393

ALVA B. CLARKE,

U. S. Geological Survey, Washington, D. C.

ALVA B. CLARKE

some degree, all of the edges as they appear inthe aerial negative and in precisely the samelocation. Basically, at present, there are twomeans which may be used for edge enhance­ment: (1) electronic image-intensifier sys­tems, and (2) direct photographic processes.It is not wi thi n the scope of this paper to dis­cuss or eva1uate systems other than the onespecifically described here in.

Photographic processes of edge enhance­ment are simple and readily applicable totopographic mapping. One method of gaining"edge-contrast" by use of a film maskingtechnique, was described by Messers. Yule l

and Clerc' in 1945 and 1955 respectively. Apaper describing the application of the samemasking technique to aerial photographs waspresented by Messrs. John St. Clair andMylon Merriam in 1960.3 Results achievedby this method are promising.

1 Yule, J. A. c., Journal of Photographic Societyof America, Vol. 11:123, 1945.

2 Clerc, L. P., Photography, Theory and Practice,3rd ed., London. 1955, p. 374.

3 St. Clair, John and Merriam,. Myl.on~ "TheTo emphasize the detail of maximum sig- Use of Photography to Achieve BasIc Pnnclples of

nificance while subduing detail of lesser im- Map Design," PHOTOGRAMMETRIC ENGINEERING,portance, it is necessary to emphasize, in June 1960. Vol. xxvi, no. 2, p. 498.

* Presented at 28th Annual Meeting of the American Society of Photogrammetry, March 16, 1962.Publication approved by Director, U. S. Geological Survey.

1: Abstract is on page 349 of 1962 YEARBOOK.

EDGE ENHANCEMENT

A Photographic Edge-Isolation Technique*!

T HE oldest of all art forms is the represen­tation of objects and ideas by lines. A

capable artist can usually project his thoughtby using a minimum of lines. The idea, then,of representing real objects by using edgesformed during a photographic process is notinconceivable. The primary difference be­tween photographic representation and artis­tic rendition (Figure 1) is that the formerfulfills one of the basic requirements of map­ping, that of measurability, while the otherdoes not.

The process of mapping from aerial photo­graphs includes observing and interpretingpertinent information contained in an aerialnegative. Photographic film acts as a memoryunit, recording information transferred to itby an optical system. Differences in lumi­nance due to color variation, sun angle, andshadows of the terrain and objects upon it arerecorded in their relative positions as differ­ences in density. If the unit distance on thefilm is small, the density differences aresharply defined and are termed "edges."These edges reveal much of the informationthe interpreter needs to locate and identifyimages on the photograph. For some pur­poses and under certain circumstances, edgesalone, without intervening tonal variations,can convey sufficient information.

I n aerial photographs, imagery of maxi­mum significance to the interpreter or photo­grammetrist is characterized by the size,shape, and pattern of the edges formed duringthe photographic process. Unbroken, straight,or smooth continuous curved lines usuallyindicate man-made or man-modified objects.Continuous but irregularly curved lines indi­cate natural features, such as drainage sys­tems. Closed erratic patterns generally indi­cate vegetation. ]\'ote that all of these fea­tures have a common characteristic-theycan be represented by a single line describingtheir edges.

FIG. 1. Comparison of a photograph (Left), a photographically outlined subject (Center), and anartists' free hand drawing (Right).

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A PHOTOGRAPHIC EDGE-ISOLATION TECHNIQUE 395

EDGE ISOLATION

In 1959, a technique was tested in theTopographic Division of the Geological Sur­vey whereby the compression of density inan aerial photograph could reach a nearmaximum while preserving and in many casesincreasi ng the con trast of fine detail. I twastheorized that to completely eliminate tonesof lesser importance, more control over con­trast was required. The ensuing process wascalled "edge isolation," and the first testsindicated that it was adaptable to negativesof extreme density which are beyond thenormal range of automatic-dodging printers.

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• Watson, Alfred J., The Fluoro-Dodge methodfor contrast ("ontral. PHOTOGRAMMETRIC ENGI­NEERING, September 1958. Vol. xxiv, no. 4, p. 638.

PRIKCIPLES

Other than film, the only equipment neces­sary for the edge-isolation process is a specialcontact printer manufactured under the tradename "Fluoro-Dodge."4 The operation ofthis printer is based on photo-luminescence,which is the property of certain substances toabsorb radiation of one wave-length and emitradiation of another wave-length. The photo­luminescent substance used in this system iszinc sulfide. In conjunction with its photo­luminescent property or, more specifically,phosphorescence due to ultraviolet absorp-

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tion, zinc sulfide is also inversely sensitive tored and infrared. In other words, red andinfrared radiation suppresses the lumines­cence of the zinc sulfide phosphor, as shownin Figure 2.

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396 PHOTOGRAMMETRIC ENGINEERING

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shown in Figure 3. The lamp (UVL) radiatesa range of ultraviolet rays with a peak emis­sion at 3650 Angstrom units, cxciting the

phosphor screeen (P), which luminesces. Thefilter (UVF) absorbs the ultraviolet raysshorter than 4000 Angstrom units and trans­mits visible light. The filter (IRF) transmitsinfrared above the visible range, emanatingfrom the incandescent lamps (IL), quenchingluminescence from the phosphor screen (P).The quenching action of the infrared througha negative (N) converts the phosphor screeninto a point-to-point image-modulated lightsource.

FORMATION OF A LINE

A determining factor in the formation of aline of finite width by the edge-isolation proc­ess is the distance between the photographicnegative and the phosphor screen. Infrareddiffusion from the areas of less density acrossthe edges into areas of greater density,emphasizes the border. The effect produced issi milar to the border effects of normal de­velopmen t (Figure 4). Figure 5 is a micro­trace across seven consecu ti ve steps of astandard density step wedge after compres­sion has taken place. By manipulation of adeveloper and exposure, it is possible toobtain any desired degree of masking on thefinal negative.

Proper selection of film and developer com­binations enables the operator to control thedegree of edge emphasis. For this application,

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FIG. 7. Stone Mountain, near Atlanta, Georgia. Large clear area, upper center, is in dense shadowformed by a high cliff. Xote the pattern on the opposite side of the mountain, indicating drainage systemsleading to the small lakes and ponds in the wooded area at lower levels.

FIG. 8. Semirural area in Southern California. Here the difference between the lines formed by naturalvegetation and those formed by manmade objects is very apparent. Areas in both figures 7 and 8 were usedin a study in contrast control presented to the Society in 1961 (Contrast Control for Diapositives by JamesC. Lewis).

398 PHOTOGRAMMETRIC ENGINEERING

FIG. 9. Part of Alexandria, Virginia, much of the culture shown on this photograph is of recentorigin and does not appear on published maps.

FIG. 10. Alexandria, Virginia, area at smaller scale. The first step of this edge-isolated photographwas made with Kodak auto-screen ortho. The edge effect is very pronounced, without total loss of inter­mediate tone.

A PHOTOGRAPHIC EDGE-ISOLATION TECHNIQUE 399

the edges should be completely isolated fromintermediate tones and rendered as clear linesagainst an opaque background. Figure 6illustrates the transition from a simple densitydifference to total edge isolation.

PROCEDURE

The emulsion for this transformation musthave extremely high density and contrastcapabilities and must be coated on a stablebase. In the first step, a positive transparencyis made on high-contrast orthochromatic filmfrom the original negative, with maximumquenching and developed in a moderatelycontrasty developer, such as Kodak D-19.The dark lines along the image borders, asdescribed above, will have a density higherthan that of the surrounding background.The clear lines adjacent to them allow addi­tional passage of infrared on the succeedingstep and thus further accentuate the darklines, which are then printed in negativeform.

The final negative is printed in the sameway as the positive transparency. After theinitial exposure, however, the exposed nega­tive remains in the printer for a short time toallow the afterglow of the phosphor screen toexpose the film in the background areas. Theextent of edge isolation depends on the char­acteristics of the developer used for thisnegative. For complete isolation or totalelimination of tone, the film is processed in acaustic developer, such as Kodalith. The

background becomes opaque, and only clearlines are left to represent the original imageedges.

The accompanying slides demonstrate thetype of line which can be expected. They are,of course, greatly magnified and show somedegradation (Figures 7, 8, 9,10).

The edge-isolation process offers someattractive possibilities for saving time andmoney, especially in producing special-pur­pose maps for which standard cartographicsymbolization is not necessary. Currently, allmapworthy features must be represented bygraphically constructed symbols, manuallylocated. The photographic processing of imagesymbols by edge isolation provides a semi­automatic method of representing prominentterrain features and in many respects, thesymbolization is superior. When made fromorthophotographs, the imagery is accuratelypositioned and the resulting product is a veryuseful planimetric map. In addition, edge­represented maps do not require halftonescreening for reproduction by lithography.Patterns val uable to geologists and othersinterested in the general configuration of theterrain are often more easily detected in edge­isolated prints than in the original photo­graph.

The relative simplicity of processing edge­isolated prints could result in their wide­spread use. It is, however, impossible topredict at this time all the benefits which mayresult from this process.

AP/ i-A New Concept zn Stereoplotting*

S. JACK FRIEDMAN,

OMI Corporation of America,Washington, D. C.

I N PREPARING this paper the original inten­tion was to briefly review the principles of

the Analytical Stereoplotter and then to givesome detailed information on its design andconstruction. And finally to discuss brieflythe uses to which the instrument could be put.I found, however, that during the past yearor two in which we have been developing theprototype this information has already beenmade available in several papers. Mr. Helava,

the inventor of the instrument itself, has pre­sented some excellent material on the prin­ciples as well as the contemplated use of suchinstrument and now I notice in a recent is­sue of "PHOTOGRAMMETRIC ENGINEERING,"that Dr. Johnson of the Bendix ResearchLaboratories Division has presented a verythorough discussion of the basic concepts in­volved in the prototype instrument.

Also I must confess that I myself have been

* Paper presented at the Semi-Annual Meeting of the American Society of Photogrammetry, October1961, New York.