ARNOLD H. LANCKTON*

Rome Air Development CenterGriffiss A ir Force Base, N. Y.

Analytical StereoplotterDevelopmentThe device has evolved from an idea of Mr. Helava in 1957to an automated version and an electronic orthophoto

and hypsoline printer.

A~ALYTlCAL STEREOPLOTTER REVIEW

BASIC PRI/,\CIPLES

T HE ANALYTICAL Stereoplotter conceptwas introduced by Mr. U. V. Helava on

August 30, 1957, at National ResearchCouncil (NRC), Ottawa, Canada, during anInternational Photogram metric Conference.Mr. Helava's paper entitled, "New Principlefor Photogrammetric Plotters" and a com­panion paper by Mr. \Y. ]. M. Moore, en­titled, "Considerations in the Design of anElectronic Computer for a PhotogrammetricPlotting Instrument," describes in detail thebasic functional organization of an AnalyticalStereoplotter. Mr. Helava's description of theAnalytical Plotter concept is: "The photo­graphs are not projected using conventionaloptical or mechanical projection. Rather, aposition is computed in which an image pointappears as if an ideal central projection hadtaken place. In addition, deviations from theideal case, such as those caused by lens distor­tions, are also derived."

This basic functional operation of theAnalytical Stereoplotter as described by !VI r.Hela\'a is still being employed in the currentstereoplotter systems. It is interesting to notethat the Analytical Stereoplotter operation issomewhat of a reverse operation as comparedto cOIl\'en tional analytical photogram metricprocedures. That is, the analytical stereo­plotter operator selects a desired model posi­tion and then the electronic system computesthe resultant photographic position. \Vhereas,in analytical photogrammetric procedures,the operator selects the photographic positionand then the electronic system computes theresultant model position.

* Presented at the Semi-Annual Convention ofthe American Society of Photogrammetry. Los:\ngles, Calif., September 1966. Prof. Lanckton ispresently teaching photogrammetry at the Collegeof Forestry, Syracuse, N. Y.

vVith this analytical stereoplotter oper­ational concept, the operator selects the de­sired model position via the hand and foot­wheel motions. The electronic system acceptsthis model data and adjusts the imagery withrespect to the index marks for the operator tosee the actual position of the floating mark.These image adjustments are based on knownperspective, lens distortions, and other sys-

Actual Positionof the

Floating Mark

Operator's

Decision to HoveFloating Hark

Novement ofHand and Foot

I,.,,'heels

Computationof the

Image POsition

FIG. 1. Information flow diagram for theAnalytical Stereoplotter

1160

A TALYTICAL STEREOPLOTTER DEVELOPME TT 1161

tematic conditions. Thus, the operator is ableto compare the actual position of the floatingmark as referenced to his perception of thestereoscopic model. This comparison can thenbe used for determining the direction in whichthe floating mark is to be moved for a newmeasurement. (Figure 1)

The reason for using this particular feed­back loop, i.e., measuring, computing, imageadjustment and operator \'iewing sequence, isto provide for a means of establishing anoptimum man-machine interface. \\'ith thisparticular instrumental configuration, theman maintains control over the machine byyisually sensing the differences between the

The major problem in actually implement­ing this concept is to compute all of the neces­sary mathematics involved in solving thephotogrammetric problem (especially the pro­jective equations), Mr. Helava recognizedthis problem in his original paper, and he dis­cussed the advantages and disad\'antages ofdigital and electrical analog computationalsystems, including the use of such analogies asmechanical, hydraulic and electronic. For anexperimental system i\[ r. Hela\'a decided onan electrical analog system as the most prac­tical approach, Mr. Helava then developedthe neces ary photogrammetric mathematicsin a form such that they were amenable fOl'

ABSTRACT; A bout a decade ago, Mr. U. J1elava began organizing in h1:s mindthe Analytical Stereoplol/er concept. In 1957 11[r. Helava introduced this COll­cept at the International Photogrammetric Conference on A nalytical ,I ero­triangulation at the National Research Council at Ottawa, Canada, in a paperentitled, "New Principle for Photogrammetric Plol/ers." Since that first intro­duction by J[r. I£elava many tecll11010gical advances have been taken in photo­grammetry and other all'ied sciences, especially those required to impleme11t allAnalytical Plol/er. 11 is the purpose of this paper to elaborate on the uuiquecharacteristics of the Analytical Stereo plotter and to review its development fromits original concept to the preseut AS-llE System, From this review it is hopedthat an understanding of the basic principles employed in the ,lnalyticalStereoplol/er may be appreciated, (Iud the influence of these principles on thecurrent trends in photogrammetric instrumentation may be realized. With thisknowledge one can theu begin to postulate tTlefnture of the science of photogram­metry and its potential infllle11ce 011 other tecll11ologies. Included in this paper isa complete b'ibliography of published and unpublished papers, whose teclmicalcontent is related to the /I nalytical Siereoplotter.

desired and the actual model position of thefloating mark. However, to implement thisinterface successfully, the differences betweenthe desired and actual posi tion of the f10ati ngmark must not be yisually apparent to theoperator. That is, the electronic system mustbe designed to sense the model position, COIll­

pute and position the t\\·o images at a ratehigher than the visual response rate of theoperator.

A generally accepted \'isual response rateof the human eye is about 30 cycles persecond. This, if the electronic system com­pletes a cycle (sense, com pu te and posi tion)at a rate greater than 30 times per second, theoperator will not be aware of his con trollingfunction within the feedback loop. That is,the operator is not conscious of his sensingand correction of the diA'erences between thedesired and actual model position of the float­ing mark. In the present analytical stereo­plotter the electronic system completes acycle e\'ery 1/100 of a second,

the analog electronics, The magnitude of thisproblem can be appreciated by noting thatconsiderable portions of Mr, Helava's paperis devoted to describing these mathematics.

AXALYTlCAL STEREOPLOTTER DESIG:-I

Somewhat simultaneously two independentgroups were stimulated by ]\[r. Hela\'a'sunique analytical stereoplotter con-:-ept,Umberto Nistri and his technical staR' atOttico Meccanica Italiana (aMI) of Rome,Italy, recognized the potential of i\1r.Hela\'a's concept and initiated a program togain licensing rights from r-:nc to de\'elop ananalytical stereoplotter. Also, at about thesame time, the U. S. Air Force initiated a de­\'elopment program to imprO\'e their chartingcapability at the Aeronautical Chart and In­formation Center. Included in this programwas the development of a stereoscopic plotterbased upon the Hela\'a concept. In June of1960, the Rome Air Deyelopment Center(RADC) contracted with the Bendix Research

1162 PHOTOGRAMMETRIC ENGINEERING

~-_....... .. ,

FIG. 2. The first AnalyticalStereoplotter AP-l.

Laboratories of Southfield, Michigan, andOMI for the development of an analyticalstereoplotter.

The original analytical stereoplotter de­sign (Figure 1), as proposed by the BENDIX­

OMI team, consisted of the stereoviewer-co­ordinatorgraph interfaced \\·ith an incremen­tal digital processing unit. This particular in­cremental system employed a magnetostric­tive device as a memory, organized to use adigi tal differen tial analyzer compu tation tech­nique. This system was able to handle thephotogrammetric problem efficiently for t\\·opri mary reasons:

(I) The delay line memory (Figure 2) offersthe unique capability of being a natural recircu­latory memory for about 3,000 bits everyonehundredth of a second.

(2) The digital differential analyzer comput­ing technique (commonly employed in servocontrolled devices) is a simple technique to pro­gram and is very accurate in solving mathe­matically continuous functions.

The stereoscopic viewer proposed by theBE~[)[X-Oyll team was a unique departurefrom com'en tional stereoplotter image trans­lation systems. The viewer was designed to beconstructed with a very light weight butstable metal. The image translation wasaccomplished by moving the photo carriageon one axis and a direct optical viewing sys­tem and illumination source on the orthogonalaxis. This particular translation system isideally suited for ser\'o control, for automa­tion, and for adaptation of other impmvedmeasuring systems.

During the next few months after theaward of a contract there were several criticaldecisions made in the develop men t of the an­alytical stereoplotter computer. Perhaps themost important of these was the incorpora­tion of a programmable system in lieu of a

fixed programmed system. This decision alsoinitiated serious consideration for integratinga modified general purpose digital computa­tion capability with the digital differentialcomputation technique.

AI' ALYTICAL STEREOPLOTTER DEVELOPyIE~TS

The design considerations resulting fromthe short study period evoh'ed into the firstprototype analytical stereoplotter, the API 1.The AP/l is a rather limited system as com­pared to the presen t analytical stereoplottersystems, and can be described as a manualanalytical stereoplotter for near verticalframe photography. \\"ith the APll, interior,relative and absolute orientation require oR'­line calculations and the system could onlyaccommodate frame photography with arange of focal lengths from 1 inch to 50 inchesand with a spatial orientation limitation of± 9° for Kappa, Phi and Omega. The API1exhibited a photogrammetric capability farmore versatile than any of the conven tionaloptical-mechanical projection stereoplottersbecause, for the first time in any stereo­plotter, the API system could be pro­grammed to correct for such variable distor­tions as earth curvature, atmospheric refrac­tion, film shrinkage and lens distortions.

Soon after fabrication and checkout inAugust 1961, the AP/l system underwent ex­tensi\'e testing and e\'aluation by both AirForce and contractor personnel. This test andevaluation resulted in establishment of re­quirements for an operational analyticalstereoplotter. In September of 1962 the firstoperational analytical stereoplotter was in­stalled at the Aeronau tical Chart and f nfor­mation Center, in St. Louis, i\Iissouri. Thisfirst operational analytical stereoplotter \\'asgiven the designation of AP-2. However,when its measuring system was improvedfrom 10 to 5 microns it was given a new desig­nation of AS-llA. (The nomenclature was in­tended to have been the AS- I1/A meani ngAnalytical Stereoplotter N umber Two/A.However, as a resul t of a typographical error,the Roman Numeral number two becameeleven). The AP/2 or the AS-IIA was reportedon in great detail during the Second Inter­national Photogrammetric Conference at the:-Jational Research Council in o tta \\. a, Can­ada, by the following series of papers:

(I) "Analysis of Mechanical-Optical Design,"Parenti, Dr. G.

(2) "Analytical Plotter," Helava, C.(3) "General Concept of the Analytical Stereo­

plotter," Lesser, B.(-1-) "Operational Use of the AP/2 at ACIC,"

Mahoney, L>r. W.

ANALYTICAL STEREOPLOTTER DEVELOPMENT 1163

FIG. 3. The production model of the r\nalytical Stereoplotter AS-llA.

Since September, 1962, the A'-ll/A hasbeen performing extremely well and the twoprototype AP/2's have been modified with a5-micron measuring system so that they cannow be considered to be AS-llA's. (Figure 3)

A TO};lATIO\, OF THE Al\ALYTICAL STEREO­

PLOTTER

In March of 1963, the Air Force set a ide anAS-ll/A for automation. The ba ic instru­mental philosophy employed in the automa­tion of the analytical stereoplotter was todesign an automated system such that itwould aid and assist the basic manual oper­ation, and would not destroy or disturb thevery successful manual system.

In June 1963, a contract for a design studyto au tomate the analytical stereoplotter wasawarded to the same BENDIX-aMI team thatdeveloped the analytical stereoplotter. Thenin December 1963, the BENDIX-aMI tean;began work on fabricating the automatedanalytical stereoplotter.

The automation of the AS-llA (Figure 3)required development of a photographicimage scanning system to convert the imagedensity information into electrical informa­tion. The resultant electrical informationfrom the two stereophotographs is processedin an electronic system called a correia tor.The correlator processes the electrical stereoinformation in such a manner that it canmeasure the amount of cross correlation(total misalignment of two homologous im­ages) and the X-parallax, Y-parallax, X­slope, and Y-slope existing within that image

area being scanned. These five types of imagedata are accepted by the analytical stereo­plotter's digital computer via the DigitalDifferential AnalyzeI'.

The five separate types of stereo data areprocessed, using anyone of several pro­grammed plotting strategies. The selectionof a plotting strategy to be used depends onthe characteristics of both the terrain and thephotography. Involved in the strategy selec­tion are considerations of tradeoffs betweenspeed and accuracy. This programmable sys­tem in conjunction with the basic analyticalstereoplotter feed back system (i.e., man­machine interface) is the key in the automa­tion of a stereoscopic plotting system.

The basic Analytical Stereoplotter conceptof incorporating a means of sensing the differ­ences between the desired and actual modelposition provides the digital system with in­formation, measured by the image correlator,for numerically evaluating a plotting strat­egy. When the system is in the automatedmode of operation, the programmed plottingstrategy is to adapt itself according to thequality of the image being examined andaccording to the amount of relief contained inthe image area. This adaptability of the auto­mated system is the major reason for its suc­cess. The entire concept can be best under­stood by sequencing through the image cor­relator-digital operation and analyticalstereoplotter feedback loop and examiningthe numerical evaluation and the logic em­ployed in the digital system. (Figure 4)

For the purpose of describing this continu-

1164 PJ-IOTOGRAMMETRIC E rGINEERING

FIG. 4. Information Aow diagram for the Auto­mated Analytical Stereoplotter

ous cycle, it is best to begin with the digitalsystem incrementing the model coordinatesunder program control (dX, dYand dZ). Theanalytical stereoplotter goes through its nor-

Operator'sDecision

Movementof

Hand and FootWheels

Actual Positionof the

Floating }lark

Computationof the

Image Position

ImageScann(!r

andCorreIa tor

Computationof

fiX, 6Y and.Z

l mal computing sequence and positions thet\\·o stereophotographs. The correlation sys­tem in the meantime has scanned the imagearea about the floating mark and measuredthe cross correlation and the amount ofX-parallax, Y-parallax, X-slope and Y-slopecontained in the image area. These five valuesare incremented into the computer where thefollowing calculations are performed for con­touring (dZ is zero):

- If sin B - W cos B

dX,.

dt

V' cos B - W· sin B

dt

tan- I [(Sy)/5x)] = Terrain Slope Angle or the angle B

Sy = Y-slope

5, = X-sfopeF,(C)l'3(P,') = Plotting Velocity or JI

K 12Px (1/5",) = Correction [or X Parallax or W

. I----;:;;=;:==;=:;:=:; = Slope magnitude or 5mv'Su' + 5,'

P, = X-ParallaxC = Average Correlation Quality

F2 , F3- and 1\..12 are pre-determined con­tants of the plotting trategy.

From these equations it is evident that theactual motion of the floating mark (bothdirection and velocity) dXm and dY,. arefunctions of

(1) Degree of I mage Correlation,(2) Magnitude of the Terrain Slope,(3) Amount of X-parallax,

FIG. 5. The first Automated Analytical Stereoplotter AS-lIB.

ANALYTICAL STEREO PLOTTER DEVELOPMENT 1165

FIG. 6. The Gigas-Zeiss orthophoto printer and reading unit.

In addition to being constrained by the vari­ous predetermined constants. Therefore,every 10 milliseconds the au tomated Analyt­ical Stereoplotter can adapt itself to the typeof image characteristics being stereoscopicallyreduced. It should be noted that this machinedecision-making procedure can always beal tered or changed by reprogrammi ng thecomputer system.

The automation of the Analytical Stereo­plotter (Figure 5) has been given the detaileddescription of the en tire system and its orga­nization was reported by Mr. F. A. Scarano ina paper entitled, "Automated AnalyticalStereoplotter (AS-ll/B)" and presented atthe Semi-Annual Meeting of the AmericanSociety of Photogrammetry held in Septem­ber 1965 at Dayton, Ohio.

CURRE:'\T A:,\ALYTICAL STEREOPLOTTER SYS­

TEMS

The \'ersatility of the Analytical Stereo­plotte'" is well illustrated by its integrationwith the following equipments:

(1) The Gigas-Ziess Orthoprojector with the

reading and storage units (Figure 6). The actualinterface is with the storage unit (SG-l) in asimilar manner that the unit is interfaced withthe C-8 Stereoplanigraph. This type of systemorganization permits the off-line production ofhigh quality orthophotos and drop line contourson the Orthoprojector and Reading Unit (LG-l).

(2) A :Vlagnetic Tape Recording Unit forrecording model coordinates at a fixed intervalof time (Figure 7). Therefore, model coordinatescan be recorded at a minimum of 2.5 micronswhen the floating mark is moved at a rate lessthan 0.25 mm per second, to a maximum of 100microns when the floating mark is moved at itsmaximum rate. These recording rates are forrote-recording of terrain data. The AnalyticalStereoplotter can be programmed to record databased upon any desirable criteria such as therate of change of anyone of the three coordinatevalues. (Figure 8)

(3) An electronic orthophoto and hypsoclilleprinter to operate simultaneously with the AS­11/8 in the Automatic Profile Mode (Figure 6).This system can produce an orthophoto usinga profile width from 0.5 mm to 5 mm. in incre­ments of 5 microns. The hypsocline output issimilar to a standard contour chart except thatthe width of the actual contour line varies as afunction of the slope of the terrain through whichit is passi ng. This particular graphic presentationis unique in the representation of terrain data ingraphic form and appears to have several po-

1166 PI-IOTOGRAM"1ETRIC ENGINEERING

FIG. 7. An AS-IIA with a Gigas-Zeiss storage unit.

tential applications. This system and its or­ganization will be reported on by Mr. J. Edmondin a paper entitled, "The Analytical Stereo­plotter and Orthophoto Pri nter."

FUTURE ANALYTICAL STEREOPLOTTER

SYSTEMS

The next Analytical Stereoplotter willmost probably employ the same basic feed­back concept as formulated by Mr. Helava.The reason being that the feedback concept isideally suited for both manual and automaticoperation. However, the equipment for im­plementing this concept will undoubtedly bedifferent. The present Analytical Stereo­plotter magnetostricti'·e delay lines ,,-illprobably be replaced by ei ther a digi tal shiftregister or possibly a thi n-film or a coatedwire memory. The pl-esent digital differentialanalyzer computing technique will probably

be replaced by conventional arithmeticdigital processors operating in parallel with afew channels operating in a fixed mode. Thesehal-dware changes are premissed on the fol­lowing technological advances that will per­mit the practical manufacture of special pur­pose systems:

(I) Advanced Integrated Circuit Technologywill reduce the cost, size and power require­ments for shift registers and arithmetic processorand will increase their operating speed.

(2) Advanced Thin Film Memory Conceptwi II become a practica I rea Iity.

An ob"ious limiting component in the pres­en t Analytical Stereoplotter is mechanicalmotions of both the Viewing nit and the Co­ordinatograph. This is especially true in oper­ating in an Automatic Mode. In fact thislimitation has already been encountered with

FIG. 8. The AS-lIB Ranked by a magnetic tape unit and an orthophoto and hypsocline printer.

ANALYTICAL STEREOPLOTTER DEVELOPMENT 1167

the AS-llB when plotting an ideal image andterrain areas.

Mr. Helaya recognized this limitation inhis paper "A Fast Automatic Plotter"(PHOTOGRAMMETRIC ENGDIEERING, Jan uary,1966) and suggests several types of imagetransd uci ng systems wi th inertial free (ornearly so) image translation systems. Ho\\'­ever, for these systems to be practically im­plemented the state-of-the-art must be ad­yanced in many areas.

An alternative to the pure AnalyticalStereoplotter, using the precise and accuratedigital computation system, is a HybridStereoplotter. The Hybrid Stereoplotter Sys­tem employs a combination of analog anddigital computation system to solve thephotogrammetric problem. I n particular, theHybrid Stereoplotter System would employ afixed program med analog system to sol ve thcprojective equations and a programmabledigi tal system to solye the variable systematiceffects, such as lens distortion, film shrinkage,atmospheric refraction, earth cu rvatu re, etc.Of course, this stereoplotter concept wouldnot have the degree of flexibili ty or versatili tyas is enjoyed with the AS-ll/A and AS-ll/Bsystems.

One of seyeral possible configurations of aHybrid Stereoplotter is presently being de­veloped by RAOC. This particular plotter em­ploys a Kelsh Plotter to solve the parallaxequations and a special platen containing adigitally controlled Z-, X- and I'-correctionsystem for removing model distortions. TheZ-correction is applied to the platen heigh t in­dependent of the Model Z-measuring systcmand the X- and V-corrections are applied tothe pencil-holding mechanisms independen tof the floating mark. The correction system isunder control of a digital computer which canbe programmed to perform absolu te orien ta­tion, to removc model distortions, and to cali­brate the basic stereoplotter.

The basic concept of the Hybrid Sterco­plotter will be reported on at a later date andthe Hybrid Plotter will be described in greatdetail.

CO:\,CLUSIOJ'S

This paper attem pted to describe the sys­tem principles of the Analytical Stereoplotterintroduced by i'vlr. Helaya in 1957. Fromthese basic principles the AP-1, AP-2, AS-11Aand the AS-ll B Analytical Stereoplottersevolved. This successful evolution was the re­sult of a unique feedback system and anequally unique type of instrumentation. Thatis, a magnetostrictive delay line organized as

a digital differential analyzer integrated witha general pu rpose digi tal processor.

Except for the influence of integrated cir­cuit technology the pure digital analyticalstereoplotter will probably remain un­changed. However, the recent development ofa Hybrid Stereoplotter may become competi­tive in manual compilation.

Automated compilation with ideal imageand terrain is already being inhibited by thei mage translation systems. This fact, willsoon force the photogrammetrist to develop abetter system concept; a few which have beensuggested by Mr. Helava in his paper, "AFast Automatic Plotter."

BIBLIOGRAPHY

1. "Photogrammetric Plotting I nstrument UsingElectronic Computation" by U. V. Helava,Internal NRC-Memorandum, 1st October1956, unpublished. (This memo started it allat NRC.)

2. Helava, U. V., "New Principle for Photo­grammetric Plotters," Photogrammetria, Vol.2 (1957-1958), pp. 89-96.

3. Moore, \V. J. M., "Considerations in the De­sign of an Electronic Computer for a Photo­gram metric Plotti ng Instrument," Photo­gramme/ria, Vol. 2 (1957-1958), pp. 97-101.

+. Helava, . V., "Analytical Plotter in Photo­gram metric Production Z Line," PHOTO­GRAMMETRIC ENGI:-iEERI1\G, September, 1958,pp.794-797.

5. "Aparato de restitllcion analitico utilizandocalcoJo digital" by U. V. Helava, Revis/aCartografica, 1958.

6. "On 'High Altitude' Photogrammetry" byU. V. Helava, June 1959, Private Communica­tion, unpublished. (Deals with the use of AI'in mapping moon and planets.)

7. "On the Transformation of Coordinates froma Photogrammetric System to a ConformalMap Projection" by U. V. Helava, Privatecommunication, 1960, unpublished (gives thefundamentals for performing this on AP).

8. "Automatic Mappi ng System Based on the useof an Analytical Plotter," by U. V. Helava,unpublished.

9. "Determi nation of Parameters of Computationin an Automated Photogrammetric Plotter,"by U. V. Helava, unpublished.

10. Helava, U. V., "Mathematical Methods inthe Design of Photogrammetric Plotters,"Photogram1l1e/ria, Vol. XVI (1959/1960) pp.41-56.

11. Yloore, W. J. i\I., and Chen, C. c., "A Studyof an Analog Computing System for an Analyt­ical Plotter," Photogramme/ria, Vol. XV (1958-1959), pp. 14-20.

12. "An Ultimate Solution of Relative Orienta­tion" by U. V. Helava, paper presented duringPhotogrammelric Weeks, 1960 in i[ unich.

13. Helava, U. V., "Analytic Plotter Using In­cremental Computer," PHOTOGRAMMETRIC E1\­GI:-iEERING, June 1960, pp. 428-435.

14. E. C. Johnson, "System Design of a DigitalControl Computer," PHOTOGRAMMETRIC EN­GINEERING, September, 1961, pp. 583-589.

15. "An Ultimate Solution of Relative Orienta-

1168 PIIOTOGRAllMETRIC ENGINEERING

tion" by U. V. I-lelava, Publication of theFinnish Society of Photogratl/lIletry n :03, 1962.

16. Parenti, G., "Analysis of the Mechanical-Opti­cal Design of the Analytical Plotter," TheCanadian Surveyor, Vol. XVII, June, 1963, pp.148-154.

17. Helava, U. V., "Analytical Plotter," TheCanadian Surveyor, Vol. XV] I, June, 1963, pp.131-148.

18. Lesser, B. S., "General Concept of the Analyti­cal Plotter," The Canadian Surve)'or, Vol.XVII, June, 1963, pp. 168-182.

19. Blachut, T. J., "Performance of the AnalyticalPlotter," The Canadian Surveyor, Vol. XV 1I,June, 1963, pp. 205-211.

20. E. C. Johnson and V. C. Kamm, "Computerand Programs for the AP-C Analytical Stereo­plotter," The Canadian Surveyor, Vol. XVI I,June, 1963, pp. 155-168.

21. Mahoney, W. c., "Operational Use of the AP­2 at ACrC," The Candaian Surveyor, Vol.XVII, June, 1963, pp. 189-205.

22. Weibrecht, 0., "A New Solution of Differ­ential Rectification Using the Helava Analyti­cal Plotter," Th.e Canadian Surveyor, Vol.XVI I, June, 1963, pp. 211-224.

23. V. C. Kamm and A. E. Whiteside, "GeneralMapping and Surveying Applications of theAP-C Computer," presented at Tenth Con­gress of Tnternational Society of Photogram­metry, Lisbon, Portugal, September, 1964.

24. E. C. Johnson and A. DiPetima, "Image Cor­relation System for Analytical Stereoplotters,"presented at Tenth Congress of InternationalSociety of Photogrammetry, Lisbon, Portugal,September, 1964.

25. Gill, c., "Relative Orientation of Segmented,Panoramic Grid Models on the AP-2," PHOTO­GRAMMETRIC ENGI:-lEERING.

26. Friedman, S. J., "Practical Considerations onthe Usefulness of the Analytical Plotter i\Tod.AP/C." Lecture held on 14 April 1965 at l.T.C

27. W. E. Chappelle, "The Automated AP-2

Stereoplotler," presented at ASP Central fewYork Section Meeting, May 14, 1965.

28. A. E. Whiteside, "Orientation Operations inAnalytical Stereoplotters," presented at ASPSemi-Annual Convention, Dayton, Ohio,September, 1965.

29. Scarana, F. A. and Shearer, R. v\'o , "Auto­mated Analytical Stereoplotter," presented atASP Semi-Annual Convention, Dayton, Ohio,September, 1965.

30. Hedden, R. T., "Lunar Charting ExperimentsUsing Ranger VI] Materials in the AS-IIAAnalytical Plotter," presented at ASP Semi­Annual Convention, Dayton, Ohio, September,1965.

31. Hedden, R. T. "Lunar Charting ExperimentsUsing Ranger Materials in the AS-llA Analyt­ical Plotter," presented at ACSM-ASP Con­vention, Washington, D. c., March, 1966.

32. Helava, U. V., "A Family of PhotogrammetricSystems," presented at ACSM-ASP Conven­tion, Washington, D. c., March, 1966.

33. Parenti, G., "The Use of Analytical PlotterOutputs for Orthophoto Printing," presentedat ACS:VI-ASP Convention, \Vashington, D.C.March, 1966.

34. R. B. Forrest, "Improved Orientation Pro­cedures for the AP-C Analytical Stereoplotter,'at the ASCM-ASP Convention, Washington,D. c., March, 1966.

35. C. \V. Matherly and R. i\l. Centner, "ATrainable System for Automatically Selecting­Photographic Pass-Points," presented atACSM-ASP Convention, \\'ashington, D. CoO:\11 arch, 1966.

36. Prescott, W., "Strip Triangulation with theAnalytical Plotter/Commercial," presentedat ASP Great Lakes Regional Meeting, Colum­bus, Ohio, 21 i\Iay 1966.

37. Herd, L., "The Analytical Plotter, Commercialin Highway Engineering," presented at ASPGreat Lakes Regional i\leeting, Columbus,Ohio, 21 May 1966.

Articles in Other Photogrammetric Journals

Vermessungstechnik Leipzig, June 1969

F. Deumlich, On the present importance of large-area levelling with maximum accuracy.J. Rauhu./, A comparator for determining the errors of graduation of 24-m invar tapes.£. Haack, Activities of the commission for the spelling of geographical names on maps.H. Glander, Purpose, con ten t and function of hydrogeological maps.J. Pie/sellner, Bases of analytical photogrammetry.H. Rellse, The determination of geocentric angular coordinates./.. S/einich, On the configuration and accuracy of observation In trigonometric sun'ey net­

works.