ELEMENTS OF VISUAL INTERPRETATION OF AIR PHOTOGAPHS AND SATTELITE IMAGERIES.pptx

Remote Sensing

1. ELEMENTS OF VISUAL INTERPRETATION AERIAL PHOTOGRAPHS AND SATELLITE IMAGERIES Mr. Piyush R. Telang Assistant Professor, Department of Geography, Sir Parashurambhau College, Pune

2. Interpretation Interpretation is the process of extraction of qualitative and quantitative information of objects from aerial photographs or satellite images . Visual interpretation D igital interpretation

3. Visual image interpretation is a process of identifying features seen on the images by an analyst / interpreter and communication of information obtained from these images to others for evaluating their significance. This process, however , is not restricted to making decisions concerning what objects appear in images but it also includes determination of their relative locations and extents . Success in visual image interpretation varies with the training and experience on the pictorial data analysis. If the interpreter has an artistic and photographic sense then information derived from the interpretation process may be more authentic and reliable.

4. Comparison of merits and demerits of visual (human) and digital interpretation techniques Visual (human) interpretation Digital interpretation Merits Image analyst’s experience and knowledge is available Very good for extraction of spatial information Time effective-requires much less time for interpretation Results can be exactly reproduced for any number of times Extraction of quantitative information is possible and easier Demerits Time consuming Interpretation results may vary with time and person depending upon their experience and knowledge Image analyst’s experience and knowledge is not available Poor in extracting spatial information

5. The photographs taken from an aircraft or helicopter using a precision camera are termed as aerial photographs . Cameras are the simplest and oldest of sensors used for remote sensing . The aerial image differs from everyday photograph in the following aspects: overhead perspective beyond visible light spectrum and unfamiliar scales and orientation.

6. Aerial photography has many applications such as cartography, urban and rural planning, environmental impact studies, civil law cases, real estate evaluations and can even be used as wall art. Following are the advantages of aerial photography : Synoptic Viewpoint: Aerial photographs give a bird’s eye view of large areas enabling us to see surface features in their spatial context. Permanent Recording: They are virtually permanent records of the existing conditions on the Earth’s surface at one point in time and are used as an historical document. Capability to Stop Action: They provide a view of dynamic conditions and are useful in studying phenomena e.g., flooding, wildlife, oil spills, etc . Three Dimensional Perspectives: It provides a stereoscopic view of the Earth’s surface and makes it possible to take measurements horizontally and vertically. Spectral and Spatial Resolution: Aerial photographs are sensitive to radiation in wavelengths that are outside of spectral sensitivity of the human eye. They also have better spatial resolution than many ground based remote sensing methods. Availability : They are readily available at a range of scales for much of the world. Economy : They are much cheaper than field surveys and are often cheaper and more accurate than maps .

7. Tone is the most basic of the interpretive element and refers to the relative brightness or colour of elements on an aerial photograph. Size of objects must be considered in the context of the scale of a photograph. The scale will help you to determine if a water body is a pond or lake or sea. Shape refers to the general outline of objects and regular geometric shapes. Texture is the impression of smoothness or roughness of image features and is caused by the frequency of change of tone in photographs. Pattern or spatial arrangement is formed by objects in a photo which can be diagnostic. Shadows aid interpreters in determining height of objects in aerial photographs. Site refers to topographic or geographic location. Association refers to position of the objects of interest in relation with the other objects. Elements of Visual Interpretation: An Overview

8. Image Interpretation Tasks The image interpretation procedure is a complex task and requires several tasks to be conducted in a methodical manner which include : Classification: is the assignment of object, features, or area to the classes based on their appearance on the images . Often the distinctions are made between three levels of confidence and precision namely- detection, recognition and identification . - Detection is the determination of presence or absence of the feature. - Recognition implies a higher level of knowledge about a feature or an object such that the object can be assigned identity. - Identification means that the identity of an object or feature can be specified with enough confidence and detail to place it in a specific class .

9. Enumeration: is the task of listing or counting discrete items visible on an image . Mensuration: Mensuration or measurement is an important function in many image interpretation problems. Two kinds of measurements are important, first, is the measurement of distance and height, and by extension, volumes and areas as well . A second form of measurement is quantitative assessment of image brightness. Delineation: The interpreter must delineate, or outline, regions as observed on remotely sensed images. The interpreter must be able to separate distinct aerial units that are characterized by specific tones and textures and to identify edges or boundaries .

10. Prerequisites for Image Interpretation Remote sensing system Knowledge of image and sensor characteristics Proficiency based on knowledge of the subject and Adequate familiarity of the geographic region and locality.

11. Elements of Visual Interpretation

12. Tone or color: Tone refers to the relative brightness of objects in an image. Generally, tone is the fundamental element for distinguishing between different targets or features. Variations in tone also allow the elements of shape, texture, and pattern of objects to be distinguished. Colour may be defined as each distinguishable variation on an image produced by a multitude of combinations of hue, value and chroma . Human interpreter can distinguish between ten to twenty shades of grey, but can distinguish many more colours

13. In general, smooth surface tends to have high reflectance than rougher surface with less reflectance. Tone in aerial photographs is influenced by the following factors: light reflectivity of the object angle of reflected light the geographic latitude type of photography and film sensitivity light transmission of filters and photographic processing.

14. Size: Size of an object can be important tool for its identification, in two ways . First, the size of an object or feature is relative in relation to other objects on the image. This is probably the most direct and important function of size, as it provides the interpreter with an intuitive notion of the scale and resolution of an image even though no measurements or calculations may have been made . Second, absolute measurement can be equally valuable as interpretation aids. You should remember that size of an object in an image depends on the scale and resolution of the image.

15. Shape: Shape refers to the general form, configuration or outline of an individual object. Shape is one of the most important single factors for recognizing object from an image. Generally regular shapes, squares, rectangles, circles are signs of man-made objects, e.g., buildings, roads, and cultivated fields, whereas irregular shapes, with no distinct geometrical pattern are signs of a natural environment, e.g., a river, forest. In a general case of misinterpretation in between roads and train line: roads can have sharp turns, joints perpendicularly, but rails line does not. From the shape of the following image, it can be easily said that the dark-blue colored object is a river.

16. Pattern: Pattern refers to the spatial arrangement of the objects. Objects both natural and manmade have a pattern which aids in their recognition. The repetition of certain general form or relationship in tones and texture creates a pattern, which is characteristic of this element in image interpretation.

17. Shadow :  Shadow is a helpful element in image interpretation. It also creates difficulties for some objects in their identification in the image. Knowing the time of photography, we can estimate the solar elevation/illumination , which helps in height estimation of objects. The outline or shape of a shadow affords an impression of the profile view of objects. But objects within shadow become difficult to interpret.

18. Texture: Texture refers to the frequency of tonal variation in an image. Texture is produced by an aggregate unit of features which may be too small to be clearly discerned individually on the image.  Texture can be expressed qualitatively as coarse, moderate, fine, very fine, smooth , rough, rippled and mottled. It is rather easier to distinguish various textural classes visually than in the digital oriented techniques. Texture is, thus , dependent upon tone, shape, size, pattern, and scale of the imagery, and, is produced by a mixture of features that are too small to be seen individually. For example, grass and water generally appear ‘smooth’ while trees or a forest canopy may appear ‘rough ’.

19. Association: Association refers to the occurrence of certain features in relation to others objects in the imagery. In urban area a smooth vegetation pattern generally refers to a play ground or grass land not agricultural land. Smoke stacks, step buildings, cooling ponds, transformer yards, coal piles, railroad tracks = coal fired power plant. Arid terrain, basin bottom location, highly reflective surface, sparse vegetation = playa. W ater body surrounded by salt pond and saline patches = salt production units

20. Site: Site refers to topographic or geographic location . It is also an important element in image interpretation when objects are not clearly identified using the previous the elements. A very high reflectance feature in the Himalayan valley may be snow or cloud, but in Kerala one cannot say it as snow . For example, sewage treatment facilities are positioned at low topographic sites near stream or rivers to collect waste flowing through the system from higher locations. The relationship of feature to the surrounding features provides clues towards its identity.