Applied aerial photography, with a chapter on Oblique aerial surveys (Canada) by A.M. Narraway

Front matter

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Preface

Tue purpose of this book is to make available under one cover a compact study of modern aerial photography. Published matter on this subject is today largely available only in scattered form in periodicals and pamphlets. Furthermore in view of the rapid development of aerial photography since the war most of this literature has become obsolete. An exception to this is the excellent work produced by the United States Army Air Corps, an invaluable source of material from which the author has obtained most of his data.

In the single decade since the world war, aerial photography has progressed from crude rule-of-thumb work to that of a science. Cameras and instruments are improved to a degree which at last makes accurate work possible. The success with which the U. S. Geological Survey, the Corps of Engineers of the U. S. Army, and the Surveys Bureau (Department of the Interior) of Canada have employed aerial photography substantially indicates its value as a surveying method of precision.

Engineers are gradually accepting aerial photography as a sound supplementary mapping method. It seems inevitable that the aerial survey must, in time, become indispensable for mapping. In mapping inaccessible territory the aerial survey has already assumed priority, as shown by the success of the Hamilton Rice Expedition in mapping hitherto inaccessible regions in tropical South America.

The author is deeply indebted to the United States Army Air Corps, not only as his primary source of information but as the school in which he received all of his training. He is also under obligations to Captain A. W. Stevens and Lieuts. Ployer P. Hill and Edward P. Gaines of the United States Army Air Corps, to Glenn Phelps, Edward R. Polley and F. O. Strowger. Particular thanks are due Miss M. E. Ferguson for her careful work on the manuscript. The book itself would never have appeared had it not been for my wife and Morton W. Royse, Ph.D., of Harvard

BG PREFACE

University, authority on aeronautics, to whom, in my absence with the Byrd Antarctic Expedition, was left all the labor of preparing the rough manuscript for publication.

Inasmuch as the Surveys Bureau of the Department of the Interior of Canada has done unique work in aerial surveying by the grid system the value of the present book is greatly enhanced by the addition of Chapter XXI by A. M. Narraway, now chief aerial surveys engineer of the Canadian Department.

A. C. McKIntey.

June, 1929.

Foreword

WHEN one opens the old family album and views the coldly severe pictures of grandma and grandpa, it is difficult to realize that these pictures marked the very beginning of practical photography. It is true that some of our ancestors, centuries and centuries ago, must have noted the tanning of the skin in the summer sun. Today it is possible, by use of a quartz lens and sufficient exposure, to make a negative upon the human body, and it is a comparatively simple matter to print through an ordinary celluloid negative, with the aid of ultraviolet light or sunlight, a positive picture upon one’s own skin.

Even Adam in the Garden of Eden must have noticed the change in appearance of the skin of an apple, where a leaf or a twig hid its surface from direct sunlight. This was a true photographic process where a chemical change was effected by light itself, even though the exposure ran into a matter of days and weeks.

Less than eighty years ago, it was discovered that certain chemical salts were far more sensitive to light action. The principle of the lens was already known and it shortly became possible to make photographs much as we know them today. However, the time still ran into minutes and seconds of exposure. Development of more sensitive emulsions and the design of better lenses made high speed photography possible at the beginning of the present century. Some aerial photography had been accomplished with the aid of balloons and kites; therefore it was natural, with the advent of the airplane, that photography, previously a ground process, should receive extensive application in the airplane, The world war gave a great impetus to aerial photography, and aerial cameras were designed by the score. These cameras were made more with the intention of getting sharp, clear pictures of enemy territory than for the purposes of accurate topographic mapping.

In the years since the war much effort has been made to use the x1 xii FOREWORD aerial camera as an aid to the preparation of accurate maps or the making of good photographic mosaics. Much of this work naturally fell to the Air Services of the Army and Navy, and chief among the photographic officers who strove for better, and still better, results was Captain A. C. McKinley. Not only did he do outstanding work in the development of apparatus and the making of successful missions, but he endeavored to school others in the most practical methods of making successful aerial photographs. It was logical that he should be chosen by Commander Byrd’s Expedition for the important pioneer work in aerial photography in the south polar regions.

Before Captain McKinley left on this mission he prepared the subject matter of this book, which is intended to give the would-be aerial photographer a practical working knowledge of modern aerial cameras, photographic emulsions, and photographic processes. While it is not possible to cover the story of photography entirely in one volume, it is believed that Captain McKinley has furnished the vital information that will put the average photographer on the right road to successful results.

It is to be remembered that at the time of publication of this volume Captain McKinley is still in the south polar regions coping with the inevitable difficulties and trials that any pioneer must meet while struggling under new and trying conditions.

Wricut Fier, A. W. STEVENS, June, 1929. Captain U. S. Army Air Corps.

Contents

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Part I. TAKING AERIAL PHOTOGRAPHS CHAPTER

Part Il. FINISHING THE PHOTOGRAPHS VIII. Tue Design or A PuHotocrapuic LABORATORY.............. 141 IEXS PHOTO GRAPHICE CBE MIST Riverine: ane ann th ener ieee 154 Xe ADEVETO PEN GN se ce Ne te es eet: en ae ete AO Vey pee 187 XCEL SPIRIN TIN Git cas hia ee erie aap pee rey ea Meer enely Sins came oe 198 DELTA COBVING Sener eee ORR ee rage ed ate, pu eine ce 217 Part III. MAPPING WITH OR FROM THE PHOTOGRAPHS QUAI Wenaved Denshonopd SGaNaays; COND IMLS 4 A sine Ae ts een Gots Geog 6 aoe oe 229 XIV. Types or AmRIAL PHOTOGRAPHIC HRRORS...............-.-- 236 DOV IN GNIS) AGNAOY INL DEIROVNDIINC Rio ng oun onuecoocsusbncecsuasdeas 244 DOV LMT Ssscolo aes Sle ree Te ean a rani Weer 251 EVA AC ONTROUSH OES eycreh re ecteat Crp eeu: He eed eno th cin re 255 POVULT CACSHMBTING cDHE VIOSATC een ts eee in nen tre arene nem 260 DOS ENE DEH ODS £ OF ASS RIMIB Tny-ceca eee teat ede are Set eae eee a 278 XX. Mrtuops or ASSEMBLING LINE Maps—RESTITUTION........... 287 OBLIQUE AERIAL SURVEYS (CANADA)

By A. M. Narraway, Chief Aerial Surveys Engineer, Department of the Interior, Canada

APPLIED AERIAL PHOTOGRAPHY

APPLIED AERIAL PHOTOGRAPHY

Introduction

AERIAL PHotocrapHy is the work of three types of craftsmen: the pilots and photographers, the laboratory personnel, and the draftsmen. The work of an aerial photographic organization, therefore, is divided into three departments. Although the duties of each of these departments are separate, it is absolutely essential that the closest possible cooperation exist between them. This can be accomplished effectively only when the entire personnel have a general understanding of each other’s duties and problems and a general knowledge of the work of the topographer. Following the work of an aerial photographic section in operation, the subject of aerial photography naturally falls into three parts.

I. The first part covers the taking of aerial photographs. It includes the duties and problems of the pilot and photographer together with a study of navigation and photographic instruments.

II. The second part covers the work involved in finishing the photographs, the work of the laboratory personnel.

III. The third part covers the methods of using aerial photographs for mapping. Two general methods are used in applying aerial photographs to mapping, namely, making a map with the photographs and constructing a map from the photographs. The former method is covered in detail in this book since it comes directly under the work of an aerial photographic organization. The latter method, however, is treated in only a general way. Compiling maps from data obtained on aerial photographs is essentially the duty of a mapping agent or topographer. A sufficient amount of material on this subject, therefore, is included in order that the personnel of the three departments will have a fair understanding of the problems of the topographer when constructing a line map from aerial photographs. This knowledge

2 INTRODUCTION will lead to more efficient cooperation and will improve the general results. The topographer should also know something of the problems connected with the work of an aerial photographic organization, especially the work of the flying department.

Aerial photographic mapping is based on ground photography and surveying. Although many readers are doubtless familiar with one or both of these subjects it is deemed essential for the sake of clarity and convenience to touch upon them, thus making it possible for the average reader to continue through the book without the necessity of constantly referring to texts on these subjects. It is recommended, however, that the student of aerial photography, if he does not already possess a working knowledge of these sciences, make a fairly complete study of them.

Every photographic project or mission will have probiems which concern that mission alone. It will be seldom that a regular routine can be followed from start to finish. There are certain operations, however, which will be performed regardless of the character of the mission in hand. These duties are performed in the following order:

1. The ground preparation by the flying crew. This will include:

a. Locating the area to be mapped in relation to the point from which the flight is to start.

b. Obtaining all necessary flying information in regard to that area, which will necessitate obtaining maps or making a reconnaissance flight or both.

c. Choosing and installing the photographic and navigation instruments.

d. A general study of the weather and light conditions to be expected over the area to be photographed.

2. Photographing the area.

3. Developing the aerial film. Due to its sensitive nature there will be a rapid deterioration of aerial film upon exposure to the air. This calls for development as soon as practicable after each flight.

4. The inspecting and numbering of negatives.

INTRODUCTION 3

5. Making the prints. This is done by one of three methods, namely, contact printing, projection printing, and rectifying. The prints will be made by contact if the scale of the negatives is to be accepted as the scale of the prints. If a uniform scale is to be desired, all or part of the prints will be made by projection, that is, the prints will be made larger or smaller than the negative from which they are printed. If a uniform scale and corrections for tilt are desired, then a rectifying camera will be used. The resultant accuracy of these three methods increases in the order named. The speed of production, however, will decrease in the order named.

6. Making the index map. 7. Finally, preparing the map from or with the prints.

In a broad sense the methods of performing the operations 3, 4, and 6 are constant. Operations 1, 2, 5, and 7 will depend to a great extent upon the following factors: the accuracy required in the finished map, the use of the map, and the time and material allotted to the project. These factors, however, are very general. There will usually be other considerations which will have a direct bearing on the methods used, namely, the relief of the terrain, the size of the area to be mapped, the location of the area, both in respect to climate and distance from the home station, and the type of equipment.

There may be some mapping projects of sufficient size and distance from the home station to warrant the setting up of a small laboratory in the vicinity of the area to be mapped. This temporary laboratory will usually do only the work of finishing the negatives and preparing rough index maps therefrom. The remaining operations will almost always be performed at the home station. Due to the time saved in shipping film by Air Mail it will rarely be necessary to use a temporary laboratory when mapping areas included in the continental limits of the United States. When a large area, however, is to be mapped and it is so situated that it is beyond reach of fast transportation or located in a hot climate, it will always be advisable to set up a temporary laboratory.

A surprising amount of accurate work can be accomplished

4 INTRODUCTION daily by a small, competent, well-trained crew. The combination of quality and quantity, however, can be obtained only by a well-trained personnel. A trained laboratory crew of four men can finish from 200 to 400 aerial photographs during a ten-hour day. Using a single-lens vertical camera the pilot and photographer during a photographic day can fly and photograph from 100 to 250 square miles. Quality, rather than quantity, must always receive prime consideration. The quantity of work accomplished with speed is a very important factor, however, since the practical value, the raison d’étre of aerial photography as a mapping method, rests upon the speed of action and the consequent wide range of operation.

Considering a map as a representation of details on the earth’s surface, the aerial camera records these details in a manner impossible to obtain by any other method. In other words, every detail of the area covered by the camera is recorded. On the other hand, when a map is considered as an accurate location of points on the earth’s surface, the work of the ground surveyor is more accurate. These citations are not intended to imply, on one hand, that the ground survey cannot record sufficient details; or, on the other hand, that distances cannot be measured on aerial photographs. They are intended rather to show that by combining the two methods the map produced will be the best possible representation of the earth’s surface. The extent, therefore, to which either of these methods is used in surveying an area will depend upon whether the resultant map is to be used to study the features existing in the area or whether it is to be used to locate peints accurately. The majority of mapping projects will necessitate combining to a greater or less degree these two methods.

Aerial photography has justified its existence as a surveying method by its two outstanding factors, namely, speed and recording of details. On the other hand, ground surveying is always needed to a greater or less extent on all photographic surveys except when the uncontrolled mosaic is used. The uncontrolled mosaic can only in a very broad sense be considered a map. In reality, it is only a composite picture of the earth’s surface.

It may be noted that no one factor has retarded the advancement of both military and commercial aerial photography so much as the assertion of exaggerated claims. The most outstanding of these claims is that aerial photography has supplanted, or

INTRODUCTION 5 will supplant, all ground surveying. No method for using aerial photographs has been devised or can be foreseen at this time which will make ground surveying obsolete.

As already stated, aerial photography when applied to mapping takes advantage of the data obtained by ground survey, but the new aerial method can be applied in so sweeping a manner that the exclusive use of ground mapping will in time become obsolete.

Part I TAKING AERIAL PHOTOGRAPHS

Chapter I The Duties And Problems Of The Pilot And The Photographer

THE subjects treated in this chapter form the foundation of aerial photography. If the operations involved in the performance of the duties of the pilot and the photographer are not accomplished with precision and knowledge of the work, a project is destined from the start to be a failure. When flying and taking aerial photographs for a mapping mission the pilot and the photographer in reality become surveyors and each part of the equipment used becomes a surveying instrument. All operations, therefore, must be performed with the same accuracy as that demanded of a surveyor and his instruments on the ground.

The Personnel.—The personnel engaged in this work, that is, the pilot and the photographer, must be men who have had a thorough photographic training. The pilot must have as a foundation a natural flying ability and must be more than an average pilot. As will be seen, the pilot, even with this general technical background, must spend many hours practicing the methods to be used in flying a photographic mission.

The photographer need not have previous air experience although this qualification will be helpful, but he must undergo a thorough course in all branches of aerial photography.

Photographic Mapping Mission.—Each photographic mapping mission will be accomplished under different circumstances and conditions than all other mapping missions. The general routine, however, encountered on any mapping mission remains constant, and can therefore be described in a systematic order. Each problem connected with this routine will be treated separately in the sections of this chapter, and also a number of methods will be included for performing problems in different ways so as to meet new conditions as they arise. It will therefore be necessary to apply the methods to the regular routine as circumstances require. Although different degrees of accuracy may be incorporated in the finished product, that is, in the mosaic or map, the photographs taken should not vary in accuracy. Each duty performed in the air must therefore be accomplished with all the precision that the personnel is capable of applying.

TYPES OF AERIAL PHOTOGRAPHS

There are two main types of aerial photographs, designated as verticals and obliques. There is also a combination of the two types, termed a composite vertical.

Verticals—When an aerial photograph is taken with the optical axis of the camera vertical or nearly vertical it is termed a vertical photograph. In general, this term is applied regardless of the amount of tilt so long as the photograph is taken with the intention that the camera shall be vertical. In making mosaics, or in constructing line maps from them, the vertical photograph is employed almost exclusively. In reality, there is no true dividing line between these two types because practically all aerial photographs are inclined to a small degree.

Obliques.—The oblique photograph is one that is taken intentionally at an angle between the horizontal and the vertical. The oblique photograph is sometimes used for mapping, especially when the country is flat and a large area is to be covered. When an oblique is taken for mapping purposes the horizon is usually included in a straight line across the upper edge of the photograph.

Composite Verticals——The composite vertical is really three pictures in one. The center picture is taken approximately in the vertical. The two wing pictures are taken with the optical axis inclined 35 deg. When these three pictures are printed, the center picture is printed as a vertical and the two wing pictures are projected through an angle to the vertical. The three pictures are then placed together as one composite picture.

ROUTINE OF A PHOTOGRAPHIC MISSION

Although every photographic mission will present some new and difficult problems, there will always be the prime factor to consider, that is, covering the area to be mapped with aerial

WORK OF PILOT AND PHOTOGRAPHER ith

Fie. 1—A VertTicaAL PHoToGRAPH

CANADIAN FALLS

| U.S. Army Air Corps | AMERICAN FALLS

Fig. 2—Osiigurt Virws or NIAGARA FALLS photographs taken as near the vertical as possible. When the area is to be covered by vertical overlapping photographs there will always be some routine procedure that will be accomplished in the same manner as in preceding photographic missions. The area is to be covered, generally speaking, by flying the airplane methodically back and forth over parallel lines of flight until the whole area has been thus traversed. While the area is being flown in this manner, the photographer makes exposures in such a way as to obtain photographs with the desired overlap and taken as near the vertical as possible. The process is continued until the area to be mapped has been completely photographed.

Fig. 3.—Errect or Onze Exposure BY THE T-1 (TRI-LENS) CAMERA

The above procedure will usually be performed in the following order:

1. Before the flight is started, all preparations on the ground are completed, such as the loading of the camera and the installing of all photographic equipment, a careful routine inspection of equipment and instruments, the making of all necessary calculations and preparing a flight map. An essential element in these preparations is the plan of cooperation between the pilot and the photographer, calling for a thorough understanding by both pilot and photographer of all maneuvers to be made and all signals to be used.

2. When the airplane has reached the area to be photographed and has attained the altitude at which the mission is to be performed, the pilot then flies a sufficient distance to the rear of the starting point of the first strip so that on the return to that point the necessary amount of crab can be obtained at which the airplane must fly in order to pass directly over the ground course

14 TAKING AERIAL PHOTOGRAPHS of the first strip. The return to the starting point should be made in the same azimuth as that in which the first strip is to be flown. The photographer, during this interval of time before reaching the starting point, has obtained the time interval between exposures for that strip and has adjusted the camera in such a manner as to compensate for the angle which the airplane, after being crabbed to allow for the wind, is making with the ground course. The position of asingle-lens camera should be such that the long diameter of the photograph forms the width of the strip and the line of flight passes through the center of the photograph and is parallel to its sides.

3. About ten seconds before the starting point is reached the pilot signals this fact to the photographer who then levels the camera and makes the first exposure as soon as the level bubble is centered. After the exposure has been made the photographer signals to the pilot. The number of the first exposure as shown by the reading on the camera is noted by the photographer, and the camera is then prepared for the next exposure. It is very important that the above duties be performed by the photographer in a systematic order immediately after each exposure. If this habit is not formed the result will be that some portion of the routine, at times, will be overlooked. During the time between the pilot’s signal to the photographer and the giving of the exposure signal, little if any adjustment should be made in the controls of the airplane, especially during the last five seconds. (See paragraph on “Level Bubbles.”’)

4, The time interval which elapses between the photographer’s signal, indicating to the pilot that the first exposure has been completed, and the next signal from the photographer is taken advantage of by the pilot to check the true course (air course), in order to make sure that it coincides with the ground course (the strip being flown). In case there has been a deviation from the ground course, the plane should be returned over the ground course and another adjustment made to compensate for drift. This change in the crab of the airplane should be closely followed by the photographer in order to readjust the position of the camera. The readjustment of the camera for any change in direction by the airplane should be made just before the next signal is given to the pilot. The photographer should also use this time to check the time interval obtained before the first exposure. If a view finder is used, this check can be accomplished by catching an object appearing on the rear line of the view finder and noting the time required for that object to reach the front line of the view finder. This time should be the same as the time interval obtained before the first exposure.

5. Approximately fifteen seconds before the next exposure 1s to be made the photographer should again signal the pilot. The pilot then knows that he has ten seconds in which to make any adjustments necessary to steady and level the plane. The final adjustment in azimuth, if necessary, can also be made before the second signal. After ten seconds have elapsed the photographer sends the second signal to the pilot. This signal indicates to the pilot that only five seconds remain before the next exposure. During this interval no change in the position or speed of the airplane should be made by the pilot unless it is absolutely necessary.! At any instant, within the last three seconds before the time for the next exposure, when the photographer succeeds in centering the level bubble the exposure should be made.? When the second exposure has been completed the pilot is again notified and the same routine is followed as given for the time interval between the first and second exposures.

6. This procedure is continued until the end of the strip has been reached. At that point either the pilot or the photographer will give the other a signal that the strip has been finished.

7. After reaching the end of the first strip the flight is continued in the same general direction for two or three minutes. A 180- deg. turn is then made, either to the right or left, as the case may be. This turn should be made so that it will be completed when the airplane is in the rear of and in line with the next strip to be flown. The interval of time, before reaching the starting point of the next strip to be flown, is used by the pilot and photographer to correct for the wind and to obtain the time interval for the second strip. If there is any wind the time interval and

1This method will reduce to a minimum the serious effect of centrifugal force on the level bubble.

2 It is not always possible to center the level bubble at the exact second when the time interval elapses.

3 When an accurate flight map is used the best results can be obtained by the pilot signaling the photographer.

the direction of crab will of course be different than when fiying the first strip.

8. When the starting point of the second strip is reached a signal is given to the photographer and the same method of procedure is started and continued as given for the first strip.

| The remaining strips of the area are flown and photographed in

the same manner as the first and second strips. This is continued

until the entire area to be mapped has been covered by overlapping

photographs.

In the operations involved in covering a designated area with

| overlapping photographs it can therefore be seen that the pilot,

| photographer, camera and airplane must, figuratively speaking,

become one machine. This machine should have perfect coordination among its parts. A movement by one part of this imaginary machine must be followed by an accurate adjustment of all other parts of that machine. The accuracy of the final result

of a photo mission will be proportionate to the approach made to

| perfect coordination of the parts of this machine. This point has been especially emphasized because the value of aerial photography

| as a mapping instrument is distinctly dependent on this factor,

namely, on the coordination between the pilot and photographer

: and their working tools.

i

GROUND PREPARATION

| All photographic mapping missions, before the flight is started,

are preceded by certain preparations on the ground. Each mis- : | sion will usually demand special ground preparations that cannot | be foreseen and given in a general outline, but there are certain routine preparations that must always be accomplished for all | photographie mapping missions before flight is begun. These routine preparations are usually carried out as follows:

1. The pilot and photographer together study the existing map or maps of the area to be photographed.! Plans should also be made as to the order in which the flight is to be carried out. The photographer must know the manner in which the pilot intends

1Tf no existing maps of the area tc be photographed are available a reconnaissance flight must be made. The method of carrying out a reconnaissance flight is explained in the paragraph under that heading, p. 44.

to cover the area. The following factors should be known by the photographer before the flight is started:

a.

b.

The strip that will be flown first and the direction in which the flight is to be made.

The time and method of the approach before reaching the starting point of each strip. This must be known by the photographer so that he will be able to determine the amount of time available in order to adjust for crab and obtain the time interval between exposures.

. The signals for the routine flying and photographing must be decided upon. (See the paragraph under ‘‘Signals.”’) A pre-determined signal should also be agreed upon for use in cases where it is necessary for either the pilot or the photographer to attract the other’s attention. It may be that the photographer will need some additional time to obtain the interval between exposures or to adjust for crab before starting a strip. If the procedure ta be followed in events of this kind is not thoroughly understood by both parties concerned great confusion in the air will result. Some means of verbal communication, of course, will make an understanding in the air much easier.