopicallybecause the foliage was buffeted from oneposition to another during the intervalbetween successive exposures, therebyproducing large parallactic distortions.At altitudes of greater than 50 feet, how­ever, neither of these factors provedtroublesome.

In the interest of brevity, only passingmention can be made of certain othervaluable uses of the helicopter in preparingthis training film. The ideal way for abeginner to learn photo interpretationafter having familiarized himself with theaerial view, as described above, is to studythe aerial photos of a great many suitableareas and then to occupy these same areason the ground, so that he may check theaccuracy of his interpretations. In at­tempting to simulate this procedure asclosely as possible in a training film, thehelicopter is useful, first in making a low

altitude reconnaissance for the purpose ofselecting suitable areas, then in photo­graphing such areas with a motion picturecamera from the most favorable aerialvantage points, and finally in permittingthe photographer to land at or near theselected areas so that he may examine eacharea carefully on the ground, preparepertinent and accurate narrations for thisportion of the film, and take supplemen­tary motion pictures on the ground, asrequired to illustrate certain essentialpoints. By this means the motion pictureviewers, without leaving the comfort oftheir theatre seats, can in effect be con­ducted on an extensive photo interpreta­tion field trip to a wide variety of carefullyselected areas, however remote, by anexpert who has thoroughly familiarizedhimself with each area through actualground observation.

In illustrating certain aspects of photointerpretation it is desirable to show howan analysis of the motion of objects duringthe interval between successive exposurescan be exploited by the photo interpreter.Whether it be an analysis of the flow oftraffic in a congested area, or the accelera­tion rate and ultimate speed of a navalvessel under full power, if aerial photosare used in the analysis, then aerial mo­tion pictures are an excellent means forexplaining the techniques. Obviously thehelicopter proves useful in the making ofthese scenes because its ability to hover oradvance slowly provides the photographerwith precisely the desired vantage pointsfrom which to take his motion pictures ofthe moving objects.

While many other important uses ofthe helicopter in taking either still ormotion picture photography might bediscussed, it is considered appropriate todevote the remainder of this brief paperto a discussion of some limitations of thehelicopter in this regard: Much of thephotography desired should be taken ataltitudes of from 50 to 500 feet. Thiscorresponds closely to the altitudina~

range considered most hazardous by heli­copter pilots since it is too low to permitany emergency landing by means ofautorotation in the event of engine failureand it is too high to permit any beneficialuse of the "aircushion" created by thestrong downdraft of air from the rotarywings. For these reasons some pilots areunwilling to hover or even to fly at speeds

HELICOPTER PJJOTOGRAl'llY FOR USE IN RESEAHCII 621

as low as 30 to 40 miles per hour whileoperating within this altitudinal range.Accordingly the photographer may belimited as to the types of photography hp.,an obtain.

With amazingly high frequency the shad­ow of the helicopter tends to appearwithin the area to be photographed. Per­haps this should not be too surprising, afterall, when it is considered that the bestillumination of an area usually exists whenthe sun is at the photographers' back; thisis the very condition under which thehelicopter's shadow is most apt to fallwithin the area to be photographed.Because of the relatively low altitudes atwhich most helicopter photography isflown, the shadow tends to occupy a verylarge portion of the area to be photo­graphed and therefore is much more con­spicuous than on conventional, high alti­tude photography. On motion picturephotography, the shadow of a helicopterfluttering across the area of interest, withits wings rotating at an observable rate,can be especially distracting. While thisdifficulty is by no means insurmountable,the limitations imposed by it should beclearly recognized.

The carrying capacity of most present­day helicopters is too little to permit thedesired flexibility of operations on certainaerial photographic missions. For exam­ple, in the project involving assessment ofcereal crop diseases, it was necessary totransport the extra film magazines andseveral gallons of high test aviation gaso­line to the target area by automobilerather than in the helicopter itself, be­cause of both weight and volume limita-

tions of the helicopter.Still another limitation which is SOI1JC­

what corollary to the above is that theperformance characteristics of a helicopterflying at slow speeds, or hovering, arcgreatly dependent IIpon air density. In theprojects that have just been described thisimposed limitations in two respects: (a)it was not possible to obtain helicopterphotography of all the desired terrainfeatures in the Sierra because some ofthem were situated at such high eleva­tions that there was insufficient air densityto support helicopter flight; (b) even at thelow elevations (less than 100 feet abovesea level) at which the test plots of cerealcrops were grown, helicopter performancewas so reduced on very hot days thatseveral "involuntary landings" were made.Although there fortunately was no damageto personnel or equipment, these landingswere indeed so involuntary as to be termed"crashes" by most observers unfamiliarwith the more technical terminology.

In summary, then, it may be said thatthe helicopter has proved to be an ex­ceedingly val uable platform from whichto take aerial photography of the typesherein described, but its versatility, greatthough it is, still is limited by its per­formance characteristics. While rotary­winged aircraft can perform certain typesof photo reconnaissance missions thatfixed-winged aircraft cannot perform, theconverse' is also true. Accordingly therelation between these two types of air­craft for photo reconnaissance purposesshould be recognized as being mutuallycomplementary rather than being com­petitive.

AIR SURVEY AND MAPPING PERSONNELWe will immediately employ experienced personnel as

Stereo Operators • SurveyorsMapping Pilots • Aerial Photographers

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