4SM results (february 2002)
This exercise is a clear proof
that the calibration of the model
using Jerlov's data
is trustworthy,
provided the effective wavelengths
are precisely specified


below : image_Z, image_B, and image-BSC for CASI Line_1

February 2002
AN EXERCISE IN CALIBRATION AND SEA-TRUTHING

• This dataset lacks a real coverage of optically deep waters

• Sea truth was acquired one year after CASI imagery in this highly dynamic environment.

• Optical modeling results reported above by Manson et al. were obtained prior any field data was communicated to Y. Morel.
  
   

The best 4SM seatruth in town
work done 14 february 2002,
using the new radiometrically and geometrically corrected CASI image
and the complete seatruth dataset

band 5 is 577 nm
So it would appear that a bad choice of deep water radiances
which are nowhere to be sampled in the image
resulted in a a bad underestimation of the depth
over the dark bottomed troughs in the blind test
mmmff....


Please, Gavin, consider some alterations to your paper!
My advice would have been :

• first try Landsat to get a feeling of the scene
• then use multispectral satellite images: they'll be good enough for bathymetry purpose
• as CASI does not carry any significant advantage fir such a sandy environment
• but, I know, in those days, going CASI was the ultimate craze!!

 By application of an input K[799nm]=4.56 m -1 in the -KK argument of the command line , 
a CoefZ=1.33 is proposed by the calibration process in calibration diagram ns-E calibration.
The details are as follows:

• Left alone, initial calibration through Jerlov's water type yields K[799nm]=6.075 m -1:

  • see ns-E calibration . An unlikely value at this wavelength of 799 nm.

• Left alone, this initial calibration yields a sea truth regression for Line6100 of ZC=0.02+0.75*ZR (N=167; r 2=0.92; RMS=0.94 m) ns-E RegressZZ Line6100 1.

• This suggests that a value of  CoefZ=1/0.75=1.33 is in order.

• ==> The application of a value of CoefZ=1.33 brings about an excellent fit for Line6100: ZC=0.03+1.00*ZR (N=167; r 2=0.92; RMS=0.36 m), as well as for most of the whole seatruth dataset  ns-E RegressZZ ALL.

• We can now derive an operational value for K[799nm]=6.075/1.33=4.56 m -1, which is virtually twice the absorption coefficient for pure water at this wavelength.

• Therefore we can accept  that K[799nm]=4.56 m -1 is a very satisfactory operational value.

• Using this last value as a seed value to the complete set of Ki/Kj ratios observed in the image, we can then derive the whole series of spectral operational K values for the whole CASI bandsetting.

• This suggests that, using published values for the absorption coefficient  of pure water in the Near InfraRed region of the solar spectrum, it might turn out that a quasi-absolute optical calibration of shallow water optical modeling may be achieved without the use of any field data.

True color composite                                                  Water column corrected true color composite

• The sea truth dataset was acquired in summer 2001,  one year after the CASI imagery. 
• Major storms have affected the scene between those two data collection exercises,
  • which have affected the location and the shape of the sand waves.
• Sand waves appear to rest on a bed of red-rich dark bottom deposit.

• In sea truth regression for all_lines ns-E RegressZZ ALL ,

  • it is clear that a group of pixels stands out of the crowd.

  • these are located over a locally dark-bottomed trough between two sand waves.

• In the sea truth profiles for the blind test ns-E_ProfileZZ ,

  • the computed depths (colored profile) across the dark patch ns-E_dark_bottoms

  • appears to have been badly underestimated (Line6400 to Line6900),

  • as the trough between sand waves is shown to be flat and shallow at approx 3.5 m in the computed depths

  • while sea truth (black profile) shows depths ranging from 4 to 6 m one year later.

• These "faulty" pixels exhibit radiance levels well above deep water radiances in the 650-670 nm range: ns-E_650nm

  • This radiance level is ~7 DNs above deep water radiance in 8-bits scaled bands at 650 nm.

• Maximum depth of bottom detection for bright sands does not exceed ~5-4 m over this wavelength range,

  • even much less for a dark bottom.

• Applying a depth 1-2 m deeper would result in unacceptable extremely bright computed bottom reflectances

  • whereas it is clear that the bottom  exhibits distinctly dark features: see ns-E_RGB where a raw RGB view is shown on left, and a "water column corrected" RGB view is shown on the right.

• Note in this last image that most bright bottoms exhibit a greenish computed bottom spectral signature.

  • either because the bottom has really changed since sea truth data acquisition

  • or because of the growth of algae/seaweeds , in which case the top of the canopy is detected instead of the true bottom.

• It is therefore likely that genuine bottom detection  actually occurs at 650 nm

WATER COLUMN CORRECTED SPECTRAL BOTTOM REFLECTANCES

This shows that the water-column corrected radiances are available for thematic classification
 

•  Scatter plot of spectral bottom reflectance inside the red polygon above

  • Left :   TOA raw radiances Ls[490] versus Ls[547].

  • Right: BOA normalized water column corrected radiances LB[490] versus LB[547].

Water column corrected spectral bottom signatures
ready for bottom ytping

Five BOA normalized water column corrected spectral bottom reflectance signatures are illustrated 
above a backdrop of true color composite of BOA water column corrected radiances

• for 10 spectral bands ranging from ch_46=490 nm to ch_55=690 nm

• for   5 distinct targets (black, red, blue, green, yellow).

• either inland waters pour out of the main inlet?
• or bottoms at the inlet are darker

NIR band at 850 nm

Water column corrected

• all water column corrected shallow bottoms are very dark
•  

Water column corrected, Normalized

• this enhances the spectral bottom signature,
  • in view of bottyom typing
• ?Shallow bottoms appear to have a greenish signature

Computed depth

Average bottom brightness

• all water column corrected shallow bottoms are very dark
• shalow sands in the inlet are the same as along the SW coast 
• bottoms just in front of the inlet are extremely dark
  • ?this extends all the way along the NE coast
• Now, ?I see no sign that the waters in the inlet would be less clear

Calibration 11_8_6_1

• Jerlov water type is half way between            Coastal 1 and 2
• shallow pixels are very dark
• Knir is confirmed at 4.5 m-1

BiDimensional histograms

• shallow pixels are the tiny group with distinct exponential decay shape, with radiances hardly exceeding 100 in all bands
• it is seen that shallow bottoms are very dark, with radiances hardly exceeding 100 in all bands,
• while the 100-255 range depicts exclusively land pixels and wave breakers