The Automatic Map Compilation System *

DR. S. BERTRAM,

Thompson Ramo Wooldridge Inc.

RW Division, Canoga Park, Calif.

ABSTRACT: This paper describes the A utomatic Map Compilation System. Ituses a small digital computer to position an electronically generated scan onestimated homologous points on a pair of diapositives. The resulting signalsare then correlated and used to measure the height error. The computer receivesthe height error and uses it to maintain the system in contact with the surfacedefined by the diapositives. The system moves through a series of profilingsequences to simultaneously expose a new photograph, with the imagery movedso that it will appear in correct orthographic projection in relation to a selectedscale, and an altitude chart showing the contour information. A complete com­pilation for normal 9" X 9" vertical photography is accomplished in about1t hours.

T HE Automatic Map Compilation Systemdeveloped at Thompson Ramo vVoold­

ridge under the auspices of the U. S. ArmyEngineer Geodesy, Intelligence and MappingResearch and Development Agency auto­matically produces orthographic projectionphotographs and altitude charts from stereopairs of aerial transparencies. These can becombined and suitably annotated to producea photomap of the stereo area.

The system utilizes a small digital com­puter to provide the equivalent of a stereomodel. Electronic scanning of small areas andcorrelation of the resulting signals is used toevaluate height errors. The computer controlsthe system through a series of profiling se­quences using the height-error signal to keepthe system "on the ground."

The principle of automatic height deter­mination from aerial photographs can bevisualized in terms of an analog model. Thesituation is diagrammed in Figure 1, where theview normal to the line of flight (projectorseparation) is shown. Suppose in a small por­tion of the field there is a high light point P.If P is estimated low at P e it would be foundto the left of its estimated position on a dia­positive exposed at C and to the right of itsestimated position on a diapositive exposed atC'. A synchronous scanning in the directionof the line of flight and centered at the esti-

SIDNEY BERTRAM

mated positions of the homologous points willtherefore produce signals from correspondingimage points that are displaced in time anamount proportional to the height error.

In the Automatic l\Iap Compilation Sys­tem, the two diapositives and a photosensi­tive film sheet used for printing the outputare mou nted on a common movable carriageas shown in Figure 2. Associated with thiscarriage are four flying-spot scanners, one foreach diapositive, one for exposure of theorthophoto and one for exposure of the al ti­tude chart. The traces on the diapositivescan ners are imaged on the two diaposi ti Yes,

* Presented at March 24-30, 1963 ASP-ACSM Convention, Hotel Shoreham, Washington, D. C.

675

676 PHOTOGRAMMETRIC ENGINEER[ G

FIG. 1. Use of stereo photographs to developheight-error signal

FLYING-SPOT SCANNER

LENS MOVES UNDERCOMPUTER CONTROL

FIG. 2. Carriage configuration of automaticmap compilation system.

For the present, the system operates onlywith diapositives whose orientation matrices

The operator is able to observe the dia­positive areas being scanned at a given timethrough an electronic stereoviewer. This isessentially a twin TV system on which theimages observed on the two diapositives arereproduced; an electronically generated cross­hair is available as a reference for use in sys­tem calibration and as a floating mark.

using lenses whose positions are undel- thecommand of the computer. As shown inFigure 3, this permits the lenses to be movedto observe the computer-estimated homol­ogous points on the two diapositives. Thelenses for the orthophoto and altitude print­out are fixed in position, making the areasprinted out dependent only upon the in­stantaneous carriage position. Conversely,the instantaneous carriage position definesthe geographic coordinate under observationat a given time.

FIG. 3. Photograph scanning system.

are available from comparator measurementsand subsequent calculations. 'vVhen the dia­positives are introduced into the system, thecomputer directs the transport carriages tomove so scan ning can take place near the firstpass-poin t. The operator, through Flexo­wri ter con trol of the compu ter, then movesthe scanning, through motion of the diaposi­tive lenses, until the selected point has beencentered on the crosshair in the stereoviewer.v\'hen this has been accomplished, the oper­ator signals the computer to take into itsmemory the system coordinates of the firstpass-point and then to move to a second pass­points where the process is repeated. The com­puter then calculates new coefficien ts for eachdiapositive that are characteristic of theorientation of the diapositives in the systemand of any drift in the electronics. As a checkit then directs the system to move to a thirdpass-point, using its geographical coordinates(X, Y, Z). If this third point is well centeredon the crosshair, the photo coordinates havebeen correctly converted to machine co­ordinates. The automatic compilation is theninitiated.

In the starting position, the carriage is at aposition X/m=xo, Y/m=yo where m is thedesired orthophoto scale. As pointed outearlier, the homologous points (x, y) and(x', y') on the two diapositives correspondingto the spatial point (X, Y, Z) are scanned byshifting the two lenses amounts correspond­ing to the differences (x-xo, y-Yo) and(x'-xo, y'-Yo). The instrument then pro­ceeds in a profiling mode, with the carriagemoving in a direction nearly perpendicular to

Yo

L ..D I~IPHOTO-

SENSITIVEFILM

SHEET

Al'TOMATIC MAP COMPILATJON SYSTEM 677

the flight line, signaling the computer wheneach new 0,010" interval has been reached,For each position the computer, with the aidof ~he altitude measured at preceding points,estimates the new altitude and directs thediapositive lenses to center on the correspond­ing homologous points, As the profilingproceeds, the heigh t-error eyal uati ng cir­cuitry operates to correct the height estimate,so that the instrument can proceed withoutlosing contact \\'ith the surface, At the sametime the video signal from one of the diaposi­tive scanners is used to create a scaled imageof the area "seen" by the scanner on the fly­ing-spot scanner associated with the or tho­photo, The computer also provides a signal tothe fourth flying-spot scanner, to adjust theexposure on the altitude chart appropriatelyto the altitude interval. Successive ranges ofelevations are delineated by white, gray orblack line segments in the contour interval.

At the end of each profile the carriage stepsover in the x direction, typically by 0,020",and proceeds to profile in the opposi te y direc­tion, so as to expose a new orthophoto stripand con tour in terval line. The process con-

tinues in this manner until the desired areahas been compiled,

As profiling progresses the operator watchesthe stereoviewer and the heigh t-error signalsreported to the computer as reproduced inlights on the control panel. As long as the in­strument maintains contact with the stereosurface the operator can relax. However, ifcontact is lost, as evidenced either by thestereo image mismatch or by a "lack of corre­lation" light or erratic behavior of the errorlights, the operator stops the profiling opera­tion and then, through the Flexowriter U (forup) or D (for down) keys, modifies the heightas it appears in the computer memory untilthe stereo image appears in the plane of thecrosshairs, The automatic profiling operationis then re-initiated,

Figure 4 shows an orthophoto and altitudechart made by the machi ne, The photographywas vertical from an altitude of 10,000 feet.The contour interval shown is 10 meters, Theautomatic profiling \"as accomplished inabout an hour and twenty minutes includingthree manual interventions, It \\'ill be ob­served that the "noise" on the contour lines is

ALTITUDE CHART

FIG. 4. Altitude chart and orthoohoto,

678 PHOTOGRAMMETRIC ENGINEERING

FIG. S. Automatic map compilaton mechanicalassembly.

primarily a "bouncing," indicating that theI::asic internal sensitivity of the system wasin excess of a "least count" of 0.001" at thescale of 1: 20,OOO-that is, the smallest de­tectable height sensitivity was within 20" onthe grou nd.

The present map compilation instrument isan experimental machine intended to provideverification of the fundamental concept. Thebasic instrument is shown in Figure S, andthe control panel and stereoviewer are shownin Figure 6.

Operation of the equipment involves anumber of subtle techniques. These are de­scribed briefly in the following sections. Thediapositive carriage moves smoothly duringthe profiling operation. This makes it neces­sary to use "stop motion" circuitry, analo­gous to the image motion compensation sys­tem in a camera. This allows scanning to cen­ter accurately at the points specified by thecomputer despite the carriage motion.

(BEHIND) CORRElATORI

1-,"'F':::-R--tI CORR~LATOR

BEHIND)

FIG. 7. Height-error sensor. (Parentheticalnotation for displaced scan.)

The computer commands the lens servosthrough preCisIOn digital-to-analog con­verters. Since the point-by-point measure­ments proceed at a rapid rate-about fiftytimes per second for the present operation­any delay in the servos' achieving the desig­nated positions would in terfere wi th the oper­ation. For this reason the instantaneous servoerror signal is applied to the flying-spot scan­ners with appropriate gain to cause thescanning to move over precisely the correctamount to compensate for the servo error.The system, therefore, operates with an es­sentially instantaneous servo response.

Immediately following a new commandfrom the computer, scanning is initiated overthe designated areas on the diapositives. Theresulting signals are applied to a pair of corre­lators through complementarily connected de­lay lines as shown in Figure 7. The correlatorsact to produce a maximum output for inputsthat are coincident in time, with reduced out-

FIG. 6. Control panel and stereoviewer.

AUTOMATIC MAP COMPILATION SYSTEM 679

puts when the signals are displaced in time.The result is to have outputs from the twocorrelators that are equal for coincidentsignals, while one increases for time displace­ments in one direction (say. positive heighterrors), the other increases for time displace­ments in the opposite direction (negativeheigh terrors). The di fference between thesetwo signals becomes a measure of the heighterror.

The height error is applied to a height errormeasuring unit shown in outline form inFigure 8. The error is integrated and the out­put of the integrator used to control a pair ofthreshold circuits, one acti ng on posi ti veerrors, the other on negative errors. If one ofthe threshold values is exceeded, the revers­ible counter is stepped in an appropriatedirection and the integrator restored to zerooutput. Associated with the counter is adigital-to-analog converter whose output isused to shift one of the diapositive scans inthe X direction. At the beginning of a newmeasurement cycle, the counter and integra­tor are reset to zero by computer command.As scanning progresses, any observed errorwill step the counter and introduce a correc­tion signal. As scanning continues any addi­tional error will cause the operation to be re­peated until equilibrium is reached. The com­puter then takes the count, indicative of therequired height correction, into its memory,and uses it to update its information so thatit can maintain contact with the surface.There is thus an independent height meas­urement at each stationary point in theoperation. This is currently made every0.010" of travel in the y direction.

At each measurement point an area of0.050" XO.050" at the orthophoto scale is ex­amined. The diapositive scans are adjustedunder computer control to an area appro­priate to the local scale, larger for points athigh altitude and smaller for points at lowaltitude. The orthophoto exposure is limitedby video switching techniques to provide anonoverlappi ng pri n tou t.

It is expected that the diapositive scanningwill be modified in the near future so that itconforms more nearly with the orthophoto

FIG. 8. Height-error measuring unit.

scanning. This will require the distortion ofthe otherwise square scan to a parallelogramwhose precise shape is a function of the slopeof the terrain element being examined and ofits position relative to the camera. The signalsrequired to control this operation are readilyobtained by additional correlation circuitrycombined with appropriate video switchingtechniques.

The Automatic Map Compilation Systemuses rather conven tional electronic, servo andoptical elements to achieve a system that isfaster operating and more flexible than anysimilar instrument. Its potential accuracy islimited only by the measuring techniques usedand the stabili ty of its various com ponen ts.Its sensitivity with good imagery is compar­able to that of a good operator; with poorimagery, it probably is better. The systemcannot, however, work unattended. Occa­sional manual intervention is necessary tostart the operation and to assist the instru­ment over areas that are unsuited to auto­matic operation. These include steep cliffswhere imagery at the two levels cannot besimultaneously in the field of the correlators,hidden areas in one or both photographs, andareas of such low image detail as to provide nouseful height signals. In such cases, theoperator, through his larger view of the area,is able to interpolate useful information tokeep the system going.

Achievement of a working system requireda team effort. The contributions of GeorgeMiller, mechanical engineer, Glen Kimball inelectronic design, and] ules Mersel in compu­ter programming were particularly significant.