To pass into history – 1917 year.
[The following article, which we reprint from our American contemporary "Aviation," very properly raises the question of the commercial future of photography in the air, in which there are certainly as great possibilities as in aerial navigation generally. The fact that one of the authors, Mr. L. J. R. Hoist, is a camera designer and instructor should emphasize to manufacturers of apparatus the field which is opened by the application of aerial machines to practical methods of surveying.-EDS. "B.J."]
The commercial future of aeroplane photography is not generally appreciated by aeroplane builders, since they have not as yet realised that preliminary surveying of high-roads, railroad, and other through routes can be done quicker and more cheaply by aeroplane photography than by any other know method. This is largely due to the circumstance that cameras have been hitherto considered as instruments of mere detail in the aeroplane industry as a whole, while as a matter of fact they will probably become one of the most important factors in the development of commercial aviation.
The most fruitful field of commercial work for the aero plans camera in the United States will undoubtedly be the completion if the topographic survey of this country, which work includes the complete detailing of maps not yet complete, as well as the mapping of territory which till now has not been charted at all. To this already extensive programmer should be added the locating of the high and low water lines along our coasts, besides work of more local interest, such as the production of correct maps of smaller communities for real estate records, the location of sites for dams for irrigation purposes, locating railroads and waterways through mountainous country, establishing aerial routes and emergency landings in vast wooded tracts, etc., all of which work can be done in a fraction of the time, and hence for a fraction of the cost entailed by a complete manual survey.
It should, however, not be understood that the aeroplane camera renders manual surveying obsolete. On the contrary, it is recognized that the camera does its most, effective work in connection with the slower but extremely accurate work of the surveyor, each one being checked up by the work of the other. In fact, it may be stated that the camera furnishes the fellahin details to a degree of perfection not attainable or, at any not obtained by manual survey whereas the latter produces a series of exact points of location which serve as control points for the data furnished by the camera.
To obtain useful commercial mapping results from an aero plane requires different methods of operation than those used to make the so-called mosaics, because maps will be made between control points often many miles apart, between which it will be necessary to fill in not only the general ground plan bat also the contour lines.
As a matter of fact, the mapping use of aeroplane photography has been to a certain extent set back by the war, for the reason that certain scientific views, which are not relevant to commercial photography, rather held the foreground. These were questions of emulsions and lens-openings, neither of which is really of as much importance as the purely mechanical sides of the problem.
Since all work enumerated before it this article is strictly of the order map-making, only such photographic apparatus as is adapted to this class of work need here be noted. This excludes all hand-held or hand-operated cameras as well as any semi-automat or non-automatic instruments which are rigidly attached to the aeroplane, since they partake with it in all its deviations from a perfectly horizontal position while in flight, a condition which is entirely incompatible with results for map-making. This leaves thus the freely suspended camera for further consideration.
The military use of the aeroplane camera, during the war has led not necessarily form correct precedents, became of the assumption that an observer is essential. This has led to undue emphasis being placed on hand-operated or semi-automatic cameras which were to be of such simplified construction that no imperfect functioning would under the hands of the operator-observed. These military conditions do not prevail, nor apply, in commercial aero plane-mapping.
A derision of opinion has existed relative to the automatic functions of aero plane cameras among the Allied Governments, but in n case were these difference caused by the requirement of aerial photographer mapping.
Paramount importance was attached the chemical side of aerial photography much attention being devoted in particular to the perfection of color sensitive emulation to more readily camouflaged objects from the real ones.
No doubt this aim is worth while but since existing emulsion used with suitable ray filters were apt to produce under the rough and ready condition of actual field photography behind the lines similar if not better remits with a far greater degrees of like hood to get any remits with a far greater degrees of like hood to get any results at all it must be regretted that these efforts placed the much more important mechanical side of the problem entirely in the shadow.
As long as the question of aero plane photography was a question of filling in maps with nearby control points the detail design of the camera was not of the highest importance. To take a series of views over well-known ground is a problem comparatively simple but commercial application of aero lane photography requires more definite information in regard to drainage, divides and general contour information at otherwise the main benefits of rapid surveying cannot be obtained. Photographic aero plane surveying will unquestionably be used largely as a supplement of the Photo-theologize and other more common methods of surveying, to fill in between control points established by the older methods. But in the case of the more or less unknown country between control points established by the more common methods and for such works a coast surveying arises the greatest need of aeroplane survey.
Surveying to be done from an aeroplane, particularly for filling in large areas, most is obtained by means of an automatic camera. On considering the problem of commercial aero plane photography it will soon be possible to pick out the necessary elements of design, and why an automatic camera should be used.
All photographs to be used in map making must be a true horizontal projection of the earth's surface. In this way all points are practically reproduced by the photograph on a scale which is equal to elevation divided by focal length of the lens. If the plane of the photograph is not approximately parallel to the plane of the earth's surface (not considering the curvature of the earth), positions will not be correctly rendered; there is distortion to location which is equal to the cosine of the angle of inclination. In actual military practice, however, any camera installation which fastens the camera to the plane so that in the average flying position the camera is approximately horizontal was considered sufficiently well for purposes of making mosaics.
Contour Mapping.
However this is by no means an answer to the question of obtaining survey information, as in the case of obtaining contours from aero plane photographs these can only be done by making use of the lens axis of the camera. The only way of establishing a true parallel position of the negative to the assumed plane of the earth is by means of the vertical position of the axis of tin- lens. Moreover, the only known factor for determining contours needed is the angular relation of the axis of the lens and its true focal length. With these two know factors the position of the earth can be graphically reproduced, and by ordinary drawing instruments a series of exposure points in the air can be determined. These exposure points are then used by transference of angles to reproduce graphically the location of points on the ground in space above or below the base line, which is usually taken at the foot of one of the axial points of the series of photographs.
To obtain a vertical position of the axis of the lens necessitates a free suspension of the camera in the body of an aero piano where it is not affected by the air current in motion. This is easily done by patting the camera in the Gimbals suspension similar to a ship's compass. However, it is necessary to maintain a constant position, as otherwise the variation in the rate of the aero plane through the air would introduce a pendulum action in the camera. There are only two ways whereby this can be done first, the use of a gyroscope, and, secondly, the use of gravity and suitable air cushions to prevent oscillation, but at the same time to allow motion relative to the swinging plane. In practice the latter method is it introduces no mechanical complications and the accuracy resulting is close enough for mapping purposes.
In to the requirements of angular position, aero plane photographic surveying requires a large number of photographs in a single flight. It is next to impossible to change plate magazines or to fill holders in flight sufficiently rapidly to obtain an unbroken series; moreover, it is necessary that photographs should be taken at what is practically a constant, interval between exposures; if not, there is very apt to be a break between two successive exposures which requires an additional flight to obtain the necessary information with which to fill the gap.
In a military sense, during the recent war, this matter was not of such importance as it becomes for photographic surveying, for the reason that usually photographs were wanted of a single spot only, and intervening places were not necessary, although at times desirable. This has led to the use of film cameras using roll films having as many as 100 exposures to each loading.
The principal difference between different makes of cameras for purposes of making military mosaics are usually matters of obtaining necessary power to change plates or film negatives.
Three methods of obtaining power have been used: the air fan electricity and spring motors.
In considering the design features of the camera it is very necessary to know to what extent photographic surveying will be carried out. If military mosaics are required, which generally are used simply to fill in small areas, a semiautomatic regardless of form of drive even a hand-operated? Camera -may be useful. If contour surveying is to be done it becomes necessary to have a very accurate timing device for the interval between exposures, as otherwise the overlapping relations between exposures will very easily be lost. The European continent is so thoroughly surveyed that the contour method has been very little used, and in the considerations of the necessary types of cameras, contour mapping has not been considered an essential feature. For this reasons the so-called automatic or semi-automatic camera ordinarily in use is not completely automatic, but simply is a power-driven mechanism which changes a plate or film negative and in which the actual exposure is made either by the or observer. There is only one completely automatic camera in existence which operates at varying rates of speed, controlled with certainty either by the pilot or the observer. This camera the Brock camera will be described later in detail.
The Triple Lens Camera.
When making flights for map-making purposes a given area of grand can only be fully covered if a number of flights in parallel courses are undertaken, each new flight following a course just overlapping the strip of land covered by the previous flight. The number of flights required to cover a given width of territory will then be directly proportional to the width of the strip embraced by the photographs, and consequently it is desirable to embrace as wide a strip of land as practicable in each view.
This consideration has led to the construction of triple lens cameras, the central lens pointing straight downward, and the lens on either side placed under an outwardly slanting angle. These three lenses are in one vertical plane at right angles to the direction of flight. The image-planes of the outer negatives are then usually set at an angle of 45 deg. to the central lens. These cameras are arranged for rigid attachment to the aeroplane. The two outer lenses will then produce pictures of strips of land extending far out at each side of the line of flight, whereas the central lens registers the views directly beneath the aeroplane and immediately adjoining regions.
Although this arrangement undoubtedly covers a width of territory not obtainable with a single lens, it introduces difficulties which seem to far outweigh its possible advantages, and at its best it is only suitable for use on fairly flat ground. The complications arising from (three different planes of projection, and the necessity of afterwards reducing the two outer planes to that of the central one, without definite knowledge of the actual position of either plane at the instant of exposure constantly serious obstacles to the usefulness and commercial adaptation of devices of this kind.
Timing Exposures.
It seems now preferable to obtain increased width of the strip of land depicted on the photography by increasing its width and reducing the focal length, of the lens, both to such limits as sound photographs and optical practice allow. A camera arranged to take negative. 5 x 7 ins., the 5-in. side in h line of flight and equipped with a lens of 8 ins. equivalent focus, embraces at 5000ft. elevation on each picture a strip of land 4,375ft. Wide and 3.125 ft. long on a scale of 1/7,500, or, if elevated to 6.666 ft., would produce pictures on a scale of 1/10,000 embracing 5,833 ft. in width by 4,166 in length. Most of high-grade lenses at 8-in. focus and openings of /'5 6 will cut a 5 x 7 in. image sharp and without distortion, and are serviceable for such work.
The succeeding exposures should be timed at intervals of distance depending on the elevation and corresponding length of ground taken by the picture. The speed of the flight translates this distance interval into a corresponding time interval. It is thus desirable to provide means by which the actual time interval can be varied according to these conditions and which will operate with dependable precision in its various adjustments.
As the manufacturers of the Brock Automatic Camera, it is probable that our actual experimental work in photographic surveying and in the design of cameras to obtain these results has been carried far beyond experimental work in this line by any individuals or by any of the Governments who have participated in the European war. The system of installation which we use has been found to permit exposures of from two to five times the exposure possible with any other system of installation, with all its attendant benefits. No other camera has been successfully built -with a system of free suspension, with the result, also, that no other camera has been built which can also do contour surveying. The importance of contour surveying in a military -sense was becoming recognized as the war drew to a close, and consequently this field of endeavour has had but a fraction of the effort put on it which will be devoted to it in the next few years.
Another feature of camera design which originated with the Brock camera was the use of film for military purposes. The Brock film camera has been made successfully to use film in two sizes: one 4x5 ins., using ordinarily a 12-in. lens, and one 8 x 10 ins., using a 24-in. lens, both types completely automatic. These cameras were the first ones used which entirely eliminated static electricity in the film, and are to-day the only ones in which static electricity never occurs. The system of obtaining large negatives on film without distortion through the use of a glass support originated with the Brock 8 x 10 in. cameras. However, the use of the large negative sizes will be confined to military purposes, as for commercial or ordinary Government mapping a very large scale is not required.
The Brock Camera.
Let us now consider some of the mechanical details of the Brock 4x5 in. camera and the reasons for the design. This camera is the outgrowth of a series of experiments to obtain a free suspension camera, motor driven, within reasonable weight, certain of operation, and controlled from a distant point, for use in a single-sealed aeroplane.
Mechanical experiments led us into the design of the spring motor-driven cameras and resulted in our finding commercially feasible means of controlling the speed of operation of the spring -motor not in any way dependent on friction. The entire engineering profession is aware of the difficulty of obtaining a satisfactory variable speed friction drive which includes both certainty of operation and certainty of speed. In the case of a spring motor this difficulty is emphasized because the control through friction must be done fey the introduction of a governor. Such a construction results in absolute loss of control of speed regulation owing partly Ito the different percentage of friction due to moisture in the air -in varying amounts. The system we use, therefore, is the control of the main spring motor by means of an auxiliary spring motor, which is in effect a clock with a speed regulation of 3*(1/2) to 1. The importance of accurate speed control in the case of the Brock camera is greater than in any other camera, because of its ability to produce contour maps. Wherever it becomes necessary to obtain contours the axial point of the succeeding and the preceding negative must show on each exposure, with the result that exposures must occur at frequent and constant intervals without interruption, other wise a break will partially destroy the value of the contour photographic flight.
Without any exceptions worth mentioning, all aerial cameras are fitted with focal-plane shutters, as the high efficiency of this type shutter, together with its extremes mechanical simplicity, renders is superior to the between lens shelter. In the Brock cameras focal-plane shutters are prevailed with a fixed slot, the speed adjustment being obtained entirely by means of the spring tension. Spends are variable from 1/50 to 200 of a second or any similar range.
Shutter Speeds.
Is connection with shutter spends of aero plane cameras it is of particular interest to mention that, owing to the almost absence of vibration obtained, shutter spends can be regulated entirely with regard to the elimination of image movement through traveling spend, and no regard need be paid to the effect of vibration. The requirements due to speed are very easily met. For instance, a camera as already mentioned, fitted with an 8ft lens flying at 5.000 ft. elevation and at 120 m.p.h., reprises no faster exposure than 1 20 of a second to give a sharpness corresponding to a circle of confusion of 14/1000 of an inch and with a speed of 1/50 of a second the circle of confusion will be only 5.6 1000 of an inch. This means that the gumball-suspension has the practical advantage of being able to photograph both earlier and later in the day than would be practical with rigidly suspended cameras, which are generally, used with minimum exposures of 1/100 of a second to eliminate as much as possible the effect of vibration. It also means that such focal-plane shatters can be made with wide slots to reduce the duration to the expense period and thereby the resulting distortion to a perfectly negligible minimum, which in turn eliminates the only meeting valid arguments against focal-plane shutters.
Evenness of Exposure.
A perfect eventide of exposure at all speeds can easily be obtained by proper design of the focal-plane shutter, in fact so that the slightest difference in exposure plainly seen is aero plane films, since maximum and minimum density adjoin in two negatives of a strip of continuous pictures side by show no observable variation. Especially in the civil or commercial uses of aero plane photography do these various features come to their fullest signification and it is only on this account that they have mentioned in detail.
Summarizing the status and development of aero plane photographic surveying is an art still in its infancy. Under the aeronautical trade recognizes the possibilities of this use of their product, and unless the engineering profession of the business realize its possibilities it is not likely that the growth will be rapid. It is an unfortunate fact that up to the present time the energy developed to the constructive surveying features of aero plane photography has been sun fined to, and understood by only a few people.
The truth of the matter is that, in the first place the emission questions which have arisen in Europe are really matters of is serious consequence although a considerable amount of time and effect has been spent to change and to make special emulsion. Efforts have also been made in Europe to make emulsions sensitive to certain colour whereas similar is not better, affects could have been obtained with existing emulsions by the use of a co1our screen and lengthened exposures. Whatever efforts have been made in the mechanical direction they have been very largely in the direction of making a camera which could be operated by the average observer without any knowledge of photography or of mechanics is safe to say that if the motion picture industry which is now in existent had had a military use and military development, the present stage of perfected would never have been leached, as the mechanical side of the motion picture industry has received a tremendous amount of attention for years. High-class motion pictures cannot be made by anyone but a skilled operator, and in exactly the way the success of the future of aeroplane surveying is dependent upon operators learning about the necessary photography and mechanics of aeroplane cameras; but it must be recognized for the future of aero plane surveying that apart from the development of the negative and the printing of the prints aero plane photography is a matter which should be entirely in the hands of civil and mechanical engineers.
Aeroplane Photography and Commerce.
The modern aeroplane has reached the static of development where the safety of operation and the certainty of operation are as good as of the average automobile of ten years ago. This means that an aeroplane can be used for some hours over entirely unknown country without any landing places: it further means that such country can be completely and correctly mapped without difficulty and at a very low expense compared to the present-day methods of surveying. It is quite possible that in the future preliminary surveys will be made for railroads, roads, and various water-works entirely by aero plane photography and that when the line has been decided upon a party will go out simply for the purpose of staking agreed-upon lines. If the aeroplane industry will devote as much attention to this subject as has been devoted to other commercial possibilities, a rapid growth can be expected. As foundation-stones have been laid, it is only necessary for a general interest in this subject to be awakened by the aero plane industry before possibilities, first as a internment enterprise, and then as a commercial enterprise for civil engineers will find general recognition.
[The following article, which we reprint from our American contemporary "Aviation," very properly raises the question of the commercial future of photography in the air, in which there are certainly as great possibilities as in aerial navigation generally. The fact that one of the authors, Mr. L. J. R. Hoist, is a camera designer and instructor should emphasize to manufacturers of apparatus the field which is opened by the application of aerial machines to practical methods of surveying.-EDS. "B.J."]
The commercial future of aeroplane photography is not generally appreciated by aeroplane builders, since they have not as yet realised that preliminary surveying of high-roads, railroad, and other through routes can be done quicker and more cheaply by aeroplane photography than by any other know method. This is largely due to the circumstance that cameras have been hitherto considered as instruments of mere detail in the aeroplane industry as a whole, while as a matter of fact they will probably become one of the most important factors in the development of commercial aviation.
The most fruitful field of commercial work for the aero plans camera in the United States will undoubtedly be the completion if the topographic survey of this country, which work includes the complete detailing of maps not yet complete, as well as the mapping of territory which till now has not been charted at all. To this already extensive programmer should be added the locating of the high and low water lines along our coasts, besides work of more local interest, such as the production of correct maps of smaller communities for real estate records, the location of sites for dams for irrigation purposes, locating railroads and waterways through mountainous country, establishing aerial routes and emergency landings in vast wooded tracts, etc., all of which work can be done in a fraction of the time, and hence for a fraction of the cost entailed by a complete manual survey.
It should, however, not be understood that the aeroplane camera renders manual surveying obsolete. On the contrary, it is recognized that the camera does its most, effective work in connection with the slower but extremely accurate work of the surveyor, each one being checked up by the work of the other. In fact, it may be stated that the camera furnishes the fellahin details to a degree of perfection not attainable or, at any not obtained by manual survey whereas the latter produces a series of exact points of location which serve as control points for the data furnished by the camera.
To obtain useful commercial mapping results from an aero plane requires different methods of operation than those used to make the so-called mosaics, because maps will be made between control points often many miles apart, between which it will be necessary to fill in not only the general ground plan bat also the contour lines.
As a matter of fact, the mapping use of aeroplane photography has been to a certain extent set back by the war, for the reason that certain scientific views, which are not relevant to commercial photography, rather held the foreground. These were questions of emulsions and lens-openings, neither of which is really of as much importance as the purely mechanical sides of the problem.
Since all work enumerated before it this article is strictly of the order map-making, only such photographic apparatus as is adapted to this class of work need here be noted. This excludes all hand-held or hand-operated cameras as well as any semi-automat or non-automatic instruments which are rigidly attached to the aeroplane, since they partake with it in all its deviations from a perfectly horizontal position while in flight, a condition which is entirely incompatible with results for map-making. This leaves thus the freely suspended camera for further consideration.
The military use of the aeroplane camera, during the war has led not necessarily form correct precedents, became of the assumption that an observer is essential. This has led to undue emphasis being placed on hand-operated or semi-automatic cameras which were to be of such simplified construction that no imperfect functioning would under the hands of the operator-observed. These military conditions do not prevail, nor apply, in commercial aero plane-mapping.
A derision of opinion has existed relative to the automatic functions of aero plane cameras among the Allied Governments, but in n case were these difference caused by the requirement of aerial photographer mapping.
Paramount importance was attached the chemical side of aerial photography much attention being devoted in particular to the perfection of color sensitive emulation to more readily camouflaged objects from the real ones.
No doubt this aim is worth while but since existing emulsion used with suitable ray filters were apt to produce under the rough and ready condition of actual field photography behind the lines similar if not better remits with a far greater degrees of like hood to get any remits with a far greater degrees of like hood to get any results at all it must be regretted that these efforts placed the much more important mechanical side of the problem entirely in the shadow.
As long as the question of aero plane photography was a question of filling in maps with nearby control points the detail design of the camera was not of the highest importance. To take a series of views over well-known ground is a problem comparatively simple but commercial application of aero lane photography requires more definite information in regard to drainage, divides and general contour information at otherwise the main benefits of rapid surveying cannot be obtained. Photographic aero plane surveying will unquestionably be used largely as a supplement of the Photo-theologize and other more common methods of surveying, to fill in between control points established by the older methods. But in the case of the more or less unknown country between control points established by the more common methods and for such works a coast surveying arises the greatest need of aeroplane survey.
Surveying to be done from an aeroplane, particularly for filling in large areas, most is obtained by means of an automatic camera. On considering the problem of commercial aero plane photography it will soon be possible to pick out the necessary elements of design, and why an automatic camera should be used.
All photographs to be used in map making must be a true horizontal projection of the earth's surface. In this way all points are practically reproduced by the photograph on a scale which is equal to elevation divided by focal length of the lens. If the plane of the photograph is not approximately parallel to the plane of the earth's surface (not considering the curvature of the earth), positions will not be correctly rendered; there is distortion to location which is equal to the cosine of the angle of inclination. In actual military practice, however, any camera installation which fastens the camera to the plane so that in the average flying position the camera is approximately horizontal was considered sufficiently well for purposes of making mosaics.
Contour Mapping.
However this is by no means an answer to the question of obtaining survey information, as in the case of obtaining contours from aero plane photographs these can only be done by making use of the lens axis of the camera. The only way of establishing a true parallel position of the negative to the assumed plane of the earth is by means of the vertical position of the axis of tin- lens. Moreover, the only known factor for determining contours needed is the angular relation of the axis of the lens and its true focal length. With these two know factors the position of the earth can be graphically reproduced, and by ordinary drawing instruments a series of exposure points in the air can be determined. These exposure points are then used by transference of angles to reproduce graphically the location of points on the ground in space above or below the base line, which is usually taken at the foot of one of the axial points of the series of photographs.
To obtain a vertical position of the axis of the lens necessitates a free suspension of the camera in the body of an aero piano where it is not affected by the air current in motion. This is easily done by patting the camera in the Gimbals suspension similar to a ship's compass. However, it is necessary to maintain a constant position, as otherwise the variation in the rate of the aero plane through the air would introduce a pendulum action in the camera. There are only two ways whereby this can be done first, the use of a gyroscope, and, secondly, the use of gravity and suitable air cushions to prevent oscillation, but at the same time to allow motion relative to the swinging plane. In practice the latter method is it introduces no mechanical complications and the accuracy resulting is close enough for mapping purposes.
In to the requirements of angular position, aero plane photographic surveying requires a large number of photographs in a single flight. It is next to impossible to change plate magazines or to fill holders in flight sufficiently rapidly to obtain an unbroken series; moreover, it is necessary that photographs should be taken at what is practically a constant, interval between exposures; if not, there is very apt to be a break between two successive exposures which requires an additional flight to obtain the necessary information with which to fill the gap.
In a military sense, during the recent war, this matter was not of such importance as it becomes for photographic surveying, for the reason that usually photographs were wanted of a single spot only, and intervening places were not necessary, although at times desirable. This has led to the use of film cameras using roll films having as many as 100 exposures to each loading.
The principal difference between different makes of cameras for purposes of making military mosaics are usually matters of obtaining necessary power to change plates or film negatives.
Three methods of obtaining power have been used: the air fan electricity and spring motors.
In considering the design features of the camera it is very necessary to know to what extent photographic surveying will be carried out. If military mosaics are required, which generally are used simply to fill in small areas, a semiautomatic regardless of form of drive even a hand-operated? Camera -may be useful. If contour surveying is to be done it becomes necessary to have a very accurate timing device for the interval between exposures, as otherwise the overlapping relations between exposures will very easily be lost. The European continent is so thoroughly surveyed that the contour method has been very little used, and in the considerations of the necessary types of cameras, contour mapping has not been considered an essential feature. For this reasons the so-called automatic or semi-automatic camera ordinarily in use is not completely automatic, but simply is a power-driven mechanism which changes a plate or film negative and in which the actual exposure is made either by the or observer. There is only one completely automatic camera in existence which operates at varying rates of speed, controlled with certainty either by the pilot or the observer. This camera the Brock camera will be described later in detail.
The Triple Lens Camera.
When making flights for map-making purposes a given area of grand can only be fully covered if a number of flights in parallel courses are undertaken, each new flight following a course just overlapping the strip of land covered by the previous flight. The number of flights required to cover a given width of territory will then be directly proportional to the width of the strip embraced by the photographs, and consequently it is desirable to embrace as wide a strip of land as practicable in each view.
This consideration has led to the construction of triple lens cameras, the central lens pointing straight downward, and the lens on either side placed under an outwardly slanting angle. These three lenses are in one vertical plane at right angles to the direction of flight. The image-planes of the outer negatives are then usually set at an angle of 45 deg. to the central lens. These cameras are arranged for rigid attachment to the aeroplane. The two outer lenses will then produce pictures of strips of land extending far out at each side of the line of flight, whereas the central lens registers the views directly beneath the aeroplane and immediately adjoining regions.
Although this arrangement undoubtedly covers a width of territory not obtainable with a single lens, it introduces difficulties which seem to far outweigh its possible advantages, and at its best it is only suitable for use on fairly flat ground. The complications arising from (three different planes of projection, and the necessity of afterwards reducing the two outer planes to that of the central one, without definite knowledge of the actual position of either plane at the instant of exposure constantly serious obstacles to the usefulness and commercial adaptation of devices of this kind.
Timing Exposures.
It seems now preferable to obtain increased width of the strip of land depicted on the photography by increasing its width and reducing the focal length, of the lens, both to such limits as sound photographs and optical practice allow. A camera arranged to take negative. 5 x 7 ins., the 5-in. side in h line of flight and equipped with a lens of 8 ins. equivalent focus, embraces at 5000ft. elevation on each picture a strip of land 4,375ft. Wide and 3.125 ft. long on a scale of 1/7,500, or, if elevated to 6.666 ft., would produce pictures on a scale of 1/10,000 embracing 5,833 ft. in width by 4,166 in length. Most of high-grade lenses at 8-in. focus and openings of /'5 6 will cut a 5 x 7 in. image sharp and without distortion, and are serviceable for such work.
The succeeding exposures should be timed at intervals of distance depending on the elevation and corresponding length of ground taken by the picture. The speed of the flight translates this distance interval into a corresponding time interval. It is thus desirable to provide means by which the actual time interval can be varied according to these conditions and which will operate with dependable precision in its various adjustments.
As the manufacturers of the Brock Automatic Camera, it is probable that our actual experimental work in photographic surveying and in the design of cameras to obtain these results has been carried far beyond experimental work in this line by any individuals or by any of the Governments who have participated in the European war. The system of installation which we use has been found to permit exposures of from two to five times the exposure possible with any other system of installation, with all its attendant benefits. No other camera has been successfully built -with a system of free suspension, with the result, also, that no other camera has been built which can also do contour surveying. The importance of contour surveying in a military -sense was becoming recognized as the war drew to a close, and consequently this field of endeavour has had but a fraction of the effort put on it which will be devoted to it in the next few years.
Another feature of camera design which originated with the Brock camera was the use of film for military purposes. The Brock film camera has been made successfully to use film in two sizes: one 4x5 ins., using ordinarily a 12-in. lens, and one 8 x 10 ins., using a 24-in. lens, both types completely automatic. These cameras were the first ones used which entirely eliminated static electricity in the film, and are to-day the only ones in which static electricity never occurs. The system of obtaining large negatives on film without distortion through the use of a glass support originated with the Brock 8 x 10 in. cameras. However, the use of the large negative sizes will be confined to military purposes, as for commercial or ordinary Government mapping a very large scale is not required.
The Brock Camera.
Let us now consider some of the mechanical details of the Brock 4x5 in. camera and the reasons for the design. This camera is the outgrowth of a series of experiments to obtain a free suspension camera, motor driven, within reasonable weight, certain of operation, and controlled from a distant point, for use in a single-sealed aeroplane.
Mechanical experiments led us into the design of the spring motor-driven cameras and resulted in our finding commercially feasible means of controlling the speed of operation of the spring -motor not in any way dependent on friction. The entire engineering profession is aware of the difficulty of obtaining a satisfactory variable speed friction drive which includes both certainty of operation and certainty of speed. In the case of a spring motor this difficulty is emphasized because the control through friction must be done fey the introduction of a governor. Such a construction results in absolute loss of control of speed regulation owing partly Ito the different percentage of friction due to moisture in the air -in varying amounts. The system we use, therefore, is the control of the main spring motor by means of an auxiliary spring motor, which is in effect a clock with a speed regulation of 3*(1/2) to 1. The importance of accurate speed control in the case of the Brock camera is greater than in any other camera, because of its ability to produce contour maps. Wherever it becomes necessary to obtain contours the axial point of the succeeding and the preceding negative must show on each exposure, with the result that exposures must occur at frequent and constant intervals without interruption, other wise a break will partially destroy the value of the contour photographic flight.
Without any exceptions worth mentioning, all aerial cameras are fitted with focal-plane shutters, as the high efficiency of this type shutter, together with its extremes mechanical simplicity, renders is superior to the between lens shelter. In the Brock cameras focal-plane shutters are prevailed with a fixed slot, the speed adjustment being obtained entirely by means of the spring tension. Spends are variable from 1/50 to 200 of a second or any similar range.
Shutter Speeds.
Is connection with shutter spends of aero plane cameras it is of particular interest to mention that, owing to the almost absence of vibration obtained, shutter spends can be regulated entirely with regard to the elimination of image movement through traveling spend, and no regard need be paid to the effect of vibration. The requirements due to speed are very easily met. For instance, a camera as already mentioned, fitted with an 8ft lens flying at 5.000 ft. elevation and at 120 m.p.h., reprises no faster exposure than 1 20 of a second to give a sharpness corresponding to a circle of confusion of 14/1000 of an inch and with a speed of 1/50 of a second the circle of confusion will be only 5.6 1000 of an inch. This means that the gumball-suspension has the practical advantage of being able to photograph both earlier and later in the day than would be practical with rigidly suspended cameras, which are generally, used with minimum exposures of 1/100 of a second to eliminate as much as possible the effect of vibration. It also means that such focal-plane shatters can be made with wide slots to reduce the duration to the expense period and thereby the resulting distortion to a perfectly negligible minimum, which in turn eliminates the only meeting valid arguments against focal-plane shutters.
Evenness of Exposure.
A perfect eventide of exposure at all speeds can easily be obtained by proper design of the focal-plane shutter, in fact so that the slightest difference in exposure plainly seen is aero plane films, since maximum and minimum density adjoin in two negatives of a strip of continuous pictures side by show no observable variation. Especially in the civil or commercial uses of aero plane photography do these various features come to their fullest signification and it is only on this account that they have mentioned in detail.
Summarizing the status and development of aero plane photographic surveying is an art still in its infancy. Under the aeronautical trade recognizes the possibilities of this use of their product, and unless the engineering profession of the business realize its possibilities it is not likely that the growth will be rapid. It is an unfortunate fact that up to the present time the energy developed to the constructive surveying features of aero plane photography has been sun fined to, and understood by only a few people.
The truth of the matter is that, in the first place the emission questions which have arisen in Europe are really matters of is serious consequence although a considerable amount of time and effect has been spent to change and to make special emulsion. Efforts have also been made in Europe to make emulsions sensitive to certain colour whereas similar is not better, affects could have been obtained with existing emulsions by the use of a co1our screen and lengthened exposures. Whatever efforts have been made in the mechanical direction they have been very largely in the direction of making a camera which could be operated by the average observer without any knowledge of photography or of mechanics is safe to say that if the motion picture industry which is now in existent had had a military use and military development, the present stage of perfected would never have been leached, as the mechanical side of the motion picture industry has received a tremendous amount of attention for years. High-class motion pictures cannot be made by anyone but a skilled operator, and in exactly the way the success of the future of aeroplane surveying is dependent upon operators learning about the necessary photography and mechanics of aeroplane cameras; but it must be recognized for the future of aero plane surveying that apart from the development of the negative and the printing of the prints aero plane photography is a matter which should be entirely in the hands of civil and mechanical engineers.
Aeroplane Photography and Commerce.
The modern aeroplane has reached the static of development where the safety of operation and the certainty of operation are as good as of the average automobile of ten years ago. This means that an aeroplane can be used for some hours over entirely unknown country without any landing places: it further means that such country can be completely and correctly mapped without difficulty and at a very low expense compared to the present-day methods of surveying. It is quite possible that in the future preliminary surveys will be made for railroads, roads, and various water-works entirely by aero plane photography and that when the line has been decided upon a party will go out simply for the purpose of staking agreed-upon lines. If the aeroplane industry will devote as much attention to this subject as has been devoted to other commercial possibilities, a rapid growth can be expected. As foundation-stones have been laid, it is only necessary for a general interest in this subject to be awakened by the aero plane industry before possibilities, first as a internment enterprise, and then as a commercial enterprise for civil engineers will find general recognition.
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