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Image interpretation, Study notes of Geography

Utkal UniversityGeography

This note explain how to interpret a digital or visual image in remote sensing

Typology: Study notes

2023/2024

Available from 06/11/2026

puspanjali-das
puspanjali-das ๐Ÿ‡ฎ๐Ÿ‡ณ

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TYPES OF IMAGE INTERPRETATION
๎˜
We have studied two major types of Remote Sensing data products, viz.
pictorial and digital. The pictorial data products, such as aerial photographs and
satellite imageries are interpreted visually. Likewise, digital data products or
digital images are interpreted mathematically by using computer software. So,
there are two ways of Remote Sensing data interpretation - 1) Visual Interpretation
and 2) Digital Interpretation
๎˜
1 VISUAL INTERPRETATION:
๎˜
Both aerial photographs and satellite imageries are interpreted visually.
Photogrammetry is the science which study interpretation of aerial photographs. To
interpret aerial photographs, a number of sophisticated instruments such as pocket
stereoscope, mirror stereoscope, plotter is used in photogrammetry for measuring
area, height, slopes of different parts of earth photographed and also for plotting
different objects.
Themes from aerial photographs. With the development of science and
technology, satellite imageries become more and more popular gradually. Satellite
image interpretation is an art of examining images for the purpose of identifying
objects and judging their significance. Interpreters study remote sensing image
logically and attempt to identify, measure and evaluate the significance of natural
and cultural features. Image interpretation technique requires extensive training
and is labour intensive. Information extraction from imageries is based on the
characteristics of image features, such as size, shape, tone, texture, shadow,
pattern, association etc. Though this approach is simple and straight forward, it has
following short comings: i) The range of gray values product on a film or print is
limited in comparison to what can be recorded in digital form, ii) Human eye can
recognize limited number of colour tones, so full advantage of radiometric
resolution cannot be used, iii) Visual interpretation poses serious limitation when
we want to combine data from various sources.
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TYPES OF IMAGE INTERPRETATION

We have studied two major types of Remote Sensing data products, viz. pictorial and digital. The pictorial data products, such as aerial photographs and satellite imageries are interpreted visually. Likewise, digital data products or digital images are interpreted mathematically by using computer software. So, there are two ways of Remote Sensing data interpretation - 1) Visual Interpretation and 2) Digital Interpretation 1 VISUAL INTERPRETATION: Both aerial photographs and satellite imageries are interpreted visually. Photogrammetry is the science which study interpretation of aerial photographs. To interpret aerial photographs, a number of sophisticated instruments such as pocket stereoscope, mirror stereoscope, plotter is used in photogrammetry for measuring area, height, slopes of different parts of earth photographed and also for plotting different objects. Themes from aerial photographs. With the development of science and technology, satellite imageries become more and more popular gradually. Satellite image interpretation is an art of examining images for the purpose of identifying objects and judging their significance. Interpreters study remote sensing image logically and attempt to identify, measure and evaluate the significance of natural and cultural features. Image interpretation technique requires extensive training and is labour intensive. Information extraction from imageries is based on the characteristics of image features, such as size, shape, tone, texture, shadow, pattern, association etc. Though this approach is simple and straight forward, it has following short comings: i) The range of gray values product on a film or print is limited in comparison to what can be recorded in digital form, ii) Human eye can recognize limited number of colour tones, so full advantage of radiometric resolution cannot be used, iii) Visual interpretation poses serious limitation when we want to combine data from various sources.

2 DIGITAL INTERPRETATION

Digital interpretation facilitates quantitative analysis of digital data with the help of computers to extract information about the earth surface. Digital interpretation is popularly known as 'Image Processing'. Image processing deals with image correction, image enhancement and information extraction. Image correction means to correct the errors in digital image. Errors are resulted due to two reasons. When errors are resulted due to defect in sensor (as for example if one of the detector out of 'n' number of detectors does not work), it is called radiometric error. When errors are resulted due to earth rotation, space craft velocity, atmosphere attenuation etc., it is called geometric error. Both radiometric and geometric errors / noise in images are reduced through different techniques with the help of computer.Image Enhancement deals with manipulation of data for improving its quality for interpretation. Sometimes digital image lacks adequate contrast, as a result different objects cannot be recognized properly. So, the image requires contrast improvement. Through different image enhancement technique,contrast is improved in digital image. After image correction / rectification, and contrast enhancement, information's are extracted from the digital image, which is the ultimate goal of an interpreter. In Information Extraction , spectral values of pixels are analyzed through computer to identify / classify objects on the earth surface. In other words, spectrally homogenous pixels in the image are grouped together and differentiated from other groups. In this way, different features of earth are recognised and classified. The field knowledge and other sources of information also help in recognition and classification processes. Visual interpretation using these elements is often a part of our daily lives, whether we are conscious of it or not. Examining satellite images on the weather report, or following high speed chases by views from a helicopter are all familiar examples of visual image interpretation. Identifying targets in remotely sensed images based on these visual elements allows us to further interpret and analyze.

Texture refers to the arrangement and frequency of tonal variation in particular areas of animage. Rough textures would consist of a mottled tone where the grey levels change abruptly in a small area, whereas smooth textures would have very little tonal variation. Smooth textures are most often the result of uniform, even surfaces, such as fields, asphalt, or grasslands. A targetwith a rough surface and irregular structure, such as a forest canopy, results in a rough textured appearance. Texture is one of the most important elements for distinguishingfeatures in radar imagery. Shadows cast by objects are sometimes important clues to their identification and Interpretation. For example, shadow of a suspension bridge can easily be discriminated from that of cantilever bridge. Similarly circular shadows are indicative of coniferous trees. Tall buildings and chimneys, and towers etc., can easily be identified for their characteristic shadows. Shadows on the other hand can sometimes render interpretation difficult i.e. dark slope shadows covering important detail. Association takes into account the relationship between other recognizable objects orfeatures in proximity to the target of interest. The identification of features that one would expect to associate with other features may provide information to facilitate identification. In the example given above, commercial properties may be associated with proximity to major transportation routes, whereas residential areas would be associated withschools, playgrounds, and sports fields. In our example, a lake is associated with boats, a marina, and adjacent recreational land.

VISUAL INTERPRETATION KEYS

Keys that provide useful reference of refresher materials and valuable trainingaids for novice interpreters are called image interpretation keys. These image interpretation keys are very much useful for the interpretation of complex imageries or photographs. These keys provide a method of organising the

information in a consistent manner and provide guidance about the correct identification of features or conditions on the images. Ideally, it consists of two basic parts' (i) a collection of annotated or captioned images (stereopalrs) illustrative of the features or conditions to be identified, and (ii) a graphic or word description that sets forth in some systematic fashion the image recognition characteristics of those features or conditions. There are two types of keys: selective key and elimination key. Selective Key Selective key is also called reference key which contains numerous examples images with supporting text. The interpreter select one example image that most nearly resembles the fracture or condition found on the image under study. Elimination Key An elimination key is arranged so that interpretation process step by step from general to specific, and leads to the elimination of all features of conditions except the one being identified. Eliminationkeys are also called dichotomous keys where the interpreter makes a series of choices between two alternatives and progressively eliminates all but one possible answer.