PRACTICAL IMPROVEMENTS IN STEREOPLOTTING INSTRUMENTS 809

Transfer Device are used by the GeologicalSurvey.

Anaglyphic spectacles have also been im­proved by the use of variable-density coat­ings. These coatings serve the operator byproviding a ready means of light balance bya slight shift in the direction of viewing fromthe eye to the stereoscopic image.

CONCLUSION

The improvements mentioned in this article

are not individually spectacular, but in theaggregate, they have enabled us to keep pacewith ever-increasing demands for immediateproduction of good maps in large quantities.It takes a great deal of thoughtful effort andpainstaking research to consolidate the gainsmade by the big breakthroughs. Unless some­one is continually working at the job of trans­lating yesterday's new principle -into today'sworkable procedure, there can be little profitin the quest for new principles.

The Photogrammetric Study of Structural Movementsin Architecture * t

PERRY E. BORCHERS,

Professor of A rchileclure

and

Research Supervisor,

Engineering Experimenl Sla.,

The Ohio Slale Univ.,

Columbus, Ohio

ABSTRACT: The measurement of small but potentially destructive movementscaused by thermal change, humidity, initial set, and loading in complex archi­tectural and engineering structures in actual use is an appropriate and rigorousapplication of photogrammetry.

Eijicient measurement requires photography from two terrestrial camerastations with strongly converging lines of sight to the structure. Movement ofstructure is measured between successive photographic plates taken at differenttimes from the same camera station. Despite careful return to established camerastations, coordinate transformations must be made by means of survey controlpoints located primarily in the plane of the camera horizon. Movement in imageplanes as measured in plates from two camera stations is resolved by intersectioninto movement in object space.

It is necessary to determine the standard error of measurement, which issometimes large in relation to the movement observed, from redundant surveycontrol and from the quality of intersections. Elements which limit accuracy arethe effect of changes of daylighting on the recognition of measuring points andthe residual lack of flatness of photographic plates and emulsions, magni­fied in effect by the wide-angle photography desirable for architectural photo­grammetry. This radial error can be isolated to a degree by an appropriatechoice of overlapping photography.

RESUME: A pplication de la photogrammetrie d I' etude des deplacements struc­turaux en architecture.

La photogrammetrie constitue une methode regoureuse et parfaitement ap­propriee pour mesurer les deplacements petits, mais potentiellement destructifs,

• Published by: Engineering Experiment Station, The Ohio State University.t To be presented at International Congress, for Commission V.

810 PHOTOGRAMMETRIC ENGINEERING

occasionnes, dans des constrltctions architecturales et du genie civil en usageactuel, par les variations thermiques, I' humidite, la mise en place initiale dubeton et les operations de chargement.

Ce mesurage ne sera elJicace que si la photographie est prise d partir de deuxstations photographiques terrestres dont les lignes de visee vers la structure sontfortement convergentes. Le deplacement structural se mesure de l'une d I' autredes plaques successivement obtenues Ii des moments differents par une memestation photographique. M algre toutes precautions prises pour retourner auxstations photographiques etabties, il est necessaire de calculer les changementsde coordonnees Ii partir de points--reperes de controte situes principalement dansIe plan de I' horizon optique de I' appareil. Le deplacement dans les plans del'image, tel que mesure sur les plaques obtenues par deux stations photographi­ques, est determine par intersection comme un deplacement dans l'espace-objet.

Etant donne que l'erreur moyenne dans Ie mesurage est pCLYfois elevee parrapport au deplacement observe, il est necessaire de la determiner au moyen derelevements de controle reiteres et par une verification de la qualite des intersec­tions. L'exactitude du mesurage est limitee par des facteurs tels que: l'effet desvariations lumineuses du jour sur let reconnaissetnce des points de mesurage ;la distortion inevitable des plaques et emulsions photographiques, grossie encorepetr la photogmphie Ii gmnde ouverture angulaire qui convient Ii let photogram­metrie architecturale. Cette erreur radiale peut eire en partie eliminee par Iechoix approprie d'une serie de photographies avec recouvrement.

RESUME: Untersuchung baulicher Beu:egungen in der A rchitektur mit Hilfeder Photogrammetrie.

Das Messen geringer, aber potentiell destruktiver Bewegungen in komplexen,in trttsachlichem Gebrauch befindlichen baulichen lind technischen Gebilden,wie sie durch tkermische Vertinderungen, Feuchtigkeit, anftingliches Sicksetzendes Betons lind Belastung verursacht werden, ist ein dankbares A nwendungs­gebiet der Pkotogrammetrie.

Effektives M essen erfordert photographische A ufnahme von zwei terrestrischenKamerastandorten aus mit stark konvergierenden Aufnahme richtungen zumCebtiude. Bauliche Bewegungen werden mittels aufeinetnderfolgender photo­graphischer Platten-zu verschiedenen Zeiten vom selben Kamerastandortaus aufgenommen-gemessen. Trotz sorgfaltiger Wiedereinnahme von denfestgesetzten Ketmerastandorten mussen Koordinaten transformationen ausge­gehend von Vermessungskont'roUpunkten aus gemacht werden. Diese sind inerster Linie in der Ebene des Kamerahorizontes zu wahlen. D~fferenzen in denBildebenen, wie sie mittels A ufnahmen von zwei Kamerastandorten aus gemes­sen werden, werden durch Vorwtirtseinschnitte in Bewegungen im Objektretumumgewandelt.

Es ist erforderlich, den mittleren Fehler zu bestimmen. Dieser ist manchmalrelativ gross im Verhiiltnis zur beobachteten Bewegung. Das Bestimmen desmittleren Fehlers geschieht durch itberschiissige Vermessungskontrollen, undPriifen der Quetlitiit der Schnitte.

Ein Faktor, der die Genauigkeit beeintriichtigt, ist die Wirkung der wechseln­den natiirlichen Beleuchtung (Tageslicht) auf die Erkenntlichkeit der Mess­punkte.

Ein weiterer beeintriichtigender Fetktor ist die nicht hundertprozentigeFlachheit photographischer Platten und Emulsionen, des sen Wirkung durckdie in der architektonischen Photogrammetrie erforderliche Weitwinkel-Photo­graphie noch erheblich vergrossert wird. Richtige Wahl der Oberdeckung derphotographiscken A ufnahmen kann den Einfluss dieses radialen Fehlersisolieren zu einem gewissen Grade reduzieren.

STRUCTURAL MOVEMENT

BUILDINGS are usually in movement withinlimits which, if properly anticipated in

design, are hardly perceptible. Buildings flexand expand and contract almost as living

things. They are given play in expansionjoints made weather-tight by accordian pleatsof metal. Slots, with concealed space above,grip window and door frames without trans­ferring the loads of deflecting lintels to shatter

STRUCTURAL MOVEMENTS IN ARCHITECTURE 811

the glass beneath. Every overlapping form ofroofing, flashing, siding, and moulding allowsand conceals movemen t.

Irregular cracks and deformation are evi­dence of failures of design. It is a principlethat where a crack must occur a neat crackshould be constructed-as where structuralfloor slabs of constan tin terior tem peratureare continued as balconies or sunshades ex­posed to variations of heat and cold in theoutside air, or \I-here basement floor andfoundation walls place greatly differing unitloads upon the bearing earth below.

Experience with buildings under conditionsof actual use provides the surest knowledge ofthe movements which can occur in structure.An engineer discovers that the exposed rein­forced concrete frame of columns and floors ofa modern hospi tal under construction deformsso much with changing direction of sunlightthat-for purposes of erecting the panels ofthe enclosing curtain wall-a vertical line onthe building must be defined as a line which isvertical at noon. One type of steel grain stor­age elevator of large diameter expands in hotsunlight-allowing the grain to settle-and incold weather contracts to failure around theincompressible conten ts. From such some­times painful experience principles of struc­tural design are distilled.

The movements of the more original anddaring forms of modern architecture areappropriate subjects for photogrammetricstudy today. Long spans and greater con ti­nuity in structure make small movementscumulative. Thin shell domes and hyperbolicparaboloids developing great strength fromthree-dimensional curvature of shallow sec­tions of material are dependent on edge stiff­ness and stability. The component materialsof reinforced concrete have differing charac­teristics of expansion and con traction andstrength. The more unusual structural formsin modern architecture are designed in re­duced scale model \\-ithin laboratories. Thecompleted structures are rarely observed andmeasured because of com plexi ty of form andrequirements of use. Under these conditionsthere are obvious advantages in the use ofphotogrammetry to accumulate measureddata upon which to base improved structuraltheory.

PHOTOGRAMMETRIC MEASUREMENTOF MOVEMENT

The photogrammetric measurement ofmovement in engineering and architecturalstructure requires, from case to case, varyingarrangements for photographic coverage andsurvey con trol and promises resul ts of varyi ng

accuracy. The structural forms and theirsurroundings permi tting photograph y vary,while the sources of movement within struc­tures are as various as short term loading,wind, adjustment to the removal of support­ing form work and the curing and setting ofconcrete, rhythmic daily or seasonal cycles ofexpansion and con traction due to change ofhumidity and thermal conditions, and long­term flow, creep, fatigue, or foundation settle­men t. I n most cases the smallness of the meas­urements sought demands the utmost accu­racy of the photogrammetric method.

In its simplest form the measurement ofstructural movement requires the use of aphototheodoli te or other precise camera offixed interior orien ta tion and flat photo­graphic plates to take two or a series of suc­cessive photographs in a fixed direction from afixed camera station showing a structuralelement-such as a beam deflecting undervarious conditions of short term loading­moving in a plane or planes parallel to theimage plane of the camera. Con trol measure­men ts in the plane of the structural elemen testablish the scale of the photography. Rota­tion of pairs of the developed plates through90° in a stereocomparator transforms verticaldeflection into horizontal parallax with rapididentification of the presence of movement­even in parts of the structure where it had notbeen anticipated-by apparent advance orrecession from the plane of the stable ele­ments within the photographs. \\lith theeffects of residual lens distortion eliminatedby the coincidence of camera axes, it mayreasonably be expected that movement of anobject can be measured wi th greater accuracythan the object itself. See "Some Experiencesfrom Deformation Measurements by Photo­gram metric Methods" by Professor BertilHallert, PHOTOGRAMMETRIC ENGINEERING,December 1954_

\,"hen movement is three-dimensional, thesingle camera station is inadequate, exceptwith some ingenious laboratory arrangementof ligh t and shadow lines providing a set ofintersecting rays to reveal variations of depthwithin a single photograph, as described in"An Application of Photogrammetry inStructural Research" by Professor K. B.Jackson of Canada in PHOTOGRAMMETRICENGli'iEERING, IVI arch 1955.

Efficient measurement of three-dimensionalmovement in architectural structure requiresphotography from two camera stations withstrongly converging lines of sight to the struc­tures_ Movement in one image plane is meas­ured from successive photographic platestaken at different times from the same camera

812 PHOTOGRAMMETRIC ENGINEERING

station. Movement in two image planes asmeasured in successive plates from two cam­era stations is then resolved by intersectioninto movemen t of the structure in objectspace.

PROJECTS AT THE OHIO STATE UNIVERSITY

The experience of three projects at TheOhio State University in the photogrammet­ric measuremen t of structural move men t hasbeen combined in this report. The projectswere the measuremen t of ini tial settlemen t ofone reinforced concrete hyperbolic paraboloidin the canopy at Scioto Downs race tracksouth of Columbus, Ohio, during the monthfollowing release of the form work in thespring of 1961; the measurement of sway anddeflection under spectator loading of the newsouth bleachers, The Ohio State UniversityStadium, in October, 1962, as confirmation ofthe manufacturer's and contractor's guaran­tee as to stability; and a recording of the pieralongside new Elevator No.4, Harbor ofMontreal, Canada, May, 1963, anticipatingsettlement under initial concentrated loads ofthe travelling towers.

CAMERA AND PHOTOGRAPHY

The camera used in The Ohio State Univer­sity projects has been a Galileo-SantoniPhototheodoJite of 13 X 18 em. format, f/6.3Aerogon Jens with diaphragm closing to f/25,

and 151.66 mm. basic focal length, adjustablein increments of 0.4 mm. for focus from in·finity to as close as 5.5 meters. The camerawas intended originally to form one half of astereocamera-Galileo Apparecchio per Rili­evo Monumenti-but it is used singly upon aGalileo Tacheometer TG2 with target andtwo tripods. The wide angle of the camera isnecessary in the usually cramped quarters ofarchitectural photogrammetrj. The variablefocal length allows close-up photography butdoes not eliminate the problem of depth offield of focus in photography of structures.Normal photographic procedure is to focussharply one-third of the distance into thedepth of field desired so that the circles ofdiffusion are equal at near and far distance inthe photograph. A better photogrammetricprocedure is to focus sharply approximatelytwo-thirds of the distance into the field of thephotograph so that resolution and pointingaccuracy are improved in the far distancewhere the small scale of the photograph exag­gerates error, and lack of resolution and inac­curacy of pointing are greatest where the largescale of the photograph minimizes the effectof error on measurements in object space.

CONTROL

Before commencing photography it isnecessary to establish permanent, preciselymarked camera stations. For recording of.. ,.

FIG. 1. Scioto Downs racetrack. Camera station A, May 6, 1961, second of series. Formwork of finalcanopy released but not removed. Measuring marks are lighting standat;ds appearing above formworkand roof line.

STRUCTURAL MOVEMENTS IN ARCHITECTURE

':--./ ••••••••L.P'"'--.-~.1'1', - F

813

FIG. 2. Scioto Downs racetrack. Camera station B, May 30, 1961, fourth of a series. Initial settlementcomplete and joint between hyperbolic paraboloids ftlled. Major control points identifted-D, V, X, Y,and Z-together with ftducial marks-F, 1, G, and H-and pressure plates.

long-term building movements the markersshould be set in concrete extending belowfrost line. Another· permanen t marker isneeded for adjusting a constant camera heightand turning constan t angles for photography.Lack of desired precision in returning to thesame camera height and the same cameraaxes-particularly when photography mustbe hurried to record a fleeting moment ofligh ting or loading-requires con trol poin tswithin the photograph.

The primary control points are in the planeof the camera horizon and must be as stable asthe camera stations and not subject to thecauses of movement in the structure beinginvestigated. At Scioto Downs racetrack (seeFigure 2), and the a.s.u. Stadium (see Figure4) the control was marked on metal fence postsindependently supported on concrete basescarried below frost line. At Elevator No.4 inM on treal the con trol consisted of markspainted on the elevator itself, which was sup­ported on Benoto pili ng down to bedrock incontrast to less adequate support for the pierbeing investigated under load. This control,however, would be subject to horizontal dis­placement with thermal expansion and con­traction of the elevator. Vertical expansionand contraction within the height of thecamera horizon is generally negligible.

The most important control point is onenear the principal point of the photograph.

Displacement of this control point in succes­sive photographs from a center determined bythe horizontal and vertical fiducial marks onthe photographic plate is the most accuratemeasure of relative orientation angle 1> or tiltw between two plates, requiring coordinatetransformations, first, for correction of dis­placemen t of the principal poin t and, second,for correction of the changes of scale acrossthe photograph caused by the turns of orienta­tion angle and tilts.

Two more poin ts on the camera horizon farto the righ t and left sides of the photographserve for effective correction of swing K. Afourth control point, also on the camera hori­zon and near the camera axis but in depthforward of the first control point, establishesany relative displacement in the height of thecamera.

MEASURING POINTS

The best measuring points upon the movingstructure are painted crosses, but generallymany other edges and marks already upon thestructure can be used. If the conditions ofsunlight are nearly similar between successivephotographs, a plain concrete surface bearingonly the marks of wood form work will containthousands of points which can serve for meas­urement of movement. Let the direction ofsunlight change and not one point of thosethousands may be identifiable. Thin pipe

814 l'HOTOGRAMMETRIC ENGINEERING

columns-as seen in Figure 3 of the O.S.U.Stadium-may seem to provide precise linesfor measurement, but such structure must bephotographed at nearly the same time onsuccessive days so that a change of location ofhighlight in the cylindrical metal surfacesdoes not suggest or conceal an equal actualmovement in the structure. The deformationsof reinforced concrete shells as bright sunlightand cold shadow follow each other around theswelling forms present particular difficulty inidentification of measuring points, and thebest surfaces for photography may be theshaded undersides of dome, \"ault, or hyper­bolic paraboloid.

The major measuring points at SciotoDowns racetrack were the intersections of thelighting standards with the top edge of theroof appearing above the formwork still sup­porting the hyperbolic paraboloid in Figure 1.I t was necessary to measure both sides of thelighting standards to compensate for thevarying effect of sky halation, which wOLtldotherwise have suggested horizontal move­ment in the structure.

Measuring points on stable structure serveto extend a slightly less reliable control be­yond the central horizon of the photograph.The concrete stadium beyond the steelbleachers in Figure 3 provides additionalcontrol, though with danger of refraction inthe air curren ts above thousands of specta-

tors. Distance red uces the scale of move men tin the photograph, and in a distant structurethere are control points of relative stability.

MEASUREMENT

Clear identification of measuring points isone factor determining the accuracy of meas­urement through its effect on the pointingaccuracy of the operator of the stereocom­para tor or other measuring instrument. Theprecision of the measuring instrument isanother. The Nistri TA3 comparator wasused for measuring the movement of theScioto Downs canopy, but this machine be­came too irregular in its electrical read-out forcon tin ued use. The O.S. U. bleachers weremeasured in the sturdy Wild A7 Autograph inwhich, however, the 40 micron measuring dotappears like a golf ball compared to the dot ofligh t in the TA3. The Nistri TA3 gave prom­ise of a standard error of 1/15,000 of the dis­tance of photography or approximately1/17,000 of the width of the building photo­graphed, while normal distribution of error atcontrol points on the O.S.U. bleachers indi­cated a standard error over the whole struc­ture of 1/9,000 of the distance of photographyfor this project as measured in the A7 Auto­graph. The plates for Elevator 1\0. 4 in Mont­real Harbor will be taken to Europe formeasurement on new stereocomparatorsthere.

- ......tJII'"

FIG. 3. South bleachers, O.S.U. Stadium. Camera station A, Southeast. Second of a series, bleachersoccupied during football game of October 27, 1962, showing unobtrusive character of photogral1lmet­ric recording.

STRUCTURAL MOVEMENTS IN ,\RCHITECTl·RE

~--_...----815

FIG. 4. South bleachers, O.S.U Stadium. Camera station B, Southwest. Third of a series, after release ofload, October 28, 1962. Parts of the structure are completely changed in scale in the two image planes.

A stereocomparator for measuring struc­tural movements should allow stereoscopicscanning of plates through the same speed oftravel over both, with electronic recording ofthe coordinates of points in one plate togetherwith parallax of the same points in the secondplate measured stereoscopically.

This measured data is then corrected forminor changes in the elements of outer orien­tation of the camera as determined at themajor con trol poin ts. Coordinate correctionfor displacement of the camera-usually inheight-requires knowledge of the depth ofimage points within object space and is acalculation less adapted to electronic compu­tation than the simple scale changes withinthe image plane for correction of orien tationangle and tilt. Care in the field in returningprecisely to the same camera height is wellrepaid by avoidance of this tedious computa­tion.

RADIAL ERROR

As pointing accuracy improves there can bevirtually complete elimination of error due tothe elements of exterior orientation, but thereis still error to be distributed a t the con trolpoints in determinations of accuracy. Thiserror must be due to elements of interiororientation of the camera-particularly tolack of flatness of the glass plate, variablethickness of the emulsion, or variable penetra­tion of the photographic image within the

emulsion. These may be considered togetheras radial error, causing displacemen ts ofmeasuring points towards or away from theprincipal point of the photograph increasingin magnitude with increase of the angle fromthe camera axis.

The possible magnitude of this error maybe calculated from the specification for Kodakultra flat glass photographic pia tes of a fla tnesstolerance of 0.0005 inch per lineal inch. Thisequals 12.7 microns deviation from flatnessper inch, times 4 inches from the principalpoint of a 5 X 7 inch glass plate to a diagonalcorner, times 2 for the worst case in whichplates are deviating from flatness in oppositedirections, equals 102 microns deviationconverted by angled light rays in the Galileo­San toni camera to 68 microns of radial dis­placement of a measuring point in the outercorners of the two photographic plates. This isalready 1/2,400 of the width of the photo­graph.

In other fields of photogrammetry radialerror may be minimized by long focal lengthcameras and narrow angles of photography,but wide angle photography is essential inarchi tectural photogram metry, and othersolutions to the problem of radial error mustbe sought.

I t was noticeable in distribution of error atthe survey control points located in the hori­zon of the photographs that the residualerrors in the vertical direction were less than

816 PHOTOGRAMMETRIC ENGINEERING

FIG. 5. Elevator #4, Harbor of Montreal, Canada. Camera station E, four of a series, May 8, 1963.Showing, together with Figure 6, a type of overlapping photography from a single camera station, as ameans of isolating radial error.

half the residual errors in the horizontal direc­tion. It is useful to state this differently-thattangential measurements had more thandouble the accuracy of radial measurements.This difference in accuracy may be expectedto increase as poi n ti ng accuracy increases.

This suggests the best patterns of photog­raphy. Vve can localize horizontal movementscaused bv wind loading of a tall building intotangen ti~l mO\'emen t across the vertical axisof the photographic plates and localize verti­cal deflection in a bridge in to tangen tialmovement across the horizontal axis of thephotographs. Finally-dependent upon ahigh degree of pointing accuracy-we can usetangential measurements in one pair of photo­graphic plates to correct radial error in anoverlapping pair of photographic plates.

Photographs overlapping by somewhatmore than 50 per cen t are taken from thesame camera station by turning the cameraaxis through an angle not quite equal to halfthe width of the photograph. [n each pair ofplates horizontal movement is measured withgreatest accuracy as tangential movementacross the vertical axis of the plates. Thesame horizontal movement measured alongthe vertical outer edge of the second pair ofplates is compared-with necessary change ofscale because of turn of camera orien tationangle--to the first results. Discrepancies areattributed to radial error and corrections are

applied to both the horizontal and the verticalmeasurement of movement along the outeredge of the plates. The corrected measure­ments of vertical movement are then com­pared with the vertical movement measured-radially-along the vertical axis of the firstpair of photographic plates. Discrepancies areattributed to radial error along this axis andare corrected. With radial error determinedalso at the fiducial marks of the plates and atthe control points along the horizontal axis ofthe photographs a systematic correction canbe attempted for relative radial error of suc­cessive plates being measured together (SeeFigures 5 and 6).

STA DARD ERROR

The corrected differences in measurementof the same points on successive plates repre­sen t movemen ts of the structure as recordedin one image plane. Though all measurementson the plates, including the residual error atthe control points, have been read in horizon­tal and vertical components, the proper ex­pression of the standard error in the imageplane may consist of a tangential componentand a radial component increasing with dis­tance from the principal point of the photo­graphs.

Movement as recorded in two image planesis resolved by intersection as movement inobject space. To avoid exaggerating error in

STRUCTURAL MOVEMENTS IN ARCHITECTURE 817

FIG. 6. Elevator #4, Harbor of Montreal, Canada. Camera station B, fourth of a series, change ofcamera axis, May 8, 1963. Expansion joints on the building serve as further control of radial error. [nobject space they expand or contract but are not displaced horizontally.

anyone coordinate axis of object space it isnecessary to have strongly converging lines ofsight from the two camera stations to all partsof the structure. This requirement and theneeds of full photographic coverage make itunusual that camera axes are parallel to eachother and perpendicular to the base betweencamera stations. Accordingly, structures usu­ally appear in very differen t scale in thephotographs taken from separate camerastations, and this difference of scale in inter­section complicates the necessary expressionof standard error as a measurement in thethree coordinates of object space. A completeexpression of standard error in object space isnecessary to establish the full accuracy and

authority of measurements of movements inarchitectural and engineering structure.

The projects at The Ohio State Universityhave shown that there are some buildingmovements-not completely anticipated indesign-which exceed by several times thepresen t standard error of photogrammetricmeasuremen t even at 300 feet distance ofphotography. By means of photogrammetrythere can be reasonable and unobtrusiveconfirmation of performance guarantees ofsway and deflection in structural elements inuse. There are yet other movements to besought in architectural and engineering struc­ture which demand the utmost accuracy ofthe photogrammetric method.

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