Aerial Photography: Uses, Mosaic and Its Treatment | Geography

In this article we will discuss about:- 1. Introduction to Aerial Photography 2. Features of Aerial Photography 3. Photo-Interpretation—it’s Planning and Execution 4. Stereoscopic Photography 5. Mosaic 6. Photo-Interpretation 7. Analytical Treatment in Aerial Photography 8. Relief Displacement 9. Specifications.

Introduction to Aerial Photography:

Aerial photography consists of taking photographs at regular time intervals from an aeroplane flying along definite lines at a certain height (flight altitude) above ground level. Economy of time and cost are the most important advantages in the use of aerial photographs. Practical application of aerial photographs in ground water studies are confined to surface geological mapping and morphological study.

Aerial photo-interpretation is of considerable use in:

(i) Identifying rock types—whether porous, massive, bedded, jointed, fractured, etc., of consolidated material and data for analysis of geomorphological features, nature of sediments of unconsolidated deposits.

(ii) Evapotranspiration studies—demarcation and measurement of lakes, streams, rivers, marshes and swamps that of 100% evaporation opportunity; differentiation of areas with various vegetation densities and land use, selection of sites for meteorological stations.

(iii) Infiltration studies and runoff—area can be divided into various infiltration components on the basis of rock type, soil cover, vegetative cover, land use pattern and intensity, surface drainage network and density, fracture and joint pattern, land form, and study of sedimentation and river regimen.

(iv) Selection of drilling sites, gauging stations, alignment of water mains and power lines; for geophysical survey study of terrain conditions for locating traverse lines, camps, access routes, etc.

Selective field investigations are a must to confirm surficial geological interpretations and carry out subsurface studies to evaluate the rocks and their water yielding properties at depths. Correlation and refinement in interpretation of geological features may be made in the field or in a follow up photo-geologic study.

Aerial photographic investigation is the first phase in planning any ground water exploration programme. Many of the features which cannot be always brought out by ground surveys can be interpreted with the help of aerial photographs. Aerial photographs provide information in respect of geomorphological features, drainage pattern and presence of springs in the area.

Features of Aerial Photography:

Some of the features as indicated by aerial photographs are as follows:

i. Igneous, metamorphic and sedimentary rocks,

ii. Joints, fissures and faults,

iii. Shales, clays or clastic rocks,

iv. Coarse grained and permeable rocks,

v. Alluvial, unconsolidated formations,

vi. Extrusive rocks (lava flows)—joint pattern, fractures, fissures and dykes, and

vii. Sink holes in Karstic topography.

Indications in Aerial Photographs:

i. Demarcated by texture and tonal changes,

ii. Appear as linear features,

iii. Show dark tones,

iv. Show light photographic tones,

v. Recognised by fluvial features such as river terraces, alluvial fans etc.,

vi. Geometrical pattern, and

vii. Pitted appearance of land surface.

Photo-Interpretation—It’s Planning and Execution:

The factors which should be taken into consideration which will guide photo-interpretation are:

A. The boundaries of the area to be photographed should be marked on a toposheet giving coordinates, and the purpose which determines the specification and type of photography should be stated.

B. For hydrological studies the specifications to ensure best results are as follows:

(i) The photographs should be absolutely vertical with a maximum tilt limit of 3°.

(ii) The photographs should be clear and free from such obscuring factors as haze, cloud, fingerprints and blurred images, as these will affect the interpretation and measurement of details.

(iii) The best season of photography is immediately after the monsoons as the light con­dition remains favourable and obscuring factors like haze and could do not mar the quality of photographs. The time of photography is very important. During morning and evening, long and deep shadows will tend to obscure details unless it is a flat and plain country. In the tropics it is necessary to limit the time of photography to between 1(1/2) to 3 hours after sunrise of avoid the atmospheric haze as soon as the temperature rises.

(iv) For interpretation purpose, photographs should preferably be in glossy prints.

(v) Colour photographs are best suited for hydrological studies but being extremely costly are rather prohibitive. Photographs taken with infra-red films and filters are generally used for hydrographic purposes, particularly in forest areas because the water spots show up very conspicuously dark owing to their absorption of infra-red light. In India at present only black and white photography is used.

(vi) The scale of photography depends on the purpose of investigation and nature of the terrain and must always be specified while ordering for photographs.

The following examples serve as a guideline in the selection of scale:

In flat terrain, the flying height is not so important but in rugged country with elevation differences, flying should be done as high as possible using the lens of largest possible focal length for the particular scale required (scale = focal length/flying height). High flight will cause less relief distortion and less scale variation.

Aerial photographs are flown in strips to cover the area with the flight direction usually kept along the length of the area. However, in areas of high dipping geological formations, if the strike is along the length of the area, it may be necessary to resort to cross stripping. Hence, the type of stripping should be specified. Minimum overlap required to have a continuous stereoscopic cover is forward overlap 52% and lateral overlap 15%. But in high relief terrain higher percentages are taken so as to cut down the relief displacement.

Stereoscopic Photography:

A pair of photographs taken from two camera stations but cover a common area constitutes a stereo pair which when viewed under a stereoscope in a certain manner with parallel eye axes give a three dimensional view of the common area.

A schematic diagram of a minor stereoscope is shown in Fig. 3.5. The distance between corresponding points is generally 240 mm large enough for 23 × 23 cm photographs. Lenses having a focal length equal to the distance from the lenses to the photographs via the mirror- prism are placed. This distance is usually about 300 mm. Since the normal viewing distance is 250 mm, the magnification is 250/300 ≈ 0.8. Oculars having an enlargement factor of 3 to 8 are placed over the lenses, resulting in a net enlargement of 2.4 (= 3 × 0.8) to 6.4 (= 8 × 0.8).

The pocket stereoscope, Fig. 3.6, usually has planoconvex lenses with a focal length of 100 mm. The parallel rays, entering the eyes converge at infinity. Since the normal viewing distance is 250 mm, the magnification due to closer view is 250/100 = 2.5. The pocket stereoscope is cheap, portable, has a large field of view and good image quality due to the simple optical system.

The limitations are as follows:

(i) They have limited magnification. Pocket stereoscope with more than three times magnification cannot be equipped with simple planoconvex lenses due to the large increase in lens aberration.

(ii) The distance between the eye and the photograph is too small for adequate illumination and working space for tracing etc.

(iii) The distance between the corresponding points on the photographs must be equal to or smaller than the eye base, which is difficult with normal photograph unless they are bent or folded.

Mosaic:

An assemblage of aerial photographs whose edges have usually been cut and matched to form a continuous photographic representation of the particular area is called a mosaic. If this assemblage is made without any control, then it is called uncontrolled mosaic. If, before being laid, the prints have been properly rectified, i.e., enlarged or reduced and fitted on adequate ground control, i.e., to fit pre-determined locations of certain important features, the mosaic is said to be a controlled mosaic.

The controlled mosaic, though more accurate, retains the changes in scale and displacements due to differences in relief within the individual prints. A contoured mosaic shows the relief by means of contours, and may be either controlled or uncontrolled. A semi controlled mosaic may be prepared from unrectified photograph assembled to have ground control; alternatively rectified photographs may be used with no ground control. Semi-controlled mosaics are a compromise between economy and accuracy.

The Surveyor General of the Survey of India, Dehradun, is the coordinating authority for all aerial photography in India. A library of the existing photographs is also maintained for restricted use and it is essential to obtain security clearance from the Ministry of Defence, Government of India.

While ordering for aerial photography the following details have to be furnished:

(a) Purpose.

(b) Extent and location (Index map).

(c) Scale of photography.

(d) Camera and focal length.

(e) Season and time.

(f) Requirements—type and number of prints, mosaic etc.

(g) Any other special requirements.

On receipt of the estimated cost from the Surveyor General of India for the type and scale of photograph, the user should provide for the necessary funds and submit a certificate.

Photo-Interpretation:

The interpretation from aerial Photographs is done in five steps:

(i) Consultation of literature, previous maps and reports and a reconnaissance field trip, if necessary.

(ii) Interpretation, hydrographic measurements and annotation of details on overlay sheets from aerial photographs.

(iii) Combination of annotated overlays to an average scale or transfer of details to a controlled base.

(iv) Field work in key areas with the interpretation map.

(v) Preparation of the final map and report.

In spite of all these steps, the whole project can be completed in a very short time.

A qualitative estimate of the grain size, degree of compaction and permeability of materials composing the land surface is possible from a study of the aerial photographs with the help of recognition elements such as photographic tone, geomorphic texture, drainage factors, shape and size of geomorphic elements, spatial association of features, colour differences, erosion pattern, etc.

The drainage pattern and drainage density are indicators of underlying geology. A granitic country develops a dendritic drainage pattern with a low drainage density whereas a sandstone or quartzite country develops deeply incised streams with moderate to low drainage density depending on the degree of permeability. The higher the permeability, the lower is the drainage density.

Similarly, a shale country develops a close knit shallow drainage and a lime stone country hardly any surface drainage. The drainage lines may be structure controlled, i.e., developed along some faults, joints, fractures systems, around domal structures, along the crests or troughs of some fold system, parallel or perpendicular to bedding, etc.

Then they develop some definite orientation. Thus, a study of drainage features from aerial photographs brings out a wealth of information regarding the water-bearing potentiality of the underlying materials. Thus, photo-geological studies are considered very advantageous in tackling various hydro-problems.

Analytical Treatment in Aerial Photography:

From Fig. 3.7, Photo-scale

Relief Displacement:

In Fig. 3.9, r₁ = a₁a₂ is the displacement of the image point of A₁ due to its relief h above the datum:

Total displacement on the two photographs r = r₁ + r₂ is the parallax displacement due to relief and is called parallax difference.

From this parallax equation, the elevation and ground coordinates of a point can be determined.

The parallax difference can be measured with an ordinary ruler but more accurately by a parallax bar, which consists of two glasses engraved with marks known as floating marks connected by a bar whose length can be varied by a micrometer screw. By adjusting the micrometer, looking through the stereoscope, the two marks are fused to land on the top of the object and on the foot of the object the difference between the corresponding micrometer readings gives the parallax difference r.

The aerial photographic flight mission is fairly expensive. Since a great number of photographs have to be taken in rapid succession while moving in an aircraft at high speed, the aerial cameras should have short cycling times, fast lenses and efficient shutters. They must be capable of faithful functioning under the most extreme weather conditions and in spite of aircraft vibrations. Aerial cameras generally use roll film and have magazine capacities of 60 to 120 m or more. The four main types of aerial cameras are single-lens frame cameras, multi-lens frame cameras, strip cameras, and panoramic cameras.

The camera mount has devices which prevent aircraft vibrations from being transmitted to the camera and also for rotating the camera in azimuth to correct for crab. Crab is the deviation in the aircraft’s actual travel direction from its direction of heading. It is usually caused by side winds, Fig. 3.11.

For topographic mapping, photo scale is usually dictated by the required map scale, required contour interval and capabilities of the instruments that will be used in compiling the map. The minimum possible contour interval that can be reliably traced with a stereoscopic plotting instrument is generally given in terms of a C-factor*, which is a ratio of flying height to the contour interval that can be reliably plotted-

C-factor = H/C.I …(3.4)

If a plotter has a C-factor of 1000 and a map with a contour interval of 2 m, then a flying height above ground of 2000 m or less is required.

In large-scale mapping, the compatible scale and contour intervals (C.I.) are given below:

Unavoidable aircraft tilts cause photographs to be exposed with the camera axis tilted slightly from vertical. This is usually less than 1° and rarely exceeds 3° in vertical photography.

A systematic study of aerial photographs involves a consideration of the basic characteristics of photographic images like shape, size, pattern, shadow, tone, texture and site.

Specifications:

Most flight plans include a set of detailed specifications which outline the materials, equipment, and procedures to be used on the project. These specifications include requirements and tolerances pertaining to photographic scale (including camera focal length and flying height), end lap, side lap, tilt, crab, and photographic quality.