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The data
Deglinting
Optical calibration
Deep water radiance
Water volume reflectance Lw and path radiance La
The "PAN solution"
Retrieved depth
Bottom reflectance
Command Line

Habitat map


The data
Seven bands are used: Purple, Blue, Green, PAN, Red, NIR and SWIR1

NIR band Note the very strong increase of atmospheric path radiance from west to east PAN band	TCC Seatruth USGS's EAARL LIDAR DTM merge_5.tif over Dry Tortugas is overlayed FCC
USGS's merge_5.tif LIDAR DTM I have resampled it to 30 resolution, by 6*6 sum aggregation

Deglinting

Profile RED and Profile Blue Also shows the glint training site in dark blue                               Deglinted TCC The location of the glint sample is shown in blue The location of profile_green is shown Glint regression based on NIR band   The atmosphere in this image is quite thick Deglinting efficiently removes this variation in path radiance from West to East: NO NEED FOR ATMOSPHERIC CORRECTION

Deglinting of Purple, Blue, Green, PAN and RED bands along Profile Red Please note the pronounced and steady increase of the signal in the from West to East: "deglinting"  removes efficiently this path radiance From 0 to 160 km, path radiance decreases continuously over open Sargasso Sea waters From 260 to 390 km, path radiance increases continuously over Gulf of Mexico's waters From 190 to 280 km, we choose to estimate that waters are optically deep for all bands: this is done through the dLsw... argument in the command line -dLsw003.0/003.0/002.0/001.0/000.9/000.0/000.0

The deep water radiance may not be ascertained with confidence at shorter wavelengths over optically deep waters

Gulf waters exhibit stronger deep water radiance relative to Keys waters Dry Tortugas area must be assigned its own set of optical parameters So does Keys area

Optical calibration
for the Dry Tortugas National Park area:
Water type OIB+0.1 of Jerlov
Spectral reflectance of the brightest bottoms:
0.154 0.196 0.230 0.242 0.286 0.387 0.324
Average coral reef sands are ~1.8 times brighter
on the scale 0-1, as calibrated using the _MTL.txt textfile

Calibration diagram for bands Blue, Green, Red and NIR	Calibration diagram for bands Purple, Green, Red and NIR
Operational wavelengths Here again the wavelength for the Red band must be set at a very low value: WLred=615 nm the wavelength for the Purple and Blue bands are set at mid-waveband: WLpurple=400 nm the wavelength for the Green bands is set at 535 nm (while mid-waveband would be 560 nm): THIS IS THE ONLY SOLUTION TO OBTAIN CORRECT RETRIEVED DEPTHS, and first time we can access this insight, thanks to the LIDAR DTM The use of Hydrolight code should provide an insight into this question, where the dependance on the average cosine of the illumination conditions must play a key role. WHAT'S NEW HERE IS THAT I HAVE A LIDAR DTM WHICH WILL ALLOW ME TO TELL THE TRUTH OVER THAT ALL-IMPORTANT QUESTION THE RESULT IS: OLI DATA FOR THIS IMAGE SUPPORT THE ABOVE ASSUMPTION: WLred must be set at a very low value WLgreen is set much lower than mid-wavelength WLblue can stay at mid-wavelength WLpurplecan stay at mid-wavelength	Operational wavelengths I suspect that this is all-important aspect  (for a "NoNeed or field data" method) very much depends on the bi-directional properties of the light field because stuffy atmospheres deliver very high levels of very diffuse illumination this is most often the case in the Gulf of Arabs (Emirates) As a result, very low diffuse attenuation coefficients are observed under very high levels of very diffuse illumination (whitish overcast skyes),  while Jerlov's diffuse attenuation data assume clear dep-blue skies As a consequence, under such illumination conditions, record shallow water penetrations are observed, which exceed by large the perfomance we can expect from using Jerlov's data.
KPAN In the absence of ubiquitous very bright bottoms in this terrigeneous environment, the conditions are not suitable for the calibration of the panchromatic band. But this is not a problem, as I seem to have established for good that the decay of KPAN may be expressed as a  generic dimensionless function of the exponential decay observed in the Blue band. We shall see that the LIDAR DTM supports this finfing beautifully	RM maximum BOA shallow bottom reflectance here in this terrigeneous environment I get 0.154 0.196 0.230 0.242 0.286 0.387 0.324 but the average coral reef sands in the Bahamas and GBR are ~1.8 times brighter: 0.259 0.356 0.429 0.443 0.502 0.678 Spring time This is spring time: healthy seaweeds are ubiquitous, and recovering from winter sadness This assumption is supported by the calibration diagram: see the white square BPL pixels in the Ls[2] vs Ls[3] diagram these actually are all Ls[2 & 3] vs Ls[5] BPL pixels: many of these exhibit a very green signature, i.e. low in blue

Tuning the deep water radiance Lsw
for Dry Tortugas Reef surroundings
Thyis tuning is most critical in 4SM

Deep water radiance values are derived using the glint regressions. By inspection of a profile of radiances over deep waters at a particular location, I can adjust the deep water radiances according to how I think things might work for the current scene at that location. -Lsw/133.5/095.2/043.1/037.2/023.1/009.6/018.2 -dLsw003.0/003.0/002.0/001.0/000.9/000.0/000.0 Red band Lswred=23.1+0.9==>24.0 PAN band Lswpan=37.2+1.0==>38.2 Green band Lswgreen=43.1+2.0==>45.1 Blue band Lswgreen=95.2+3.0==>98.2 Purple band Lswpurple=133.5+3.0==>136.5 The result is: Lsw   136.5  98.2  45.1  38.2  24.0   9.6  18.2   This tuning is very sensitive. As it allows to set threshold values at fairly low values, and therefore to extend the effective depth range

Before tuning After tuning: Deep water radiances have been adjusted The result is: Lsw   136.5  98.2  45.1  38.2  24.0   9.6  18.2

 

link to  Water volume reflectance Lw Path radiance La The Soil Line for Dry Tortugas National Park surroundings