Interpretation of Air Photographs: Role of Scale, Tone, Color, & Light Intensity (Especially for GATE-Geospatial 2022)

Glide to success with Doorsteptutor material for competitive exams : get questions, notes, tests, video lectures and more- for all subjects of your exam.

Examrace Books on Mapping, GIS, and Remote Sensing prepares you throughly for a wide range of practical applications.

The identification of detail from air photographs and the interpretation of information given on them require experience for accuracy. Even then, considerable care and common sense must be exercised, or erroneous conclusions may be reached. Information was given by an air photograph and the ease with which it can be read accurately depends on several factors such as the angle of photograph, season, weather, scalt, development and printing. While it is possible to give certain examples here as a guide to the correct identification, it should be realized that a few hours՚ practices in reading air photographs and in subsequent checking on the ground will indicate much more clearly what may be expected. Oblique photographs, although not much used for large-scale mapping, are easier to interpret than vertical photographs because they present the usual view from the top of a hill or building. Oblique and vertical views of the Houses of Parliament, London, given in below Figure, show quite clearly how the oblique may be more easily interpreted by the inexperienced observer. He has a conventional mind-picture of objects as they appear from his “worms-eye” view; while the skilled interpreter will have adapted himself to expect the “birds-eye” view in a vertical. Oblique photography has been used extensively because it presents an accurate detailed view which can be appreciated easily by such persons as lawyers and members of non-technical committees. The value of the pictorial photograph to those who are developing estates is emphasized by Collins.

Complete Guide to LiDAR: Light Detection and Ranging

Interpretation Background

“A well-placed oblique of a housing estate may often (depending, of course, upon the estate) be worth columns of advertisement. While depicting the boundaries of the property with unquestionable fidelity, the map fails to distinguish between a disreputable backyard and a garden of roses; or between a field of mangold wurzels and verdant pasture. A prospective purchaser is often more concerned with the general aspect and salubrious nature of the district than with several decimal places of the exact acreage.”

Two excellent examples of the useful employment of obliques are given by Wills.

  • Brewery companies with foresight will purchase possible sites for hotels in undeveloped areas or along new arterial roads. When development has passed the early stages, the application is made to the Licensing Magistrates for a licence and evidence must be produced that such an application is justified, together with plans of the area. Air photographic views taken for this purpose show the roads and buildings clearly over an area of approximately a square mile and are often studied instead of the plan.
Oblique and Vertical Iews of the House of Parliament, Showin …
  • The other instance given is that of a Parliamentary inquiry. The Monmouthshire County Council in opposing the Cardiff City Extension Bill 1937 had aerial photographs taken of two or three parishes which Cardiff desired to incorporate. These photographs showed the disputed areas so clearly that it was amicably agreed by the parties to use them so that the opposing Counsel could demonstrate their points.

The air photographer, when commissioned by persons such as Parliamentary agents, must choose the angle and direction of photography with some care in order that he may best illustrate the important details. For legal purposes, photographs cannot be joined and they are usually mounted consecutively on linen.

In the examination of air photographs, the value of stereoscopic examination cannot be overestimated. Man can see in the “solid” because each eye records a slightly different view of the same object in space, which enables the brain to interpret shape and size. There are, however, other factors, such as the known relative size of well-known objects, by which it is possible to judge size and shape without making much use of this stereoscopic power.

Stereographic Interpretation

Salt mentions that it is only carnivorous animals which have a stereoscopic vision; the eyes of herbivorous animals being situated so that they view the world as a flat panorama. Beasts of prey have a rather larger eye-base than human beings which gives them a sharper appreciation of relative distances. When the ground is observed from an aeroplane at a height of several thousands of feet most of the relief disappears. Therefore, instead of using the view which an observer would obtain from the aeroplane, two overlapping photographs were taken from different camera stations, which may be a mile apart, are viewed in a stereoscope. The (effect is a view which would be seen by a giant having one eye at each end of the airbase between the two photographs.

Such a pair of photographs is shown in below Figure it will be noticed that in each photograph the apex of the pyramid is in a different position relative to its base. This is due to the displacement of the image resulting from its height. The length of the shadow of an object thrown by the sun on to the ground gives some idea of this if it is imagined that the sun is the camera station.

To examine a single photograph for detail is like going about with a shade over one eye. Objects which cannot be picked out in a single photograph often stand out very clearly in a stereoscopic view. For example, by stereoscopy, it is easy to differentiate a cutting from an embankment or to separate a tree from its shadow. Again, it is easy to make wrong assumptions from a single photograph. The writer, on one occasion, was out fixing points for ground control, and working from a single photograph found what looked like a very suitable road junction in a small group of new houses. On arrival at the site, it was discovered that the supposed group of houses was a stack-yard and the road a very rough and muddy farm track. This was clearly shown by subsequent stereoscopic examination of a pair of photographs. Slight irregularities made it clear that the objects were stacks and not cottages. Hotine has remarked very true that the study of a pair of aerial photographs under a stereoscope gives the most detailed and comprehensive view of the earth՚s surface yet obtained by man.

While the oblique photograph gives a picture which can be appreciated by all and is thus of great value in evidence or as an indication of progress at a given date, it does not show as much detail as the vertical photograph due to some of the background being hidden by objects in the foreground. On the other hand, the abundance of detail on a vertical makes it more difficult to read. Maps, which have the same viewpoint as a vertical, indicate the various objects by their conventional signs and are therefore more easily interpreted.

Stereo-Pair of Pyraminds

Role of Scale

There are several factors which are likely to give to the same object widely varying appearances under different conditions. In the first instance, the scale must be considered because the basis of interpretation will differ according to the size of the image in the photograph. As will be shown later, the scale of a vertical photograph is the ratio of the focal length of the camera to the Hying might at exposure: thus if the exposure is made at a height of 15,000 feet with a lens of 7 inches focal length, the scale will be about 1/25,000 or 2 inches to 1 mile; and if from 7,500, with the same focal length, it will be at 1/12,500 or 5 inches to the mile. A cottage 24 feet square will be 012 inches square if the scale is 1/2,500 but only 0.012 if it is 1/25,000.

On a large scale, therefore, it is possible to pick out quite small objects such as fences, walls and details along roads while on small scales the interpretation must be more general. A railway can be differentiated from a road because of the greater lengths of straights, long uniform curves, the absence of ribbon development and the presence of stations and sidings. In such cases, it is the topography rather than detail which is being studied.

Role of Shadows

Shadows play an important part in the study of air photographs, and in order to avoid a pseudoscopic effect in which everything is seen inside-out, the shadows should run towards the observer and away from the source of light. Trees, in particular, are apt, in the single view, to mingle with their shadows, but in the stereoscope, the separation is at first quite startling. For example, at a scale of 1/5,000, a foot-bridge at a railway station will stand out very clearly from its shadow. Where there are shadows there is some danger of obscured detail and accordingly, the films are developed very carefully to retain as much information as possible. An object which cannot be picked out in the photograph can frequently be identified by its shadow. In the vertical view of the Houses of Parliament, the shadow of the well-known tower can be seen. Fence lines, or electric power pylon lines, can often be followed by their shadows. Air photographs taken when the shadows are long will often assist interpretation in such cases. A number of important archaeological discoveries have been made owing to the flat rays of light accentuating slight irregularities in the ground in the same way as the headlights of a car show up corrugations of a road. Ancient roads, camps, and irrigation lines may show up quite clearly and can be checked by subsequent examination although they have not been noticed on the ground.

Role of Color

The air observer has not the assistance of colour which is so valuable on the ground, because the landscape tends to appear monochromatic from a height and similarly the photograph is a monochromatic representation of the landscape. Detail can thus be identified only by its tone, and this is one of the reasons why an object may appear indistinguishable from its shadow in the single view. The tone is determined by the quantity and quality of light which imprints the image on the negative.

The modern panchromatic film is sensitive to light of long wavelength, and thus differentiates between red, orange and yellow, particularly if filters of the last named colours are used to retard the blue light. Photographs taken through such filters have been found to be easier of interpretation, and Hemming records the use of special filters in the survey of the Northern Territory, Australia, in order to differentiate more clearly between various types of vegetation, soils and geological formations. The tone values recorded on an infra-red film are completely different from those shown on the panchromatic stock, and a different technique of interpretation is required. The use of any filter necessitates- an increase in exposure and, at present, the exposures demanded by an infra-red filter are too long for the method to be used successfully in air survey. ″

Effect of Light Intensity: Reflectivity, Tone, Seasons, Time of Day

The quantity of light reaching the film is dependent upon the reflecting power of the object. On a vertical photograph the obliquity of the ray՚s results in a very different light value from that which is registered on a photograph taken from a ground station. Atmospheric conditions also greatly influence the tones. A field of corn will appear light on a windy day, but if the air is calm the photograph will show the ground at the bottom of the stalks, and as this has a lower reflecting power, the field will then appear dark. Water when calm may appear anything from white to black even on adjacent photographs. The amount of light reflected depends upon the direction of light, and upon the depth of water, which influences the amount of light reaching the camera. This effect of depth on tone has been found very useful in hydrographic surveying. One way of ascertaining if the area observed is water is to notice in the stereoscope if it is a smooth level surface. When the surface of the water is disturbed it will appear grey. Dark objects, such as wet rocks, will often appear lighter in tone if the light is reflected into the camera. Variations of the tone of ripening crops in apparently uniform fields have been found to indicate the presence of remains under the soil, and it has been shown that this difference is due to the varying surface compression of the soil. Where the soil is more compressed, as, for instance, over an old foundation, the crop receives less moisture and ripens earlier than over the rest of the area, causing a difference of tone.

These effects also have considerable military value because that part of a meadow which has been pressed down by the passage of men and materials will show up unmistakably, and many a gun position was discovered in the War by photographing the track of the ammunition line made the previous night. Incorrect shadows thrown by dummy trenches were also easily seen in the stereoscope.

It is important when examining a photograph to know at what time of the day and season it was taken. Survey photographers when working on large scales sometimes try to take the pictures early in the year before the leaves cover the trees and hide the detail beneath them. Often, however, the points of economic importance are more easily appreciated from the photographs when the trees are not bare. Photographs of the same area taken in winter and summer will appear very different, and it is remarkable that in a photograph it is often the least significant detail which shows up the most clearly. If at the time of photography, fields are being ploughed or harrowed or hay or corn is being cut, the line of demarcation will be much bolder than anything else. In the case of the crop being cut, that which is left standing will be dark, while the remainder will appear light.

Economic Use

Robbins has done much pioneer work in the economic use and interpretation of air photographs. His ecological work that is in relating the topographical features to geology, forestry, agriculture, etc. has been of great value in advancing the science. He makes the following observations: “The actual aids that the interpreter uses in his work are multitudinous, as they include every possible aspect of the effect of all natural forces. First and foremost is a general impression and 1 can best illustrate the ecologist՚s attitude of mind and powers from that of our own photographers. The latter, on looking at a print will note subconsciously all its good points and its deficiencies, and will frequently discard a print for some fault that the ordinary layman would never notice. There may be some slight darkness or lightness or lack of contrast that is not visible except to the specially trained eye and it is such slight differences that really occur on the ground and can be seen on the photograph that aid the ecologist in making his conclusions.”

The economic interpretation of air photographs, particularly in connection with geological and prospecting surveys has been comprehensively dealt with by Gill.

Developed by: