Types of Image Interpretation

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.