582 PHOTOGRAMMETRIC ENGINEERING

2. MANUAL OF PHOTOGRAMMETRY, AmericanSociety of Photogrammetry, 1952.

3. Church, Professor Earl F., Bulletins Nos. 15and 19 on Aerial Photogrammetry; SyracuseUniversity, Syracuse, New York, 1945,1949.

4. Shewell, H. A., "The Use of the CambridgeStereo Comparator for Air Triangulation";The Photogrammetric Record, London, Vol.II, September, 1953.

5. Herget, Paul, "Interim Report on GroundPosition Computation from Shoran-Con­trolled Photography"; WADe TechnicalPaper 179, Wright Air Development Center,January, 1954.

6. Schmid, Helmut H., "An Analytical Treat­ment of the Problem of Triangulation by

Stereo Photogrammetry"; Ballistics Re­search Laboratory Report 961, October, 1955.

7. Anderson, Ralph 0., "PhotogrammetricControl Extension"; Edwards Brothers Inc.,Ann Arbor, Michigan, 1947.

8. Merritt, E. L., "Control Extension withGround Control at the Flight Ends, andControl Extension Based on Shoran Estab­lished Nadir Points"; Photogrammetry Inc.,Silver Spring, Maryland, 1954. Air ForceContract No. 33(616)-2140.

9. McNair, A. ]., Shu, C. S., Moeller, C. M.,and Rutledge, J. R., "Reports on AnalyticalAerial Triangulation" for U. S. Army Engi­neer Research and Development Labora­tories; Cornell University, 1955, 1956.

Strato-Lab Balloon Photography *

EUGENE P. GRIFFIN,

U. S. Army Engineer Research and Development Laboratories,Fort Belvoir, Virginia

ABSTRACT: A n account is presented in narrative form of the first-knownattempt to obtain and utilize mapping-photography taken from an alti­tude considerably in excess of the operational ceiling of present-day re­connaissance aircraft. A free-flight unmanned plastic baUoon was usedto carry a mapping camera aloft in a pressurized gondola to an altitude ofapproximately 89,000 feet. The results of limited photogrammetric in­strument tests which were conducted using this photography are givenalong with general observations on photographic Quality and the am1u-:,tof detail obtainable.

As THE operational ceiling of modern air­craft is steadily increased, it becomes

increasingly important that investigationsbe conducted on the feasibility of utilizingvery high-altitude photography in thepreparation of charts and topographicmaps. In the field of commercial mappingthe problems which may be encounteredin mapping from high altitudes are under­standably of little or no concern, certainlyat the present time. In this field the flightaltitude can readily be adj usted as dictatedby the precision of the methods and equip­ment employed to meet the particular map­accuracy requirements. This is not thecase, however, with the military. The situ­ation may arise in which mapping-photog­raphy, must, of necessity, be taken fromextremely high altitudes. Then, too, withthe advent of more precise plotting equip-

ment it is conceivable that in the not-too­distant future, mapping from extremelyhigh altitudes could become economicallyfeasible even from a commercial stand­point, especially for small-scale mapping.Up to the present time no high-altitudemapping-photography has been availableto test the feasibility of employing presentmapping methods and equipment in thepreparation of maps. As a result, estimatesof the capabilities of present mappingequipment at altitudes of 50,000 feet, andabove, have been obtained for the mostpart through extrapolation from data ob­tained at lower altitudes. Obviously, thisleaves much to be desired.

Data derived from theoretical studieson the effects of atmospheric haze andrefraction upon high-altitude photographsare invaluable, but this does not alleviate

* Presented at Society's 23rd Annual Meeting, Hotel Shoreham, Washington, D. C., March6, 1957.

STRATO-LAB BALLOON PHOTOGRAPHY 583

EUGENE P. GRIFFIN

the need for actual test photography.As conditions at high altitudes have un­

folded unique problems in the realm ofaero-medicine which were not encoun­tered at low altitudes, likewise, there maybe hidden obstacles awaiting the aerialphotographer and the photogrammetrist.It is entirely possible that the conventionalmapping cameras or some of the auxiliaryequipment will, for some unforeseen reason.not function properly at extremely highaltitudes, thereby necessitating the de­velopment of new equipment. On theother hand, it may be found that con­ventional mapping cameras will suffice. Inthis case successful mapping from strato­spheric regions would hinge upon the de­velopment of high-altitude reconnaissanceaircraft and more accurate photogram­metric plotting equipment and methods.

In anticipation of a need for means ofmaximum exploitation of extremely high­altitude photography in the future, ad­vantage was taken of a rare opportunity toobtain a limited amount of experimentalmapping-photography from altitudes con­siderably in excess of the operational ceil­ing of present day reconnaissance air­craft. Because the number of photographsobtained, which were suitable for photo­grammetric evaluation, was extremelylimited, the results of the investigation areby no means conclusive. The data ac­quired, however limited, and the mannerin which the photography was obtained,should be of considerable interest tophotogrammetrists since this is the first

known attempt to obtain and utilize map­ping-photography taken from such highaltitudes.

In February 1955 the Engineer Re­search and Development Laboratories wasgiven the opportunity to obtain very highaltitude mapping-photography by par­ticipating in the avy STRATa-LABprogram. This program, which was de­signed to facilitate research from a manned"space" laboratory attached to a plasticballoon, afforded a unique opportunityto obtain experimental mapping-photog­raphy from an altitude far above anypreviously attained with mapping cameras.

ERDL cooperated with the Office ofNaval Research and its contractor, Win­zen Research, Incorporated, Minneapolis,Minnesota, in the planning and executionof a photographic mission in which astandard 6-inch focal-length T-ll map­ping camera was mounted in a 30-inchdiameter pressurized gondola and carriedaloft by a free-flight plastic balloon to analtitude of approximately 89,000 feet. Thisparticular flight was one of the feasibilitydemonstration flights in the STRATO­LAB system development program. Al­though much of the photography was notusable because of the rotational positionof the camera at time of exposure, foggingof the film, or excess cloud cover, a sufficientamount of clear stereo-coverage over arecently mapped area was obtained topermit photogrammetric instrument test­ing of several stereo-models.

The material comprising the balloonitself was polyethylene plastic film only0.002 inch thick. The material, which isavailable in continuous sheets up to 9feet in width, was cut and pieced togetherso that, when fully inflated with helium,the balloon would take the general shapeof a sphere with a conical lower section.The balloon was 92t feet in diameter witha total volume of 427,500 cubic feet. Adiagram showing the balloon, the pres­surized capsule containing the camera, anda radio beacon used in tracking the ap­paratus is presented in Figure 1.

The camera was rigidly mounted in a30-inch diameter spherical gondola whichwas in turn suspended below the balloonusing an unpacked but collapsed para­chute as a means of suspension. The topof the parachute was connected to theapex of the lower conical surface of theballoon through a mechanism which auto-

584 PHOTOGRAMMETRIC ENGINEERING

92.5' DIA. BALLOON•427,500 FT. VOL.

FIG. 1. The Strato-Lab vehicle which wasused to obtain mapping photography from analtitude of approximately 89,000 ft.

camera window, the camera with itsauxiliary equipment, and, on the outside,a shock absorbing structural framework forprotection of the window and camera uponlanding. Selective painting of the gondolahelped to maintain the ambient tempera­ture within reason, so that neither thefilm nor the camera operation would beadversely affected.

The glass used for the camera windowwas a plane parallel glass blank 1 inchthick and 10 inches in diameter. Themounting flange with sealing gaskets util­ized a t inch portion of the outer circum­ference, leaving a 9-inch diameter opening.The glass blank, which was purchasedfrom Optron Laboratories, Dayton, Ohio,was not originally ground for this specificpurpose, but it possessed all the requiredoptical characteristics. I t exceeded thespecifications for "Group M" camerawindows, the most accurate used by theAir Force, which calls for parallelism of4 seconds of arc and flatness to 2t fringesof mercury light. This particular windowwas parallel to 4 seconds of arc and flatto i fringe of mercury light.

A view of the bottom half of the gon­dola, showing the camera with magazineremoved, is shown in Figure 3. The camerawas rigidly mounted with adjustments forproper positioning relative to the window.This simple type of rigid mount was de­cided upon after serious consideration hadbeen given to various means of camerastabilization. Of primary concern was theuncontrolled rotation about the verticalaxis of the balloon and camera betweenexposures. Such a random rotation couldresult in "crabbing" to such an extent thatthe photography would be practically use­less for photogrammetric work. Becauseof the complexity and added weight of asatisfactory stabilizing system the ideawas given up in favor of short exposureintervals, thereby obtaining a large num­ber of photographs from which stereo­pairs could be selected with the desiredoverlap and base-height ratio. This lattermethod proved to be very successful.

The balloon was launched at 5: 23 A.M.

CST on 1 June 1956 from a point nearMinneapolis, Minnesota. The actuallaunching is shown in Figure 4. Althoughthe envelope was only partially inflatedat the time of launching, it became fullyexpanded at ceiling as a result of the de­crease in atmospheric pressure.

\~"l'

LRADIO BEACON

+--T-II AERIAL CAMERA

...-TlMER AND RELEASE MECHANISM

+-- PARACHUTE

t12'-,-----

I55'

L

matically released the balloon and per­mitted the gondola to parachute back toearth after a preset time lapse.

The gondola assembly is shown in Figure2 being made ready for the fligh t. Thegondola shell, which was fabricated ofspun aluminum in two sections, was com­pletely sealed following the installation ofthe camera. Storage batteries were sup­ported above the top half of the sphere bymeans of a special battery rack. Thesebatteries supplied electrical power foroperation of the intervalometer, cameracycling, and for operation of the vacuumpump. The vacuum pump, which pro­vided vacuum for flattening the filmagainst the camera back, was turned on 6seconds prior to each exposure, and shutoff immediately following the exposure bya circuit of relays. This was done to con­serve power and to effect a savings inweight by reducing the number of storagebatteries to be carried aloft. The top halfof the sphere also contained an electricalpass-through which was the sole connectionto the outside of the sealed gondola. Thebottom half of the sphere contained the

STRATO-LAB BALLOON PHOTOGRAPHY 585

FIG. 2. The sealed spherical capsule, contain­ing a T-ll 6-inch Metrogon camera, is beingmade ready for the flight. A rack containingstorage batteries straddles the top half of thesphere, and a shock-absorbing structural frame­work was built around the bottom half.

Figure 5 shows the balloon immediatelyfollowing its release as it began its ascentat an average rate of 675 feet per minute.It drifted southward approximatelyseventy-five miles to the vicinity of AlbertLea, Minnesota, before reaching an alti­tude of 70,000 feet. At this point cameraoperation was begun automatically by adelayed action starter. The in tervalometerbegan cycling the camera at regular two­minute intervals. The ascent to approxi­mately 89,000 feet was quickly made andthe balloon then leveled off and remainedat or near this ceiling for 7 hours and 25minutes.

After following roughly a west-south­west course for a distance of approximately125 miles the balloon was released, thegondola and camera descended by para­chute, and were recovered. Recovery ofthe camera was made immediately uponimpact about two miles north of Marcus,Iowa, at 3 :40 P.M. on the afternoon of theday of launching. Some damage was suf­fered by the gondola on impact due tolanding in a tree but the camera was un­damaged.

The majority of the exposures were madein the vicinity of 88,000 feet on Kodak

Aerographic, Type la, Class L film, at asetting of f/11 at 1/150 of a second. Be­cause a red filter matching the opticalcharacteristics of the camera was not avail­able, :1 minus blue filter was used. Thefilm was processed in D-76 developer forabout 30 minutes. The first thirty ex­posures of the roll contained scatteredcloud cover, and the end of the roll wasfogged, apparently during unloading of thefilm. The remainder of the film, containingapproximately 150 exposures, was accept­able, but of slightly less density than de­sired.

The indications were that the rate ofdrift was quite constant after the balloonhad leveled off near the 88,000 foot level.The two-minute exposure intervals pro­vided an average forward gain of slightlyless than one-half mile between exposures,indicating a ground speed of approximately15 miles per hour. The photography showeda continuous rotation of the balloon andcamera but did not reveal any appreciabledeviation of the camera axis from the ver­tical. The normal base-height ratio forvertical photography could be achieved inmost cases through the selection of approxi­mately every twentieth exposure. Thenumber of satisfactory stereo-pairs thatcould be selected, however, was greatlyreduced by the random rotation of the bal­loon, and the resulting variations incamera orientation relative to the flightline.

The wind currents were such that theballoon was carried over only one smallarea containing sufficient ground-controlfor photogrammetric instrument evalua­tion of the geometrical characteristics ofthe photography. Due to random cameraorientations, only three stereo models atall suitable for the purpose could be found.The best model of the three was selectedfor a terrain model flatness test using aWild A-7 Autograph. This ,model con­tained approximately 20 per cent cloudcover and had the rather low base-heightratio of .51. The scale of the photographywas approximately 1: 175,000 and, using2.5 times enlargement in the instrument,the stereo model scale was established at1: 70,000.

The model was oriented to horizontaland vertical control selected from theAlbert Lea quadrangle, USGS, 1: 62,500scale. Four photo identifiable horizontalpoints and 66 spot elevations were selected.

586 PHOTOGRAMMETRIC ENGINEERING

FIG. 3. A top view of the camera, with magazine removed. showing thetop of the camera and some of its auxiliary equipment.

After scaling and leveling, the instrumentelevation of all vertical check poin ts wereread and corrected for earth's curvature.After a conversion of the corrected instru­ment elevations to feet and a comparisonwith the map elevations, it was noticedthat a very slight tilt correction to the levelsolution wa";; desirable. This condition wasbelieved to have been brought on by thecombined effects of earth curvature and theassymmetric location of the level points. Amathematical tilt correction and re-indexwas performed. The root mean square errorwas found to be 12.89 feet or about 1/6,800of the exposure altitude.

I t is believed that compilations could besuccessfully accomplished, although nonewere attempted because of the lack ofground relief in the test area. The largeamount of earth curvature encounteredat such high altitudes. could conceivablyhave some effect on compilation accuracysince the corrections are very critical,especially at the model extremities. Be­cause the earth's curvature is not automati­cally compensated in standard plottingequipment, compilations could becomemore tedious and time consuming, andperhaps decrease in accuracy as the ex­posure altitude is increased. Correctionsfor spot elevations read during flatnesstests are not difficult to make, but therapid faIl-off at the model extremities

would require frequent index correctionsduring contouring, making automatic com­pensation almost a necessity.

The tilt of both exposures used in themodel flatness test was found to be lessthan 1 degree, indicating that the gondola,as supported by the baIloon, was quitestable. The plotting instrument was alsoused to determine the exposure altitudeof both photographs. One exposure wasfound to have been made at 87,000 feetabove sea level and the other at 88,400 feet.This confirmed the balloon track datawhich indicated that the balloon hadreached its maximum altitude of about88,800 feet, in the vicinity of these two ex­posures.

Several of the cloud-free models werestudied with a hand stereoscope to deter­mine the general features that may beidentified on photography of this scale.Adequate topographic maps to confirmidentification were not available to permitthe gathering of numerical data on photoidentification, However, the following ob­servations were made:

a. All major roads and railroads appearto have been resolved and were identifiable.

b. Many farm roads were identifiable.c. In built-up areas, normal dwellings

and business establish men ts could not beindividually identified. Identification inthese built-up areas was limited to deter-

STRATO-LAB BALLOON PHOTOGRAPHY 587

. '.. I

'; , '4i!,\ - 11719

FIG. 4. The actual launching.

mining approximate density of the build­ings. Minor streets were not generally dis­cernible.

d. Groups of farm buildings could beidentified. The individual buildings couldnot be seen, but the typical farm dwellingarea of several buildings and trees couldbe identified as a unit.

e. Vegetation, in general, could be iden­tified only by tone and texture. Relief ofvegetation was not visible.

f. The identification of ground detailappeared to be limited solely by the smallscale of the photography. No evidence waspresent to indicate that other high altitudeconditions reduced the ability to recordground detail.

If serious consideration is ever given tomapping from stratospheric regions, eitherbecause of a military necessity or becausethe development of more precise plottinginstruments and techniques make it eco-

FIG. 5. The gondola, parachute and balloonimmediately following its release.

nomically feasible for small-scale mapping,certainly, additional experimental photog­raphy must be obtained. Photographymust be obtained which would permit teststo determine the effectiveness of aerotri­angulation as well as compilations. If thecamera is carried aloft by a balloon, pro­visions must be made for obtaining theproper overlap between successive photo­graphs to insure adequate stereo-coverage.The utilization of a free-flight unmannedballoon as the camera vehicle, with thecamera rigidly attached, leaves many ele­ments of success of the photographic mis­sion to chance.

The photography obtained on this firstattempt, although not up to usual photo­graphic standards, may be considered asa successful completion of the photographicmission for it has permitted the first test­ing of photography at this altitude inphotogrammetric instruments.