FAQ : NO NEED FOR FIELD DATA?

NO NEED FOR FIELD DATA?

This has been presented at the ERIM Marine RS conference in 1998.

4SM only relies on the information contained in the bareland and marine parts of the spectral imagery in order to derive scene-dependent modeling parameters.

Seed value: then 4SM uses the table of diffuse attenuation coefficients for downwelling irradiance in marine/coastal Case I waters worldwide published in 1976 by Jerlov in his reference book "Marine Optics" (Elsevier), in order to derive a seed value for the operational attenuation coefficient for the water column under study which applies to the central(?) wavelength of each of the spectral waveband.

Spectral K: then, using that seed value, spectral operational K values are derived from Ki/Kj ratios observed for all possible pairs of visible bands i and j of the image itself.

Operational wavelength: this requires effective visible wavelengths to specified correctly, as it would seem that wavelength at mid-waveband are not suitable

Therefore, 4SM does not require, neither does it uses, any preliminary field data for the estimation of any of the modeling parameters.

As a consequence, the only thing that is needed is the imagery itself and the specifications of its band setting.

But all computed depths still need to be multiplied
by a final depth correction factor
to be derived from sea truth evidence when available:

Final_Z = CoefZ*Z - Htide_to_datum

BAND SETTING: WHICH WAVELENGTH?

As of early 2014, I have come to realize that K values derived from Jerlov's data depend to a large extent on the geometry of the diffuse sun and sky downwelling irradiance light field.
Jerlov's data apply to a "clear sky, sun high in the sky" conditions.
- This is not allways the case in real life, by far.
Hence the recurrent complaint that, from inspecting the calibration diagram, I very often need to specify the effective operational wavelength away from mid-waveband, on a scene-specific basis [1 , 2, 3].
Therefore the claim that "no need for field data" is a bit of a stretch of the operational realities.
- Until this question receives some sholarly attention.
- Until a formal link can be explicitly formulated between the observed illumination geometric conditions and the particular diffuse attenuation coefficient that applies to the upwelling photons which make it into the near-nadir viewing sensor's telescope.

Problem solved