To Measure Is To Know-Or Is It?

ROBERT N. COLWELL

School oj Forestry, University oj Cal£jorniaBerkeley, Calif.

"TO MEASURE IS TO KNOW." It matterslittle who first uttered this oft-quoted

saying, or whether photo measurements werewithin his purview. But it does matter greatlythat many photogrammetrists believe im­plicitly in the truth of this saying, even whenthey are by no means certain of what it is theyare measuring. It matters equally that manyphoto interpreters, in their reI uctance tomake photo measurements, deprive them­sel ves of whatever knowledge photo measure­men ts can provide.

The primary objective of this paper is todemonstrate why all users of aerial photos, bethey mensurationists or interpretationists,should think more critically about the usesand limitations of photo measurements; forby so doing, they may be able to avoid seriouserrors they currently are making, either inmeasuring or in abstaining from measuring.As a consequence they may greatly improvetheir ability to extract useful informationfrom aerial photographs.

The most convincing way to present thistopic is through the consideration of speci ficexamples. Consequently this paper will heillustrated with seyeral aerial photographs, sochosen as to be (1) representative of a widevariety of military and civil applications ofaerial photography, and (2) illustrative of theextent to which photo measurements can berelied upon to increase our knowledge of thearea photographed.

Among the many kinds of measurementswhich photo users commonly make on aerialphotographs are those designed to provide amore accurate knowledge of the heights,depths, shapes, sizes, volumes, tones, andrates of motion of the various objects whichare imaged on the photographs. This listingserves to govern the organization of the re­mainder of this paper. Under each of theheadings which follow, examples will first begiven of the genuine value to be found inphotograph measurements, properly made;then examples will be given of the erroneousconclusions which can be reached when thephoto measurements are either improperlymade, or made in ignorance of important as­sociated facts.

RELIEF MEASUREMENTS

Figure 1 illustrates the great value of photomeasurements to a military commander whowishes to make an amphibious landing withinan enemy-held area, as shown here. From thisstereo photography, flown under combatconditions, it was possible to measure waterdepths, in the lower right portion of thestereogram, and to obtain valuable informa­tion on the configuration and gradient of theocean bottom. Predictions were then made ofthe distance offshore at which various typesof vessels, each of known draft, would runaground during an amphibious landing.Measurements also were made above thewaterline shown in this stereogram. Fromthese measurements the width and gradientof the beach were determined, and the rough­ness of the uplifted coral reef was estimated;in addition, measurements were made todetermine the height of each bluff that wouldhaye to he scaled by our landing forces whenseeki ng an exi t from the beach. The steepnessof slope and height of vegetation atop thehluffs also were estimated. Each of these de­terminations was made with the aid of accu­rate measurements of stereoscopic parallax.

Later, an amphibious landing was success­fully made on the beach, part of which isshown in Figure 1. Subsequent ground check­ing by the writer established that, for waterdepths up to thirty feet (the maximum depthat which underwater detail could be dis­cerned), the average error of depth estimatesbased on photo measurements was less thanone foot, and the maximum error, less thantwo feet. Comparable errors resulting fromphoto measurements made above the waterline were six inches and one foot, respectively.Clearly, in this example "to measure was toknow" to a far greater order of accuracy thanif the photos had merely been interpretedinstead of measured.

Even in this straightforward example, how­ever, the danger of placing too much confi­dence in mere measurements existed. Forexample, had the photo measurer failed tocorrect his underwater parallax readings forthe refraction of light rays at the water-airinterface, the average error in his answers

71

72 PHOTOGRAMMETRIC ENGINEERING

FIG. 1. For a discussion of the value to a military commander of photo measurements that might be madeof this area where an amphibious landing is contemplated, see text. (Department of Defense photo.)

would have been increased by a disastrousthirty-three per cen t. Ina si milar way if thephoto measurer had failed to report height ofthe tide at the time the photos were taken,his depth measurements, however accurate,might have been dangerously misleading topersonnel planning for making an assaultlanding here at some other stage of the tide.In this instance, then, the photo measure­ments ,,-ere clearly essential, but it wasequally important to know exactly whateach such measurement signified.

Figure 2 shows an area representative ofthose in which foresters commonly measuretree heights as a step in the determination oftimber volumes. After measuring accuratelythe stereoscopic parallax for Tree" A " (on theoriginal photos, which are shown in Figure 2at reduced scale) a forester computed theheight of the tree to be 138 feet. A secondforester measured the height of the same treefrom its relief displacement on the left photoonly,-an equally legitimate method in thisinstance. He obtained an astounding value of583 feet, even though he made no significanterrors in either his measurements or his calcu­lations. A third forester, by measuring theshadow lengths cast by the tree, and by atelephone pole of known height imaged else­where on the photo, obtained a height of only42 feet for the tree. Finally a fourth forestermeasured the relief displacement of Tree "A"on one of the photos of an adjacent flight line,on which the tree was equally well imaged. Hearrived at an incredible height of minus 112feet for this tree! A ground check made by the

writer revealed the true stem length of thistree to be 161 feet.

An examination of Figure 3 shows that theerror made by each of these photo users wasnot in his photogrammetry but in his interpre­tation; each assumed the tree to be vertical,when actually it was leaning. This is a farmore common type of difficulty than isgenerally realized. Few trees are truly verti­cal, and they certainly need not have as muchlean as Tree "A" to introduce significanterrors in photogrammetric determinations oftree height.

When working with aerial photographs, itis rarely recognized, except through the mostcareful photo interpretation, that a tree orsome other "normally vertical" object is lean­ing. We expect objects to appear somewhatdistorted on an aerial photo, particularlywhen imaged near the edge of the photo. (Seealso Figure 7.) This example, then, shows thatunless we correctly interpret photo images, wedo not know the true meaning of our photomeasurements, once we have made them.

Corollary to what has just been said, anequally serious error can arise when the photo­grammetrist, in attempting to determine thetrue horizontal ground distance between twopoints, measures the apparent distance be­tween them very precisely on aerial photos.If, because of an error in photo interpretation,he has misidentified either of the points on thephotographs, a commensurate error will, ofcourse, appear in the calculated ground dis­tance, despi le the high precision of his photomeasurement.

FIG. 2. Two overlapping vertical aerial photographs of an area in which most tree heights can be accurately measured photogrammetrically.Very sizable errors result when attempting to measure the height of tree A, however. Compare with Figure 3.

..,o:::t=1;l>Ulc::;;0t=1-Ul..,o:;;Zo:i1Io;;0-Ul

,.,'-v

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74 PHOTOGRAMMETRIC ENGINEERING

FIG. 3. Ground view showing the amount of lean in Tree "A" of Figure 2. Even if this tree had muchless lean, very significant errors might result from attempting to measure its height photogrammetricallywhile assuming it to be truly vertical. '

SHAPE AND SIZE MEASUREMENTS

Figure 4 illustrates the value of makingaccurate measurements of an unidentified ob­ject and thus of facilitating its identification.This stereo-pair of photographs, taken overenemy-held territory, reveals peculiarly­shaped objects of a type that had never beforebeen seen by our military photo interpreters.The images were merely "interpreted" ratherthan measured (a common failing amongphoto interpreters) and the objects were notcorrectly identified. If accurate photo meas­urements of these objects had been made int he x, y and z directions their true size and

three-dimensional shape would have beenmore fully appreciated. It then would havebeen apparent that each of these objects hasa deep V-notch at one end and only a shallowV-notch at the other. This is a common designfeature of the "cradles" used for transportingsmall boats. Such a shape for objects of thissize, is rarely, if eYer, encountered elsewherein the physical universe. The boat keel, whichslopes downward from fore to aft, fits snugglyinto the V-notches which, because of theirdiffering heights, permit the boat to be trans­ported in a horizontal position.

In the instance shown in Figure 4, the

FIG. 4. For an explanation of the value of photo measurements in identifying thesehighly important objects, see text. (Department of Defense photo.)

TO MEASURE IS TO KNOW-OR IS IT? 7S

FIG. 5. Twer "suicide boats" after removal fromthe caves in which they had been hidden. Only byidentification of the objects shown in Figure 4might the presence of these important weapons ofwar have been detected. Each carried a "warhead"containing 640 pounds of high explosive. (Depart­ment of Defense photo.)

enemy's clever plan was to keep his totalsupply of 400 of these secret weapons, knownas "suicide boats," hidden in caves on a small,in~ignificant island, a few miles offshore fromthe main island on which we were about toland 100,000 troops. Then, under cover ofdarkness, the boats were to be transported 011

these cradles to the nearest beaches forlaunching. During high speed runs at nightthey were to ram their high explosive war­heads directly into our supply ships, thussinking them and leaving our troops strandedashore, at the mercy of a numerically superiorenemy ground force. Historically, it has been

well-documented that failure of our entiremilitary operation might well have resultedfrom the inadequate extraction of informa­tion from the photos shown in Figure 4, had itnot been for a most fortunate com pensati ngfactor that worked to the advantage of ourforces.

It is difficult to make an unbiased judgmentwhen viewing this photo interpretation prob­lem in retrospect. It is the author's opinion,however, that had he and his colleaguesmeasured these images instead of merelyinterpreting them, the objects would havebeen correctly identified prior to the landingand thus the entire enemy plan for their usewould have been deduced.

Figure 6 provides an illustration in whichhasty photo interpretation procedures mightlead to the conclusion that there is a trainproceeding from right to left near the center ofthe stereogram, while in the top portion,automobiles are busily threading their wayalong a freeway system. The photo inter­preter has ready access to information on theflight altitude and focal-length of photogra­phy. Nevertheless, either his laziness or his,-e1uctance to calculate scale, make photomeasurements, and determine the actual di­mensions of the objects he is examini ng, isabout to lead him into making a ridiculousmistake. For if he were to practice the mostelementary photogrammetry, he would notethat the entire train, including its engine andseven cars, is little more than 150 feet long,and the au tomobiles on the freeway in the topof the stereogram are only six to seven feet

FIG. 6. Objects shown in this photo might readily be misinterpreted unless carefulphoto measurements were made of them, as explained in the text.

76 PHOTOGRAMMETRIC ENGINEERING

long. In this instance, then, "to measure is toknow" that the area shown here is only a landof make-believe, (actually a portion ofDisneyland in Southern California) in whichthe trains and automobiles are merely smallscale models used strictly for play purposes.

I t should not be inferred from this example,however, that the only way for the photo in­terpreter to have avoided the error he wasabout to make would have been throughphoto measurement. More careful interpreta­tion often will serve to accomplish the sameobjective. For instance, with a bit more carehe might have noted the full-scale automo­biles and freeway lanes in the bottom of thestereogram, and rectified his hasty interpreta­tion of the upper portions of the stereogramwithout having made any actual photo meas­urements.

The top stereogram of Figure 7 is included,lest too much credence be placed in photomeasurement as the cure-all to identifyinguninterpretable objects. Countless measure­ments might be made with great precision onthe stereogram shown here, without leadingto correct identification of this object as adirigible mast. The difficulty resides in thesimple fact that few photogrammetrists havehad prior experience \vith this type of object,even on the ground. The solution to thisproblem is not to be found through morenumerous and precise measurements. Itprobably is best found through (1) the "teamapproach," wherein a group of photo interpre­ters, each drawn from a different type of back­ground, meets to exchange ideas as to whatvarious unidentified objects might be, and (2)reference to photo interpretation keys whichillustrate and identify various kinds of ob­jects (including dirigible masts) and whichalso set forth in some systematic fashion a\\'ord description of their photo recognitionfeatures.

The stereogram in the middle of Figure 7shows the ease with which we might errone­ously conclude that the water tower has onlyfour peripheral legs instead of six, if we weregiven only the right photo of the stereo-pair.I n this instance, proper photo measurements,even if only the right hand photo had beenavailable, would have obviated the error. Thebottom two photos of Figure 7 show"dummy" objects whose lack of fidelity, inshape and size, to the objects they simulate,is best established by photo measurementwhen only small scale aerial photos are avail­able. However, it does not follow that "tomeasure," in this instance is "to know" theultimate significance of that which is meas-

ured. In some instances dummies such asthese are cleverly constructed in the hopes ofluring the enemy into wasting his efforts bybombing and strafing the dummies. But inother instances the dummies are made obvi­ously asymmetrical in the hope that the enemywill be certain to recognize them as dummies,and will thereupon carefully avoid bombingand strafing the area. Beneath such dummiesvital supplies of ammunition and fuel maythen be stored, unmolested by aerial attackuntil such time as the double deception isdeduced.

TONE MEASUREMENT

Figure 8 illustrates the potential value oftone measurement on various film-filter com­binations as an aid to knowing water depths,vegetation types, and kinds of ground surfacematerial. As this example suggests, there fre­quently is a need to detect and identify vari­ous objects and conditions in the physicaluniverse solely from "tone signatures" ob­tained by cameras or other sensors that areremotely situated with respect to the areabeing investigated. The energy that is eitheremitted or reflected from objects in the area isrecorded by the sensors, usually in photo­graphic form. Quite commonly, as illustratedin Figure 8, one portion of the desired infor­mation is best obtained when sensing theenergy returns in one part of the electromag­netic spectrum, while another portion requiressensing in a different part of the spectrum.

These considerations have given rise to areconnaissance method known as "multi bandspectral reconnaissance," by means of whichtwo or more remotely-situated cameras orother sensing devices, each specially suitedfor the sensing of energy in its own spectralband, can be used in concert to provide in­formation that no single sensor could haveprovided (Colwell, 1961). Frequently, forsuch a data-gathering process to be of maxi­mum value the tone values must be measured,not merely interpreted, so that truly quanti­tative tone signatures can be relied upon(Fischer, 1961).

Microdensitometers have recently been per­fected with the aid of which tone densitiescan be accurately measured, either on theoriginal negatives, or on opaque positiveprin ts. Si milarly useful instruments some ofwhich are known as "densichrometers" or

-"densichrons," are currently being used toquantify measurements of hue, value andchroma on color photography. Such instru­ments have demonstrated the validity of theconcept, "to measure is to know," for a large

TO MEASURE IS TO KNOW-OR IS IT? 77

FIG. 7. "To measure is not necessarily to know," as illustrated by the photographsin this figure. For explanation, see text.

variety of data-gathering problems whichrely on the use of aerial photgraphy.

Figure 9 demonstrates the need for exercis­ing precaution when relying on photographic

tone to identify various objects and condi­tions. Figure 10, by way of further precau­tion, shows an instance wherein the mostprecise measurements of tone may be of little

78 PHOTOGRAMMETRIC ENGINEERING

FIG. 8. An example of the value of tone measurement on multiband aerial photography. The film­filter combination used in taking the left photo reveals underwater detail to a depth of nearly twentyfeet readily reveals the presence of a road, and provides unique photographic tones for certain importantpla~t species not identifiable on the right photo. On the other hand, the right photo reveals underwaterdetail only to a depth of about 5 feet, accentuates water courses rather than roads, and provides uniquephotographic tones for certain important species not identifiable on the left photo. It follows that muchmore complete information is obtainable from both photos interpreted in concert, than from either onealone, particularly if the tones of certain of the objects are actually measured rather than merely inter­preted.

or no value in helping us to know exactlywhat is imaged.

MOTIo~ MEASUREMENT

Figure 11 is a graphic illustration of thepossibility of making measurements on "dis­crete-frame" aerial photos to determine thevelocity of flow in various parts of a stream.The photogrammetrist must know the timeinterval between exposures. He then can cal-

culate the rate of stream flow merely fromstereo parallax measurement of floating ob­jects (in this instance, chunks of ice) on thetwo overlapping photos. Both the direction offlight of the aircraft and the direction of flo\vof the stream must be considered in makingthe calculations. In Figure 11 the portions ofthe stream having the highest velocity arethose in which the ice appears to be floatingat the highest level when the stream is viewed

TO MEASURE IS TO KNOW-OR IS IT? 79

\

FIG. 9. The danger of relying on photographic tone measuren{I!9":ts without due regard to seasonalstate is illustrated by these two photos of an area in the central Netherlands. That which is light-tonedon the left view is dark-toned on the right view, and vice-versa, even though both photos were taken withthe same film-filter combination. The left photo was taken on November 29 at a time wh~n .drainageconditions in this area were very poor because drainage pumps were I)ot working alfcr~~\:Je shaltow depres­sions were almost Aooded. The right photo was taken on the following January 5 'aft;er a heavy frost.In this photo the depressions are white, but on the low ridges the ice or snow has melted.t~consequentlythe ridges are wet and appear in dark tones. (Photos courtesy Dr. P. Buringh.) . ,; " .. --"

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stereoscopically. It \\'ill be noted, when thisfigure is studied stereoscopically, that the is­lands in the stream appear to be embeddedfar below the surface of the stream, itself. Thisis because the chunks of ice in the stream ex­hibit a false parallax, because of their motion,

thus making them appear nearer to thecamera than they actually are.

Figure 12 illustrates the difficulty of makingaccurate motion measurements on "continu­ous strip" photography. The elongation ofthe images of automobiles when they are mo\,-

FIG, 10. Despite the unusual light tone of the structure near the center of this stereogram, great doubtremains among the experts as to exactly what is imaged here. The area shown is in the middle of a dense,inaccessible tropical jungle where ground truth is lacking. The normal tone of the lake in the lower rightcorner further mystifies the photo interpreter who attempts to identify the light-toned area.' In thisinstance, unlike Figure 8, precise measurement of tone would be of no value whatever in identifying thelight-toned area.

80 PHOTOGRAMMETlUC ENGINEERING

FIG. 11. Stereogram showing many chunks of ice floating in a Canadian stream. The motion of theice provides an artificial parallax which, when accurately measured, permits a precise determination ofstream velocity in various parts of the stream, as explained in the text. (Photos courtesy of Royal Cana­dian Air Force and Dominion Forest Surveys of Canada.)

ing in one direction, and their foreshorteningwhen they are moving in the opposite direc­tion, is well illustrated in this figure. If thetrue dimensions of each car were known, thevelocity 6f each could be measured on suchphotography, from the amount by which eachappeared to be foreshortened or elongated. Indoi ng this it would be necessary to take into'consideration the velocity of the photographicaircraft and the amount of image motion com­pensation introduced into the film drivemechanism, but photogrammetrically theproblem would be quite straightforward.Since the dimensions of the automobiles arenot known in this instance, photo measure­ments on this single stri p photography donot permit us to know the velocities at whichthp automobiles are travelling.

VOLUME MEASUREMENT..,Figure 13 shows several objects whose

volumes we may wish to determine throughaerial photo measurement. The volume ofwood chips in .each of the piles on the left sideof this stereogram is readily determined bycontouring the piles photogrammetrically at aknown vertical interval and determining, fromarea and height measurements, the volume ofeach layer delineated by adjacent contours.

Determinations as to the volume of eachcylindrical tank (near the top of the stereo­gram) and of the centrally-located pile of logslikew.ise are straightforward problems in

FIG. 12. Portion of a Sonne continuous stripphotograph in which. image Illotion compensationcauses the ground to' be -registered in sharp detail,but in which automobiles moving in one directionare elongated (see A) and those moving in theopposite direction are foreshortened (see B). If thetrue length of each vehicle is known, it is possiblethrough accurate photo measurement, to determineits velocity. (Photo courtesy of Chicago AerialSurvey.)

TO MEASURE IS TO KNOW-OR IS IT?

FIG. 13. For a discussion of the value of photo measurements in determining thevolumes of various features shown in this stereogram, see text.

81

photogrammetry. Highway engineers con­cerned with cut and fill calculations have longsince discovered the advantages of using aerialphoto measurements to obtain this informa­tion.

A more complicated problem in volumedetermination is illustrated in Figure 14. Thevertical aerial photo shown here was takenfrom an altitude of approximately 8,000 feetabove the floor of Yosemite Valley in whichStand "A" is growing. The forest photo inter­preter having only a rudimentary kno\\"ledgeof photogrammetry can make measurementson this photo and its stereo mates, from \\"hichhe can determine the volume of timber inStand "A" quite accurately. To do so, he mostlikely:

(1) will calculate the scale, S, of the photog­raphy from the relation

Js=_·11'

(2) will measure, with a floating dot instru­men t or parallax wedge, the average height, h,of the dominant and codominant trees in

Stand "A," using the relation,

11 X dP!I=---

P+dP

(3) will measure the average crown diam­eter of these same trees from the relation,

Actual Crown Diameter

Photo Measurement of Crown Diameter

Photo Scale

(4) will measure or esti mate the crownclosure of the stand with the aid of acetatetemplate overlays on which crown closures ofvarying densities have been simulated so thatthe apparent crown closure of Stand"A" canbe compared with this guide.

(5) will measure the acreage occupied byStand "A," with the aid of an acetate tem­plate overlay on which dots have been draftedat such a spacing that each dot represents,say, one acre at the scale of the photography.The forester will then consult a previously­prepared aerial photo volume-table whichrelates actual timber volumes-per-acre (aspreviously established from ground survey)to tree-height, crown-closure and stand-

82 PHOTOGRAMMETRIC ENGINEERING

FIG. 14. It is possible to measure the volume of Timber Stand "A" quite accurately by means ofphoto measuremen ts. I f the same t'ypes of measurements are applied in attempting to measure the volumeof timber Stand "B," however, an error of as much as 3,200 per cent may result, as explained in the text.

density, as measured or estimated on thephotos.

Experience has shown that, by the meansjust described, a forester can measure thevolume of timber, stand by stand, throughouta sizable area, such as the entire Iloor ofYosemite Valley, to an average error of con­siderably less than 10 per cent. This comparesvery favorably with the accuracy likely to beobtained by conventional methods of "timbercruising" on the ground.

Thus encouraged, the photo user might nextattem pt to use the same types of measurementand the same mathematical factors in deter­mining the timber volume of stand "B."

Since this stand is on the same aerial photosas stand "A," it would at first seem that thesame mathematical relationships should ap­ply. But upon ground checking, the photouser probably would find to his consternationthat he had overestimated the volume ofstand "B" by a staggeri ng 3,200 per cen t! Alittle rellection on how this could be possiblewill show that most of his difficulties stemfrom the fact that his camera was only 4,000feet above stand "B" while it was 8,000 feetabove stand "A"; consequently he probablyhas overestimated the acreage of stand "B"by 4-fold, its tree-heights and crown-diameters,each, by about two-fold, and its apparent

TO MEASURE IS TO KNOW-OR IS IT? 83

crown-closure by another two-fold or more(because of the excessive apparent lean oftrees viewed as obliquely as these from onlyhalf the altitude as that from which stand"/I"is viewed). Nor can any comfort be found inthe thought that whatever errors he mightmake by such oversights will be compensating.They will not be in this instance, because allstands growing at an elevation of 8,000 feetabove sea level in the Sierra (such as stand"B") have far different merchantabilitycharacteristics and species compositions thanstands such as "A," which are growing at anelevation of only 4,000 feet above sea level.

SUMMARY AND CONCLUSIONS

The foregoing examples are only a fewamong many that might be given to illustratewhy all of us, as we employ aerial photographs,need to think more critically about the usesand limitations of photo measurements. If

these examples will prompt even a few of us,through wiser use of photo measurements, toimprove our ability to extract useful informa­tion from aerial photographs, the paper willhave served its primary purpose.

LITERATURE REFERENCES

American Society of Photogrammetry. "j\i[A\;UAL01" PHOTOGRAPHIC INTERPRETATlO;," 875 pages.George Banta Publishing Co. 1960.

Colwell, R. N. "Some Pradical Applications oflVIultiband Spectral Reconnaissance," AmericanScientist, 49(1) Pp 9-36. 196J.

Fischer, W. A. "Color Aerial Photography inGeologic Investigations." PHOTOGl<AMMETRlCENGINEERING XXVIII(I), pp. 133-139. 1962.

Ray, R. G. and Fischer, W. A. 1960. "QuantitativePhotography-a Geologic Research Tool."PHOTOGRAMMETRIC ENGINEERING XXVI( 1) pp.143-150. 1960.

Young, H. E. and Stoeckler, E. G. "QuantitativeEvaluation of Photo Interpretation Mapping."PHOTOGllAMMETRIC ENGINEEIUNG XXll(1) pp.137-143. 1956.

Geologic Interpretation of Airborne

Infrared Imagery

LA U RENeE ]I. LATTMAN,

Department of GeologyThe Pennsylvania State Univers·ity

University Park, Fa.

ABSTRACT: N!odern airborne infrared (IR) imagery allows the photogeologist toview the earth's surface in a l'ast, new range of the spectrum. This imagery is afunction of the energy emitted from, not reflected by, the terrain object. As tltisenergy is a function primarity of the object's emissivity and temperature, thephotogeologist must retrain himself to interpret terrain features in a new aspect.

A photogeologic interpretation of an I R image of Mt. Nittany, Pennsylvaniareveals that shale may be distinguished easily from sandstone, and that valley­side springs are more clearly shown than on conventional aerial photography. Asmore I R imagery becomes available, more geologic information will undoubtedlybe obtained from this medium.

INTRODUCTlO:\T

G EOLOGIC interpretation of aerial photo­graphs taken by means of visible light is

a well developed art. Visual photographyhas been extended slightly into the infraredregion by use of special films but most infra­red radiation cannot be directly photographed.Within the past few years airborne equipmentcapable of producing good terrain images inthe infrared range of the spectrum has beendeveloped.

It is the purpose of this paper to discuss thepotential geological value of infrared (lR)imagery and give an example of geologicinterpretation of an IR image.

TECHNIQUE OF OBTAINING I R IMAGERY

Because the terrain is not photographeddirectly by airborne IR sensors, the term "IRimagery" is used to describe the final photo­graph obtained.

The airborne IR sensing equipment is