Automatic Point IdentificationMarking and Measuring Instrument *

EDWARD R. DE METEld

AS THE name implies, the Automatic Point.1""1.. Identification, Marking and MeasuringInstrument is an integrated system for theselection, identification, marking and meas­urement of pass-points and control-points.It is intended primarily for use in controlextension by analytical triangulation. Thetypical problem in selecting pass-points is asfollows:

PROBLEM

Figure 1 contains a highly idealized repre­sentation of the area location of pass­points on a photograph and their location onoverlapping and sidelapping photographs.The problem is to rapidly locate pass-pointson the center photograph within the tintedareas numbered 1 through 9 that also appearlegibly on the overlapping photographs.

Conventional selection techniques aretedious and restricted to the selection ofpoints which stand out as readily identifiableto the human eye. The process requires ahighly skilled technician and is very time­consuming. Often there are no readily identifi­able points and compromises must be madewhich result in a lowering of the accuracywith which points are located. Often pointsmust be selected in locations of sharp changesin relief; this condition makes recognition ofthe point difficult as its appearance in differ­ent photographs is radically different be­cause of the change in perspective. The se­lected point is sometimes a bush or someother vague, easily misidentified object.Often points must be omitted altogether andthe net result is a weakening of the controlextensions. To compound the problem thistime-consuming, scrutinizing operation in­volving the identification of a pass-point isrepeated over and over again by differentindividuals performing various tasks beforecompilation of the area is complete.

In test areas for research work, highlyidealized pass-points or control points areestablished by placing clearly definable panelmarkers in open flat areas such as shown inFigure 2. This type point is readily identifiedin all photos and can be located and measured

EDWARD R. DE METER

with extreme accuracy. This can be done athigh speed and with practically no risk ofmisidentification. The Automatic PointIdentification, Marking and Measuring In­strument simulates the panel marker bysimultaneously marking the conjugate imageson all overlapping photos, with an identicalmark. This operation retains all the desirablefeatures of the panel-point plus the fact thatthe photo-coordinates are measured at thesame time.

SYSTEM DESCRIPTION

The'"optimum complete system (Figure 3)for covering the triple overlap and sidelapconsists of three precise air-bearing typeX - Y comparators; three unprecise X - Ycomparators; six cathode ray tube scanning­correIa tor servo systems for driving the com­parators; six marking systems for marking theconjugate photo-images being scanned; and acontrol console containing a stereo and monomonitor viewing screen and a referencescanner. The measuring system is optionallyeither a Ferranti photo-electric diffractionmeasuring system or the Link Fringecountinterferometric measuring system with type-

* Presented at Semi-Annual Meeting of the Society, New York City, October 5, 1961.t Geodesy, Intelligence and Mapping Research & Development Agency, U. S. Army Engineers.

82

POINT IDENTIFICATION MARKING AND MEASURI G INSTRUMENT 83

\\Titer-paper tape read out. As shown inFigure 3, the experimental instrument pres­ently being constructed to prove out theminimum system concept contains only twoprecise air bearing type comparators and areference scan ner.

PRINCIPLES OF OPERATION

The principles of operation are based uponthe techniques of scanning and correlation ofphoto-images developed at Fort Belvoir dur­ing research in automatic map compilation.The basis of operation is the ability to slavethe comparators such that the images beingobserved and su bsequen tly measured arealways conjugate images of the image underthe measuring reticle of the ~lanually con­trolled comparator. This comparator is calledthe "master"; the comparators that areservo con trolled to the same image are calledthe "slave" comparators. The operator op­tionally, at any time, may select anyone ofthree precise comparators to be the manuallydirected "master" comparator. For use withcontrol-point photos, the operator may op­tionally select the fixed reference scannercontaining the control-point photo, to be the"master" image. The three unprecise com­parators are for precise marking of conjugateimages on sidelapping strips only. They haveno measuring capability.

FIG. 1. Pass points.

OPERATING SEQUEKCE

The normal sequence of operation is toselect, mark and measure pass-points and con­trol-points on Flight Line No.1 (Figure 4),with the precise measuring comparators andto simultaneously mark conjugate images asthey appear in the sidelap area on FlightLine No.2. After completion of Flight LineNo.1, Flight Line No. 2 is placed on theprecise measuring comparator and FlightLine No.3 is placed on the unprecise compara­tor and marked in its sidelap area. This op­eration is repeated until all the flight linesare marked and measured.

FIG. 2. Panel point.

84 PHOTOGRAMMETRIC ENGINEERING

FIG. 3. Automatic point identification marking and measuring instrument.

I t is obvious that when there is only a sin­gle flight line the unprecise marking com­parators are not required. It also should beapparent that ideally three precise measur­ing and marking comparators are needed toobserve the triple overlap for simultaneousselection of an acceptable pass-point in thisarea. However, in practice it may be thatonly two comparators will be necessary. Anacceptable pass-point is one in which theconjugate image as it appears in all photosis sufficiently clear and defined to permit theimage correlators to function properly. vVhenthe correlators are functioning properly theconjugate images are correctly positioned formarking and measurement.

It should also be pointed out that themeasurements of the pass-points on eachdiapositive plate must be completed beforemoving the plate. The sequence of plate place­ment and removal is similar to that used with

a conventional three-plate comparator suchas the Nistri TA-3. That is, after completingthe selection of points on diapositive No.1,diapositive No.4 replaces No. 1. After com­pletion of No.2, No.5 replaces No.2, etc.

OPERATION

Figure 5 is a basic representation of theoperator's control console. A simplified typicalstep-by-step operation sequence is as follows:the operator would initially have available acomplete set of contact prints on which thegeneral areas where pass points are desiredare marked as well as the general areas of thecontrol-points. He would also have a set ofsmall 2-X2-inch control-photo-plates or filmchip with the control point location accu­rately marked. There is one plate for eachcontrol-point. The present plate being experi­mentally used for this purpose is the standardmultiplex diapositive plate.

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POINT IDENTIFICATION MARKING AND MEASURI G INSTRUMENT 85

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The operator's initial step would be toplace the diapositives in their respectivecomparator plate holders and to secure themwithout any special effort to precisely line upthe photo fiducial marks with the measuringsystem axis. Then using the controls on theconsole and observance of the imagery on themonitor screen or stereo viewing screen. hewould manually bring each photo in turn intoposition so as to view a common area. Thearea selected would be reasonably level. Hewould then manually, using the dove prismcontrols on the console, rotate each com­parator's image to remove the effects oflarge amounts of crab.

He is now ready to select, mark and meas-

ure pass-points. Depressing the automaticbutton causes the comparators to slave to themanually controlled comparator which isselected by the "master" selector switch.Viewing the moni tor screen or the stereoview screen, the operator directs the measur­ing reticle of the master comparator to thefirst pass-point area. The slave units simul­taneously move to the conjugate areas ontheir respective photos. A panel of ligh tsglowing green informs him the area on eachcomparator is correlating satisfactorily. Hethen depresses a "fine" scan button which re­duces the size of the area being scanned andresults in the positioning of the conjugateimages to a higher order of accuracy. The

86 PHOTOGRAMMETRIC ENGINEERING

FIG. 6A. Comparator.

lights should remain green; hO\\"ever, if a lightis red the master is moved slightly until apoint is located that is satisfactory as indi­cated by the green lights. He then sets theidentification number for the pass-point anddepresses the record button automaticallyrecording the location of the point as it fallson each comparator and the mark bu ttonwhich automatically marks the location oneach diapositive. The operator then proceedsto the other pass-points and the fiducialmarks in turn, measuring and marking their

locations. To mark and measure the locationof a control-point, a library diapositive con­taining the marked location of the control­point is placed in the reference scanner and iscentered to the index mark on the monitor ­screen. This index mark is the common refer­ence for all the comparator measuring sys­tems. The pass-point area is then manuallybrought into view on the measuring compa­rators, and the control-point image is rotatedand adjusted to scale on the reference scanner.The measuring comparators are then swi tchedto slave to the reference photo-image. Whenpositioned satisfactorily the measure andmark buttons are depressed recording thelocations of the control-points simultaneouslyon all diapositives.

TECHNICAL HIGHLIGHTS

COMPARATORS

As shown in Figure 6A and 6B thecomparators employed use air-floated tablesof a unique design. They are for practicalpurposes, friction-free.

As shown in Figure 7 five-inch cathoderay tubes are employed for scanning the dia­positives. The scan size is continuously vari­able from 80 microns to 800 microns in thefine-scan position, and from 800 to 8,000microns in the coarse-scan position. Thisgives a continuous range from 0.08 millimeterto 8 millimeters. In the fine-scan position

FIG. 6B. Comparator.

POINT IDE TlFICATlON M.\RKI 'G AND J\lE.\S to G INSTl{UMENT 87

FIG. 7. Photo scan system.

tion error of less than 4 microns and a maxI­mum error of less than 10 microns.

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STATE OF DEVELOPMENT

A can tract \\'as a warded to Link Di vision ofGeneral Precision to build an experimentalt\l'o-comparator \"ersion of the system. Thecontract was awarded in March 1961 anddelivery is scheduled for iIarch 1962.

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ACCURACY

The accuracy of entire system includingmarking is expected to ha\'e a standard de via-

there is a 100 ti mes red uction of the scan as itappears on the face of the cathode ray tube;in the coarse scan position there is a 10 timesreduction.

MARKING SYSTEM

The marking system consists of a smallheated die that burns a mark in the diaposi.tive emulsion. The dies are interchangeable.Experimentally several sizes and markingpatterns will be tested. A satisfactory mark isbeing produced as shown in Figure 8. Tem­perature and pressure are non-critical. Tem­peratures from 300 to 500 degrees Fahrenheitat pressures from 1 to 16 ounces produceaq:eptable marks. The dies presently beingused produce a 50 micron dot with a 2 milli­meter circle of 50 micron thickness. Dies ofsmaller sizes and different outside circle sizesand widths are being fabricated. The die\I"hich is most suitable for a particular job willbe selected by experimentation.

FIG. 8. Marking system.