a wideband may be considered to be comprised of several closely spaced narrow bands - First problem: multispectral bands are very wide
- we can't assume that the diffuse attenuation properties remain unchanged over the whole depth range
- as the bottom depth increases, the upward flux of photons probably gets depleted of its longest wavelength component
- as a result, the operational wavelength for wideband images would be seen to decrease as the depth increases
- this would seem to be the case for the Red band
- Second problem: the Green band spans well over the 575-600 nm range where gradient is strongest in Jerlov's data
- As a result, the optical behaviour of the Green band of ETM and ALI images would be quite complex indeed
- ETM and TM and ALI
- IKONOS
- SPOT no difficulty was observed
- Third problem: illumination conditions
- Jerlov's dataset refers to measurements at sea level, clear blue skyes, sun high in the sky
- this is not allways the case in real life
- scenes at high altitude: Greenland at ~1200 m Bolivia at ~3700 m
- overcast skies, adjacency effect: Red Sea, Persian Glulf, Bahamas
- the contribution of the skydome to the downwelling irradiance might well be the clue to our problems
- Fourth problem: does the existence of biofilms at the bottom surface alter the spectral bottom reflectance?
- See an illustration of the problem using ETM at RasHatibah, Red Sea
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- First problem: multispectral bands are very wide
- we can't assume that the diffuse attenuation properties remain unchanged over the whole depth range
- as the bottom depth increases, the upward flux of photons probably gets depleted of its longest wavelength component
- as a result, the operational wavelength for wideband images would be seen to decrease as the depth increases
- this would seem to be the case for the Red band
- Second problem: the Green band spans well over the 575-600 nm range where gradient is strongest in Jerlov's data
- As a result, the optical behaviour of the Green band of ETM and ALI images would be quite complex indeed
- ETM and TM and ALI
- IKONOS
- SPOT no difficulty was observed
- Third problem: illumination conditions
- Jerlov's dataset refers to measurements at sea level, clear blue skyes, sun high in the sky
- this is not allways the case in real life
- scenes at high altitude: Greenland at ~1200 m Bolivia at ~3700 m
- overcast skies, adjacency effect: Red Sea, Persian Glulf, Bahamas
- the contribution of the skydome to the downwelling irradiance might well be the clue to our problems
- Fourth problem: does the existence of biofilms at the bottom surface alter the spectral bottom reflectance?
- See an illustration of the problem using ETM at RasHatibah, Red Sea
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