Fundamentals of Soil Science (1+1)

Factors affecting aerial photography

Factors affecting aerial photography:

Atmospheric conditions : The presence of particles (smoke or dust) and molecules of gases in the atmosphere tends to reduce contrast because of scattering, therefore, the best time photography is when the sky is clear (November-February).

Scale: Scale is the ratio of distances between two images on an aerial photograph and the actual distance between the same two points/ objects on the ground, in other words the ratio of f/h ( f- focal length of the camera lens and h- flying height above the mean terrain). Due to variations in flying height , the scales of different photographs may vary. Scale may also vary because of the effects of tilt and relief displacements.

Camera/Film/Filter combination: To ensure good image quality,modern distortion free cameras are used. Depending upon the requirements different lens/ focal length/ film/ filter combinations can be used.

Flight direction: Aerial photography is flown in strips to cover the designated area. It is advisable to keep the number of strips to minimum. The flight direction of strips is therefore kept along the length of the area.

Time/ Season of photography: Aerial photography should be flown when the sun’s elevation is 30 degrees above the horizon or three hours before and after the local noon time.

Remote sensing:

• Remote sensing is the science of obtaining information about objects or phenomenon in the enviornment through the use of sensing devices located at a distance without any physical contact between the object and the sensing device.
• Electromagnetic energy is the means by which information is transmitted from an objects to sensor.
• Earth orbitting satellites equiped with sensors, including cameras provide imagery and digital data.
• Remote sensing technology makes use of visible (0.4-0.7um), infrared (0.7-3.0um), thermal infra red (3-5 um and 8-14 um) and microwave (0.3 – 3.0um) regions of electromagnetic spectrum to collect information about various objects on the earth’s surface.
• The Indian Remote Sensing Satellite ( IRS-ID & P6) launched at an altitude of 817 km with an inclination of 98.70 in the descending mode cross the equator at 10.30 AM.
• The Indian Remote Sensing Satellite ( IRS & P6) takes 101.35 minutes to complete one revolution around the earth and thus completes about 14 orbits a day.
• The entire earth is covered in 341 orbits during a 24 day cycle. Earlier, the planned life of satellites was three years, but it is 5-7 years presently.

Indian Space Research Organization has launched 14 Indian Remote Sensing Satellites:
IRS -1A March ,1988 ;IRS -1B August ,1991;IRS-1E September, 1993; IRS-P2 October, 1994;IRS- 1C December, 1995;IRS P3 March ,1996;IRS – 1D September, 1997 ;IRS – P4 May ,1999;Technology Experiment Satellite (TES) Oct ,2001; RESOURCESAT-1 (IRS-P6) October, 2003;Cartosat-I May,2005;Catosat-2 January, 2007;Cartosat- 2A and Indian Mini Satellite (IMS-1) April,2008

Basic characteristics of features used in Interpretation:

Shape:

• It refers to general form, configuration or outline of the individual objects. Numerous components of environment can be identified with reasonable certainty, merely by their shapes or forms.
• This is true for both the naturl features (e.g. Geological structures) as well as man made objects (e.g.) different type of industrial plants and buildings.

Size:

• The size of an object is one of the most useful clues in the possible identification. Objects can easily be picked up from photos if the scale is large.
• An irrigation ditch and an anti tank ditch are very much alike except in size, and simple measurement may be sufficient to make the identification.

Pattern:

• Repetitive arrangements of both natural and cultural features are quite common. Patterns are clearly visible on images and also capture many small but significant patterns which might be over looked by the ground observer. Cultural features are conspicuous because they consist of straight lines or regular configurations.
• A road and railway may look much alike in a photography/imagery, but an interpreter can separate them by the slight configurations required by their function. For example, a road may have fairly steep grades, sharp curves and many intersections while a railway has gentle grades, wide curves and few intersections.

Shadow:

• It defines outline of an object and its length may help to estimate its height as well as elevational out line of an object.
• Shadows are especially useful in geomorphological studies where micro relief features may be easier to detect under conditions of low angle solar illumination than when the sun is high the sky.

Tone or color:

• Tone refers to the colour or relative brightness of objects in aerial data. Different objects emits or reflect different wavelengths and intensities of radiant energy. Such differences may be recorded as variations of pictures tone, colour or density.
• In black and white images, the tone varies from light grey to dark. The terms light, medium and dark are used to describe the variations in tone.

Texture:

• Texture is the frequency of tonal change on photographic image and is created by tonal repetitions of groups of objects which are two small to be disurned as individuals.
• Common photography textures include smooth, rippled, mottled, lineated and irregular. This is very important unit for rock identification.

Site:

• At an advanced stage in a photo interpretation procedure, the location of objects with respect to terrain features of other objects may be helpful in defining the identification and classification of certain picture contents. For example, certain tree species would be expected to occur on well drained upland sites, whereas other tree species would be expected to occur on poorly drained lowland sites.

Interpretation methods:

1. Visual interpretation using keys
2. Digital interpretation using computers.

Images Interpretation keys (visual)
1) Selection key :

• It contains numerous photography examples with supporting text. The interpreter selects the features or conditions found an the image under study.

2) Elimination key:

• It is arranged so that interpretation proceeds step by step, from general to specific and leads to the elimination of all features or conditions except the ones being identified.