Landsat 8 OLIP from DryTortugas to Key West, Florida Keys
6000*2000, 30 m pixel size, UTM zone 17
scene LC80160432013102LGN01, April 12th 2013. Image courtesy of the U.S. Geological Survey
with USGS's EAARL LIDAR seatruth dataset merge_5.tif
With a DTM at hand,
I can decide
which operational practices in 4SM should be ruled out
and which are confirmed

Work done jan/feb 2014 for Dry Tortugas National Park area
Using the Panchromatic band for water column correction
to derive water depth and spectral bottom signature:

Landsat 8 OLIP bandset used for this work
Purple_1Blue_2Green_3PAN_4Red_5NIR_6 and SWIR1_7

This is spring in Florida
see Hyperion and ALI at Florida Keys

1 - NO NEED for field data, nor for atmospheric correction
2 - this is demonstrated in this website, using a variety of hyper/multi spectral data
Requirements are
1 - homogeneous water body and atmosphere
2 - some coverage of optically deep water
3 - some coverage of dry land
Problems are
1 - the precision on estimated depth is found wanting, because the noise-equivalent change in radiance  of accessible data is too high for shallow water column correction work 
2 - radiance data should be preprocessed by the provider at level 1 in order to improve S/N ratio
3 - exponential decay: the deeper/darker the bottom, the poorer the performances
I keep digging
until suitable data
become available
The data
Optical calibration
Deep water radiance
Water volume reflectance Lw and path radiance La
The "PAN solution"
Retrieved depth
Bottom reflectance
Command Line
File:Dry tortugas94.jpg
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

merge_5.tif over Dry Tortugas
is overlayed


USGS's merge_5.tif LIDAR DTM
I have resampled it to 30 resolution,
by 6*6 sum aggregation

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:

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
  • 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.

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.

  • 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
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 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.

  • 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 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