- WIDEBANDS: I have long recognised that the optical calibration cannot be achieved to satisfaction
- over the years, I have come to suspect that the effective operational wavelength for the GREEN wideband of Landsat TMneeded adjustment away from mid-waveband
- and blamed the problem on the GREEN wideband, for lack of suitable seatruth evidence
- SHUWAYHAT study case using Landsat OLI data: then in october 2014, I realized that
- just by reducing 2K for all bands over the orange-red range
- one achieves a satisfactory fit in the calibration diagrams
- and modeling yields correct depths: no need for field data
- NARROW BANDS: then I set out to look at hyperspectral data for a test of that finding
- it is possible to achieve an impeccable optical calibration for all visible bands of an hyperspectral image
- this is illustrated below for HYPERION data at Fakarava atoll, French Polynesia
- in fact, I realize that I have overlooked this aspect in the past
- and therefore indulged in somewhat lousy practices and comments
- To achieve an impeccable optical calibration, I need to decrease substantially the effective two-way diffuse radiance attenuation coefficient 2K in the orange-red range of the solar spectrum
- This is illustrated below
- Please refer to Jerlov's quotation in respect of diffuse irradiance attenuation coefficient the 0-10 m depth range under high solar elevation
- it should mean that the diffuse downwelling irradiance attenuation Kd is expected to be just a bit higher than the absorption coefficient a over the Red range
- it remains to be seen how this possibly translates to the diffuse two-ways radiance attenuation coefficient 2K which most of us operate in passive shallow water work
- This warrants an investigation using HYDOLIGHT, under the full range of atmospheric, illumination and viewing conditions of commercially available satellite imageries, whether MULTI or HYPER spectral.
It would appear that, subject to this observation,
the optical calibration results in a correct estimation
of spectral 2K for all visible bands.
In other words, no need for field data for shallow water work.
This is likely to be applicable to all passive spectral imageries.
All 4SM work done prior to october 2014,
and presented in this website,
suffered from confused misconceptions in this regard.
|
