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Remote Sensing - a tool for environmental observation

Geometric accuracy the image should have the least

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Unformatted text preview: - geometric accuracy, the image should have the least possible systematic and stochastic geometric distortions. - temporal resolution, the frequency of image acquisition should match with the temporal changes of the objects of interest. Apart from the above-mentioned considerations, the following points are important: the position of the atmospheric windows and the spectral region in which the source of electromagnetic 19 energy emits. 1.7 Colour technology The first reconnaissance or the interpretation of remote sensing images is most often performed with the human eye. Apart from grey tones, texture and patterns, colour and colour patterns play an important role in the interpretation. Therefore, it is important to have some knowledge of colour, mixing of colours and colour film technology. The average human eye can discriminate many more shades of colour than it can tones of grey. Hence, the use of colour increases largely the interpretability of images. The colours seen by the human eye are in fact described by three variables intensity, hue and saturation (IHS): - Intensity (also referred to as value or brightness) relates to the total brightness of a colour (amplitude of the wave); - hue (colour tone) refers to the dominant or average wavelength of light contributing to a colour (frequency); - saturation (or chroma) specifies the purity of the colour. Hence, a three dimensional space (international colour cone) is required to describe these three variables (figure 1.14). Image processing normally uses additive colours likewise televisions. Additive primary colours The visible part (0.4-0.7 μm) of the spectrum can be divided into six gradational colours: violet, blue, green, yellow, orange and red. Almost all colours can be synthesized by adding different portions of blue, green and red. These are called the three additive primary colours because synthetic white light forms when equal amounts of blue, green and red are superimposed. The range of each additive primary colour spans one-third of the visible spectrum. A colour image can be produced by adding these three primary colours on a television screen or computer monitor. Subtractive primary colours Colours on e.g. film can also be produced by subtracting specific parts of the spectrum. In order to mix colours for films, the three subtractive primary colours must be used: yellow, magenta and cyan. Colour films Colour prints are colour photographs with an opaque base. Colour film is a transparent medium that may be either positive or negative. On conventional negative film, the colour present is complementary (!) to the colour of the object photographed and the density on the film is the inverse of the brightness of the object....
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