IRA B. CURRENT* General Aniline & Film Corp. Binghamton, N. Y. 13902 Sensitometry In Color Aerial Photography

Once the standards are established, sensitometry provides the best means of control of any aerial photography or processing system

(Abslwcl on nexl pnge)

INTRODUCTION not be substituted for Anscochrome film in a system involving printing or duplicating HE SCIENCE OF photographic sensi to­ without running practical and related sensito­ T metry is very helpful in color aerial metric tests to adj ust for color balance and photography. In a broad sense, this science is printing-time differences introduced by the concerned with the measurement of all photo­ substitu tions. graphic steps between the actual terrain and The term C!/(/raclerislic CIIYl'e is applied to the observet', or photographic interpreter. It each of the individual curves derived by is essential in the manufacturing processes of plotting the densities measured through red, all photographic materials, but the discussion blue, or green filters (corresponding to cyan, here is Ii mi ted to the use of photographic magneta, or yellow layers of the final pro­ materials in color aerial photography: opti­ cessed film) against the logarithm of the ex­ mum film exposure, color balance adjust­ posures given to the combined three layers ments, processing control, utilization of (or tri-pack). As it is usually best to make densi ty-con trast characteristics, and con trol use of the film man ufactu rer's sensitometric in the preparation of reflection of trans­ data and processi ng recom mendations as a parency pri n ts. starting point for establishing a controlled PRACTICAL SENSITOMETRY photographic system, USA Standards should be employed wherever practicable. For ex- The practical use of sensi tometry in color aerial photography will t'eq uire the establish­ ment of standard sensitometric references based on practical judgment of aerial photo­ graphs made under Ii ke condi tions. These standards, then, will serve as the basis for control of any of the parts or all of the photo­ graphs made under like conditions. These standards, then, will serve as the basis for control of any of the parts or all of the photo­ graphic system. Although the same principles may be applied to all of the color photo­ graphic systems-negative or reversal-one must be careful not to substitute in parts of, the system, such as the fil m of one design (manufacturer) with that of another, without re-establishing the standard of reference. Another brand of film, for example, should * Presented at the Annual Convention of the American Society of Photogrammetry in Washing­ ton, D. c., March 1967. TRA B. CURRENT 1143 1144 PHOTOGRAlI1l11ETRTC ENGINEERING

ample, densities should be spectral diffuse camera performance standards can lead to densities as defined in USA Standard PH2.1­ false interpretations of the sensitometry. 1952. USA Standards, by the way, is the new Variables extraneous to the sensitometric name for what was formerly known as AA system can best be eliminated by making sure Standards. They are produced by the United that the sensi tometry is related to the actual States of America Standards I nstitu te, the light meters, camera shutter speed settings, new body that replaces the American Stan­ camera lenses, vignetting filters, and the other dards Association. (Filters different from elements of the photographic system em­ those given in PH2.1-1952 may be required in ployed for the photographic mission. special applications, and if this is so, the filter used should be incorporated in the PROCESSI:-

ABSTRACT: Sensitometry is useful for the practical determination of film speed and setting correct exposure, control of processing conditions, adjustment of color balance, and control of printing. J11easurement of densities and plotting of characteristic curves provides data to carrying out these aims. From the practical viewpoint, sensitometric techniques have limitations, but these are relatively in­ significant if they are recognized and the relationships between sensitometry and practical photography are established for color photographic systems. Choice of density range,jor example, and therefore speed rating, can lead to some control of contrast and gradation in the final photographs. Sensitometry is also essential in making black-and-white negatives, color separations, or duplicates from color positives or negatives.

EXPOSURE sing machine is employed, then the sensito­ The quality of light used for sensitometric metric exposures should be processed wi th the exposure, exposure time, processing, etc., same condi tions as those for the photography. should be as nearly identical as can be at­ If the processi ng method involves a wind-re­ tained to the conditions employed in the ac­ wind processor, then sensitometric strips, or tual photography. This will eliminate most of preferably exposures on non-image areas of the failures that are introduced by: (1) a the actual pictorial ftim, should be included sensitometric exposing source of different at both ends and in the center portion of the q uali ty from that ill u mi nati ng the terrai n roll when processi ng is accom plished. The roll photographed; (2) the use of an exposure ti me length, even if it is a dummy, should be equal different substantially from that of the to that to be used in the actual aerial photog­ camera shutter; and (3) using processing con­ raphy. This procedure will not only reRect the ditions that differ from those actually used average level of the processing throughout for processing the aerial photographs. the roll, but will also indicate the variation One of the problems in sensitometry is that existing between the ends of the roll and the of correlating the sensitometric exposures to center. the light meters employed to measure the terrain being photographed. This problem DATE RECORDING may involve meters that are directly coupled The sensitometric data deriyed in any to the camera system, in which case the re­ aerial photographic or sensitometric system ferenced values used in the meter system should be carefully preserved. This can then should be very carefully determined and re­ be referenced at any time that the need lated to the exposure given to the film in should arise. The problems that require a sensitometry. Other factors, such as efficiency solution on any given day can be related to of shutters, transmission of lenses, etc., should the experience gained with previous aerial be established. These must then be taken into photography; or it can be referred to in order account when relating sensitometry to practi­ to arrive at exposure and processing condi­ cal photography. Failure to recognize the tion. Sensitometric curves and other informa- SE 'SITOMETRY IN COLOR AERIAL PHOTOGRAPHY 1145 tion relating to them should be carefully doc­ such as water, the influence of clouds, the umented. Such data as exposing light and necessity to penetrate shadow, and flare light filtration, exposure time, processing, date, in optical systems, all contribute to the related sortie, etc., should be available im­ rationalizations that must be made in es­ mediately when any curve or other data are tablishi ng the speed of a fil m, and then mak­ consulted. To save time and confusion, the ing use of thc assigned speed for photography relative log exposure scale should be filled in in a given situation. For high altitude and the fiducial mark corresponding to the photography, an integl'ated measure of any mark on the sensitometric step should be sizable area should provide a suitable lumi­ indicated on all the density-log-exposure nance value that can be related to the exposure graphs. The density scale should also be filled required to produce a given density on the In. film. Under these conditions the luminance range is relati\'ely small. INTERPRETATfON OF RESULTS For low altitude photography the lumi­ The standards of sensitometric reference nance range will usually be much greater and and control will depend on the aerial color an integrated luminance may not be as useful photographic system bei ng used. For exam pic, in determining the exposure required to the sensitometry may be related to pictures register the various luminance values in the that ar.e to be interpr~ted by direct viewing; scene within the desired density range on the or it may be related to pictures that are to film. It may be desirable, for example, to ex­ subsequently be duplicated on transparent or pose more for the shadow areas, sacrificing opaque base material; or to pictures that are density in the highlight areas of a reversal to have black-and-white (monochrome) or film; yet, on the other hand, highlight areas color separation negatives made for further may represent the primary information to be study or printing. recorded. Color negatives may be used for dil'ect printing to produce black-and-white posi­ SPEED AND CONTRAST tives using a suitable color positive printing At the present stage of the art, it seems material on ei ther transparen t or opaq ue that past performance in aerial photography base. The color negatives may also be the with a given system is the most useful source source of separation negatives for further of information that can be called on to arrive study or printing. Although the duplicating at an arbitrary film speed value. The sensito­ or printing step for color negatives may seem metric reference standard related to this to be the same, the sensi tometric reference performance can then be used, along wi th any and standards may be quite different from necessary correction tables and computers to those for color positive originals. determine the shutter time and lens aperture For example, the two different systems for any particular terrain to be photographed. may make use of pri nti ng materials (re­ \\'i th a gi ven photographic system (camera­ ceivers) made up of layers having different exposure meter-fil m-processi ng-i nterpreta­ spectral sensi tivi ty peaks. Thus, the densi tom­ tion), then, if one emulsion number is sub­ eter measuring filters will require different stituted for another, the ratio of the sensito­ absorption peaks to make the sensi tometry of metric speeds of the reference emulsion and the system really significant. The same situa­ the new one can be applied to the exposure tion holds true with refel'ence to the printing system. If another type of film (another correction fil ters. If the absorption peaks do manufacturer, for example) having similar not correspond to the spectral sensiti vi ty contrast is substituted, then the ratios may peaks of the layers maki ng up the pri n ti ng be less valid; and if a film of entirely different material, then the sensitometric response contrast (and sensitometric curve shape) is may not be equal to the practical response of substituted in the system, then the ratios may the materials. not hold true at all. Several factors enter into the assignment of No standard method of determining aerial speed to color films for aerial photography. color film speed has yet been established. It is One of the most important and most difficult doubtful that a single film speed value would aspects is that of relating the speed of the be suitable for all types of photography be­ film to the brightness of the scene being cause the characteristic curve shapes of films photographed. The measurement of lumi­ of different manufacturers tend to be quite nance of the terrain would at first seem to be a varied. With a given film, the film speed re­ straight forward process. However, the effects ference poi nt on the characteristic curve may of specular reflections from bright surfaces depend on the type of photography being 1146 PHOTOGRAMMETRIC ENGINEERiNG

4.0 criterion. In the second instance, b, a density ANSCOCHROME 0/200 AERIAL COLOR FILM of 0.8 might be more appropriate. The ap­ 3.6 proximately 0.6 difference on the log exposure ~, '''''"'~.. scale corresponding to these two densities rep­ ~ resents two lens stops, or arithmetic speed 3.2 ''\ "alues having a ratio of 1 to 4. As a given Red sensitive (Cyan dye)·...·...... ·..... '~.. speed may be assigned to a given film, and the 2.8 Blue sensitive (Yellow dye)---- two si tuations handled by sui table factors, it I \\, Green sensitive (Mooento dye)-'- is evident that the needs for aerial color 2.4 ~~ photography present serious problems in I establishing a single speed for films of this I ~. kind. 2.0 Although considerable latitude is available >. I \\ in the higher density of a typical aerial film .~ I 1.6 a ~ for photographing at high altitudes, con­ I~~ siderably less latitude in exposure is present 1.2 \\, if long-scale scenes are being photographed, I such as at low altitudes. The subject lumi­ 0.8 I ~"" nance range of high altitude terrain is quite '\ ", small, and considerable deviations from the I ~ ['\. optimum exposure can be made without seri­ 0.4 ~ ous change in gradation (only a density '-- ~ f-l)-- ~ change). On the other hand, although greater 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.-, 2.7 latitude of recording scene brightnesses is Relative 100 exposure permitted for photography with the lower densi ties, very Ii ttle latitude exists for overall FIG. 1. Characteristic curves for the three primary exposure variation without sacrificing infor­ colors on one type of film. mation at one end of the scale or the other. A third condition might be to base speed on done (low altitude versus high altitude, for a medium density, say 1.6, representative of example), as well as on the type of informa­ the average of the range available with a tion desired from the finished photographs. given film. As the density range of the aerial In high altitude photography, it may be de­ film might approach 3.2 and represent a scene sirable to record the image with densities that brightness ratio on the order of 1: 500, a would fall on the steepest portion of the great deal of latitude is available for recording characteristic curve to provide maximum con­ information. Aerial photographs made utiliz­ trast. By using densities in this high range, ing the full density range of the material may maximum image contrast amplification is be difficult to read without adjusting viewing achieved in the original. Any problems of intensity for particular areas of interest. If, viewing can be compensated for by increasing however, the films are subsequently dupli­ the illumination from the viewing table; or by cated, or separations made, printing expo­ duplicating the film to a lower density. sures can be selected for best reproduction of If, on the other hand, photographic latitude the densities of primary interest. is required when photographing terrain of The determination of speed of color nega­ wide subject contrast, then the pictures tive films presents a somewhat different prob­ should be recorded with densities falling on lem inasmuch as the characteristic curves the lower portion of the characteristic curve tend to be individually somewhat more nearly of the film, and on which the average grada­ Ii near than those for reversal processed ma­ tion is considerably lower than in the example terial. The determination of film speed may gi ven above. I n this region, the characteristic be more nearly related to that of black-and­ curve departs considerably from linearity, white aerial films, and a reference point near with highlights being registered with rela­ the threshold of the curve may be appropri­ tively low gradation, whereas the shadows are ate. A proposed USA Standard is under con­ registered with relatively high gradation. sideration for determining the speed of films Both of the situations described above will intended for amateur photography, and this call for different density criteria for establish­ may be limited to that determined for the ing film speed (Figure 1). In the first one, a, in slowest emulsion layer where the agreement of which heavier densities are utilized to make characteristic curve shape among the three up the image, a density of 2.4 might be the layers is at an acceptable minimum. SENSITOMETRY IN 'OLOR AERIAL PHOTOGRAPHY 1147

COLOR BALANCE 36 r---r----r------,------,-,------, When aerial photographs are to be judged 32 f----'----,-r--- by visual inspection, the film should be balanced to provide as nearly as pos ible a neutral rendition of a gray scale. This will not 28 necessarily mean that the sensi tometric curves obtained with exposures to light hav­ 241--f----\-;J- ing a quality equal to that used for the prac­ tical photographs will lie on top of each other. 2.0 rn fact, they will generally be displaced by ~ some amount, and this is governed by the .~ 16 sensitivities of the emulsions and the absorp­ a tion characteristics of the image dyes in the 12 aerial film in question. The relative position of these curves is also determi ned by the choice 08 if filters used in making the density measure­ ments. A set of sensitometric curves, which 04 _ represent the exposure and processing condi­ tions that provide a neu tral rendi tion of a 00 L--.J__-L_L-L_.-L_L-----l._.....L_-'-_-' gray scale as obtained by practical photog­ Relative log e'Xposure raphy under similar film and processing condi­ tions, should serve as the basis for judgment FIG. 2. Sensitometric curves for different fdtt;rs. for color balance. This will hold true only for Because the sensitivity regions and color correctIOn filter absorption regions are not sharply defined, one given photographic film product, as part the shift on the log E scale may amount to only of a given exposure processing system. Other about 0.06 units. (The exposure scale is the same as films and/or systems will require different that shown in Figure 1.) references for neutrality. Color balance differences, where one fresh color discrimination, only 50 per cent to 70 emulsion number of a given aerial color film is per cent of the filtration is effective. Hence, compared to another, may amount to the the shift on the log E scale may amount to equivalent of a 0.10 density color correction only about 0.06 units where a color correction filter, in yellow, magenta, or cyan (or any filter having a density of 0.10 is applied combination of these). Aging may increase (Figure 2). this to the equivalent of a 0.20 density I n both origi nal aerial photography and filter. This may require some correction if duplicating or printing the exposure ti.me exact uniformity is required. Also, over a should be taken into account when conductIng given period of time the processing may sensitometry. This is made necessary because vary, and a further correction of color balance the reci proci ty characteristics of the three of a given film may be required. Thus, if it is emulsion layers making up the tri-pack may desirable to balance a film-emulsion-process­ not al ways be si milar. \"lide variations in ex­ ing system to produce results equal to those posure time may affect both the color balance achieved wi th another system, an esti mation and the relative gradation characteristics for of the camera or printer filtration that will be the individual dye layers, resulting in lack of required is in order. This is done best with curve confonnily in the sensi tometry. If the trial-and-error practical or sensitometric ex­ ti me used in the sensi tometric exposures ap­ posures using color correction filters of proximates that to be used in the practical various color and density. The log E dis­ photography, the problems of establishing placement of given densi ty poi nts of curves corrective filtration and speed adjustment made with various filters can be used as refer­ are simplified. Also, if there is some departure ence for estimating the amount of filtration from conformity of the layer gradation charac­ required with a given system. Caution must teristics, as shown by the curves, the best ap­ be exercised to make such comparisons only proximation of speed and filtration for a given at or near a given density level, because the density range to be employed in the photog­ effects of color correction filters are different raphy can be determined if the sensitometry at different density levels. ti me corresponds to the practical exposure Because the sensitivity regions and the time. color correction filter absorption regions are If lack of cnrve conformity exists as the not sharply defined and have low efficiency in result of the color-film-processing system, 1148 PHOTOGRAMMETRIC ENGINEERING then the densities employed in making the integral densities in terms of the receiver, the pictures will also affect the color balance. If human eye, if a print or duplicate for correct the curves cross somewhere near the middle viewing is to be achieved. densities, the color balance of the heavier Curve 1 in the upper left hand corner of densities will tend to be opposi te to the color this tone reproduction diagram represen ts an balance of the densities below the cross-over original transparency made utilizing camera point. Thus, different color balance treat­ film densities ranging from about 0.30 through ments may be required if the films are used about 2.0. If this is printed with an exposure for photographs having high densities, com­ that would make use of the lower portion of pared to those for photographs having low the duplicating film densities, shown on the densi ties. cun'e in the upper right hand quadrant, For judging the neutrality of aerial ftlms, it with the minimum densities approximately is important to take into account the viewing matched, the projected curve is similar to conditions. For example, the subjective inter­ that shown at 1 in the lower left hand quad­ pretation of color balance can be largely rant. As can be seen, considerable photo­ affected by the color of the surroundings at graphic distortion exists in this curve, with the time the pictures are viewed. If the sur­ practically one-half of the original brightness rounding area is painted green, for example, scale reproduced as a Rat, low gradation toe. A the mi nd will tend to correct for this, and considerable increase in the number of tones sensitometric strips and pictures may seem to in the original transparency would be re­ be magenta. Likewise, a yellow surrounding produced as highlights lacking in modulation. will cause the strips or pictures to seem to be If the printing exposure is decreased so as to more cool than they would otherwise. Ideally, utilize the upper densities of the duplicating a gray surrounding should exist in the vicinity film, less distortion is obtained. This may be of a viewer used for j udging aerial trans­ represented by Curve 2 in the upper left hand parencies, and the sensitometric strips and quadrant which, if projected to the duplicat­ photographs shoule be viewed together, if ing film in the right hand quadrant, yields a they are related. The spectral energy distribu­ curve like that shown as 2 in the lower left tion at the illuminator should be approxi­ hand quadrant. This may be considered to be mately 3,800o K as recommended in USA a quite acceptable rendition of the tonal scale Standard PH2.23-1961. of the origi nal aerial color photograph, with moderate amplification of density scale or D PLICATING contrast. When making duplicates of aerial color If this type of treatment is employed for photographs, the products of gradation of the high altitude photography where the original original camera film and the duplicating film camera densities are within the range of 1.0 to can be utilized to control the final result. If, 3.0, and are printed on a duplicating film in for example, the full density scale of the the same range of densi ties, a rather dense-ap­ camera film has not been utilized in making pearing duplicate is produced, but the ampli­ the original photography, some modification fication of contrast, the approximate product of photographic characteristics of the dupli­ of the average slope of the two curves, is cate can be affected by the choice of printing greatly increased. Thus the ability to detect density employed with the duplicating film. minor differences of both color and intensity The tone reproduction diagram in Figure 3 levels in the original scene would be increased. is used to illustrate the effects of gradation of (See Figure 4.) the characteristic curves on the final result. On the other hand, if low altitude recon­ The curves in this diagram are those obtained naissance photographs have been taken, wi th thevisual response fil ter on the densi tom­ utilizing the lower portion of the characteris­ cter. They are a practical approximation to tics curve to achieve maximum latitude, a the relative gradation characteristics of the print made on the low density portions of the original, the duplicating material, and final duplicating film would retain the maxi­ transparency. Actual tone reproduction studies mum latitude. Considerable distortion, or of color films are much more complex than reduction in contrast, may occur in the this, and are beyond the scope of the present highlight portions of this duplicate, printed in discussion. They would involve the use of this way. However, should it be desirable to integral densities for the camera film, and an­ amplify a certain small portion of the lower alytical densities for the print film. The latter, contrast (lighter density original) it could be then, must be computed and converted to printed to high densities on the duplicating SENSITOMETRY IN COLOR AERIAL PHOTOGRAPHY 1149

T Density r---'0T'A'--_0"fB'--'i1r2 -;11'-6__2T·O"------'2""A'-----"'2.8"----~3.2

~--+-_._+

::Ji'---+--+- I _-l 1 I 1 rri--+--l- ~r- I I I I ----j J----{- i I

0 0 0.3 I -j [W .~ I 1 L~ c Log expoSlKe -----__ 11:p- ---, I'

\\.3

FIG. 3. This tone reproduction nomograph illustrates the effects of gradation of different portions of the characteristic curve on the final result. film, thus amplifying the particular detail in black-and-white photography, and sensitom­ question, even though a sacrifice in both con­ etry can be used to determine contrast trast and color balance may have to be ac­ effects and ftlter factors, based on practical cepted in other densities than those of tests. primary interest. The science of sensi tometry can prove to be 'vVhere monochrome negatives are printed a very useful tool in aerial color photography, from aerial color positives, the usual mono­ duplicating, printing, and separating. How­ chrome film sensitometric control can be ever, in all instances the necessary references employed. The desired exposure and grada­ need to be established based on practical tion level should be established through trial photography or printing tests, using sensitom­ and error to provide sensi tometric references etry that is processed at the same time. Al­ for control of printing of negatives. Also, the though filtration requirements can be esti­ use of contrast and separation filters can be mated from sensitometry, even here closest used, similar to those appropriate for original approximations are made where sensito- ...­ Ut" e

"C ::I1 ...,o 1. 2. o CJ ~ :> ~ ~ ...,t:rl ~ n t:rl Z CJ Z t:rl t:rl ~ Z CJ

3. 4.

FIG. 4. These black-and-white reproductions from color aerial photographs illustrate the effect of exposure (average density in the color reversal aerial photograph) on contrast. Photograph 1, made at 8,000 feet but exposed to utilize densities above about 1.6. gives good contrast. Photograph 2 was exposed to record most of the details of densities below 2.0 on the film. Here the contrast is considerably lower, and might be considered to be inadequate. Photograph 3, made at an altitude of 4.000 feet and exposed to utilize densities above 1.6, has contrast that may be higher than desired. Photograph 4, made at the same elevation as Photograph 3 but utilizing densities below 2.0. has considerably lower contrast and preserves good detail in both shadow and highlight areas. SENSITOMETRY IN COLOR AERIAL PHOTOGRAPHY 1151

metric references have been established, using BIBILIOGRAPHY tests with the actual filters that will be used in practice. The Theory of The Photographic Process, Edited by Once the standards have been established, T. H. James Chapter 21, J. C. Simonds, Mac­ sensi tometry provides the best means of con­ Millan, New York (1966). Principles of Color Sensitometry, (Committee trol of any aerial photography or processi ng headed by Dr. Francis H. Gerhardt), Society of system. Motion Picture and Television Engineers, (1962)

RESUMEN

RESUMEN DE LOS ARTiCULOS TECNICOS PUBLICADOS EN EL MES PREVIO*

Vol. XXXII' No.9 Septiembre 1967

PRINCIPIOS DE FOTOGRAFIA AEREA A COLOR Por Allan L. Sorem La adicion y substraccion de los tres colores satisfactoriamente en la cartografia estereo­ primarios, son conceptos basicos en el disefio y sc6pica. Hay disponible un computador de ex­ la utilizaci6n de los materiales fotograficos a posicion que ayuda al fotografo aereo, y el uso color. Una reproducci6n no debe tener necesaria­ correcto de los filtros quiza requiera decisiones mente, yen la mayoria de los casos, no tiene, diflciles, mientras que el proceso a colores no las mismas caracteristicas espectrales de la presenta ninguna dificultad seria. Las carac­ escena original. Los materiales de negativos a teristicas de color de la luz de enfoque, pueden color, pueden ofrecer ventajas significativas de limitar la cantidad de informacion recuperable flexibilidad sobre los tipos de color, reversibles. de un sistema. Las placas diapositivas de color, se utilizan PHOTO. ENGR., SEPTIEMBllE 1967, PAGINA 1008

FOTOGRAFiA AEREA M ULTI-ESPECTRAL A COLORES Por Edward Yost y Sondra Wenderoth Se ha construido una camara para obtener se han obtenido, indican aplicaciones poten­ fotografias multi-espectrales en cuatro bandas, ciales de este sistema a la agricultura, con­ en la porci6n de 360 a 980 nm del espectro. La taminacion de las aguas usadas en silvicultura, fotointerpretaci6n se efectua en un visor aco­ analisis de suelos, determinacion de profundidad plado que presenta una version a colores com­ de aguas, asi como en los problemas militares de puesta de las cuatro fotografias por medio de adquisici6n de blancos y detecci6n de camuflaje. tecnicas de adici6n de color. Este sistema de La aplicaci6n satisfactoria de la tecnoJogia camara y visor combina la sensibilidad espectro­ multiespectral en una gran variedad de condi­ fotometrica con los principios de la colorime­ ciones fotograficas halladas en la deteccion re­ tria, permitiendo la detecci6n de las diferencias mota practica, depende de la precisi6n de las geofisicas apropiadas del terreno. tccnicas empleadas. Un procedimiento foto­ Se han efectuado numerosos experimentos grafico preciso, es esencial para obtener resulta­ utilizando sistemas con equipos basados en dos continuos correctos. estas tecnicas. Los resultados promisorios que PHOTO. ENGR.,SEPTIEMBRE 1967, PAGINA 1020 (Concluded on page 1166)

* Nota: Traducido por la Seccion de Tracucciones de la Escuela Cartogriifica del Servicio Geodesico Interamericano (fAGS).