Earths gray values, a sixteen bits representation has
Earths surface (i.e. the land, atmosphere, and oceans) which can be achieved withno direct physical contact. Information is obtained from capturing the reflected Electromagneticwave from the earth surface, and its difference as a function of wavelength,angle direction, phase, location, and time. A variety of sensors are generallydeployed both passive (i.e. which depends on reflected solar radiation or emittedradiation) and active (i.e. which generates its own source of Electromagnetic radiation).These sensors operate all over the Electromagnetic spectrum from visibleto microwave wavelengths. There are different platforms on which these sensorsare attached such as earth-orbiting satellites, aircraft are by far the most common,helicopters, and balloons are also used. Normally, remote sensing images are categorizedby four different characteristics based on the type of sensors technology.The first characteristic is the spatial resolution where the higher is the resolution themore details about objects on earth can appear in the image (the highest resolutionis few centimeters while the lowest can reach 9 kilometers). The lower is the resolutionthe more area is captured. The second characteristic of the remote sensingimages is the spectral resolution. It defines the capacity of the sensors to differentiatebetween wavelengths in the Electromagnetic (EM) spectrum (bands). The finerthe resolution the more images (bands) are collected which means that it is possibleto obtain more details about the reaction of sensed objects to the light. There aretwo technologies hyperspectral and multispectral where the first can have a spectralresolution of 1 nm with hundreds of bands compared to coarser spectral resolutionto the last one that can reach 100 nm with few bands. The third characteristic is thetemporal resolution which defines the period of time needed to revisit the same spoton earth. This is only applicable to satellite remote sensing . The less time needed torevisit a specific area the higher is the temporal resolution. The fourth and last characteristicis the radiometric characteristic. The radiometric resolution specifies howwell the differences in brightness in an image can be perceived; this is measuredthrough the number of the levels of the gray value. The maximum value related toradiometric resolution is defined by the number of bits (binary numbers). Eight bitsrepresentation has 256 gray values, a sixteen bits representation has 65,536 greyvalues. The finer or the higher the radiometric resolution is the better is the recordedreflected waves, but the volume of measured data will be larger.