Stages of Photo-Interpretation: Stage 1: General Examination, Stage 2: Identification, Stage 3: Classification (Especially for GATE-Geospatial 2022)

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In the foregoing discussion, it has been noted that photo-interpretation proceeds in stages, and for high efficiency, a systematic approach is preferred. A three-stage procedure is usually followed:

Introduction to Remote Sensing

Stage 1: General Examination

This aims at getting a general impression of what the area as shown in the photographs like the general patterns of relief, vegetation, and cultural development is to be established. This really includes Vink՚s detection phase of photo-interpretation.

Stage 2: Identification

Concentration on more specific areas or features is required in order to identify and recognise individual objects. This has to be done in a systematic manner. Normally, the following two approaches have to be used:

1. By Significant Areas

The most significant areas established in the general examination stage are examined in detail one by one. For example, a built-up area may have been identified in the first stage as the most significant. in the second stage, the characteristics of the thin built-up area are studied to identify the street pattern, building types, and the location of some special structures such as churches or temples, cemeteries, public parks, etc. After this, the second most significant area is similarly studied in great detail, eg the agricultural area; and then the third most significant and so on, finally ending with the least significant.

2. By Uniform Sub-Areas Within a Significant Area

Within a significant area (or if the photograph shows a uniform type of landscape) , a further sub-division into smaller areas is arbitrarily made, and each such area is then examined in detail in turn. This facilitates a systematic coverage of the whole photograph during interpretation.

The identification and recognition of objects are helped by knowledge of the characteristics of photo-imagery as recorded by the black-and-white panchromatic film. These include tone, texture, pattern, shape, shadow, size and situation. 8

  • Tone represents a record of light reflection from the land surface on to the film. On panchromatic film, it will be ranging from black, through greys, to white. In general, the more light reflected by the object, the lighter its tone on the photograph. The nature of the materials affects the amount of light reflected. Thus, bare earth usually appears to be light in tone; but if it gets wet, its tone will become darker. However, the angle of the sun appears to have over-riding importance in determining the tone of an object. Thus, one may say in general that water surface appears to have a fairly dark tone on one photograph, but on the next photograph, it can be white if the sun՚s rays are all reflected from it on to the film. Usually, the steeper the angle at which the sun՚s rays reach the surface of an object, the brighter is its illumination. Consequently, in areas of dissected relief, slopes with different degrees of steepness and different aspects are illuminated differently, thus producing different tones on photographs. Therefore, the tone of an object as imaged on the photograph is influenced by many factors and one needs to exercise great care in evaluating it. plate 16, which shows a fishpond area in the rural New Territories of Hong Kong, illustrates very well the usefulness of tone in determining the depth of water inside fishponds, the degree of sedimentation in the river and the wetness of the soils. Similarly, asphalt-surfaced roads and concrete-surfaced paths can be differentiated.
  • Texture is the frequency of tone change within the image which arises when several small features are viewed together. Obviously, this ‘smallness’ of the feature is determined by the scale of the photographs used; hence the texture of a forested plot as seen at the photo-scale of 1: 50,000 will be different from that as seen at the scale of 1: 1,000, as the tree crowns, will contribute to the overall textural appearance of the image at the large scale. In general, one can distinguish between the smooth, mat and rough textures as represented by the calm water surface, ploughed field and forested area respectively (Plate 16) . Another particularly good example is given in Plate 17 (Kent) from which one can differentiate, using texture alone, the different types of crops - mature small fruit trees (A) , hops (B) , mature large fruit trees (C) , young fruit trees (D) and vegetables (E) .
  • Pattern is a familiar characteristic of the landscape to geographers as it results from the spatial arrangement of objects. Normally a macroscopic viewing of the whole photograph is required to detect the significant pattern. Examples include field patterns, settlement patterns, street patterns, drainage patterns, etc. In Plate 16, the patterns of the paddy and vegetable fields can be compared with those in Plate 17. The compact and regular layouts of the Chinese Villages should also be noted.
  • Shape is a qualitative variable describing the general form, configuration or outline of an object which is particularly useful as a single factor for the identification of objects. But in vertical aerial photographs, the shape of an object mistimes the unfamiliar plan view from the top. With the addition of the third demarcation, i.e.. height achievable by the creation or a stereo model, objects are more easily identified in conjunction with the shape variable. However, one should note that the shape of a vertical object will vary according to its position on the photograph (i.e.. near the principal point or not) . In Plate 18 (Scunthorpe) round objects of different sizes and heights can be easily detected but they represent very different things which include cooling towers (A) , gas storage tanks of varying sizes (B, C) and a concentration tank (D) .
  • Shadows of objects usually enhance the usefulness of the shape factor in identification and recognition, especially for linear vertical features such as a row of trees, lamp-posts, telegraph posts, stone walls, fences, hedges, etc, and also structural weaknesses such as faults, joints, fracture lines, etc, on the earth՚s surface. On the other hand, excessively long shadows can cause trouble to photo-interpreters and photogrammetrists by obscuring the ground surface and other features in the paths of the shadows. In Plate 18, all these points have been very well brought out.
  • Size (the dimensions of an object) needs to be carefully related to the photo-scale. Usually, the best approach is to compare an object of unknown size to some common objects of standard dimensions on the photograph, eg a football field, a car, etc, if such are available. Otherwise, the nominal scale of the photograph has to be noted and a rough measurement made directly to the object on the photograph for an estimate of its size.
  • Situation involves considering the object՚s position in relation to others of known significance in its immediate vicinity. The method of association is utilised in this connection to identify features. In reverse, the presence of some special features implies the fulfilment of the prerequisite conditions for their existence. Thus, the positive identification of a mangrove indicates that the area covered by this vegetation has a coastal situation and is periodically flooded by sea water.
  • All these seven characteristics of the imagery serve as useful aids to photo-interpretation at this identification and recognition stage; and with experience, their employment becomes automatic.

Stage 3: Classification

This stage includes Vink՚s analysis, deduction and classification types of photo-interpretation. As has already been pointed out, classification involves assigning photo-images into their Proper groups. For example, roads may be classified into a first, second and third class according to their width and functions; similarly, a typology can be developed for crops, buildings, settlements, etc. In this stage, the evaluation of the significance of individual images as a whole is made, from which generalisations (or idealisation according to Vink) can be formulated.

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