WV2 4SM at Gulf of Laganas Greece

Optical calibration for PAN band
AutoCALIBRATION

mBPL=1 vs mBPL=2

Optical calibration for MULTI bands
certainly a "not for beginners" study case

First look at the 0-5 m depth range under Mask_4 Brown: land areas are flagged in special effects mask any scattered "sea" pixel inside land areas must be forced to land Green polygons mask_1: then, radiometric artifacts over "sea" areas must be eliminated upon extraction of calibration data: such as boats and their wakes Mask_4 is the red area for some reason, things only become convincing when restricting the extraction of calibration data to pixels flaged at Mask=4 (red color). Deglinting of sea pixels is mandatory upon extraction of calibration data, in order to remove adjacency effect which is quite strong here. One stands a null chance of grasping an acceptable optical calibration for this scene, unless the above precautions have been identified and applied cautiously: certainly a "not for beginners" study case	First look at the 0-5 m depth range under Mask_4 Special effects mask See AutoCAL
under mask_4 Calibration diagram for bands 2, 3, 6 and 7 X[2] vs X[3]: we apply K_blue/K_green=0.95 although data do not clearly support this choice X[2_3] vs X6: this diagram, which covers the 0-5 m depth range, clearly shows that K_blue/K_red ~= K_green/K_red, in other words K_blue~=K_green this confirms K_blue/K_green=0.95 This at least applies to the 0-5 m depth range, but obviously does not apply over the 5-30 m depth range	under mask_4 Calibration diagram for bands 2, 5, 6 and 7 Over the 0-5 m depth range, the scenario is quite clear and straightforward : we observe a fairly tight fit of the BPL pixels to the Ki/Kj optical model for a OII+0.9 water type of Jerlov for bands 2, 5, 6 and 7 this is obtained by applying a ratio K_green/K_blue=0.95 , then deriving spectral K values for all visible bands strictly from Jerlov's data.
Brightest shallow pixel LsM -LsM/248.5/267.1/209.4/204.2/206.6/208.0/146.8/125.6/118.0 TOA for Laganas beach Clearly, the brightest shallow bottoms over the 0-5 m depth range are very well estimated. this is ~0.25 times the brightness of coral reef sands	Brightest shallow pixel LsM -LsM/248.5/267.1/209.4/204.2/206.6/208.0/146.8/125.6/118.0 TOA for Laganas beach We shall assume that this is also the case over the 5-30 m depth range: this is at the heart of 4SM assumptions i.e that the brightest shallow bottoms are present at all depths, if only as small isolated patches.
Then pay attention to the 5-30 m depth range under Mask_4	Then pay attention to the 5-30 m depth range under Mask_4
Calibration diagram for bands 2, 3, 6 and 7 the optical model now includes spectral diffuse 2K value for all MULTI bands at 1100 discrete depths over the 0-50 m depth range here this describes homogeneous OIII water from 0 to 5 m, then clearing progressively to OIB from 5 to in excess of 30 m.	Now we must figure out what is going on over the 5-30 m depth range: see diagram [X2] vs [X3] we shall now assume that waters become clearer as depth increases, from 5 m down to ~30 m and find a way to account for this assumption in the optical model For this we use calibration pixels -CP... in the command line by which to force the model into fitting the ovserved data so that the ratio K_blue/K_green increases steadily from K_blue/K_green=0.95 at 5 m to K_blue/K_green=0.54 at ~30 m -CP/210.9/195.9/123.8/077.2/064.7/051.7/029.5/019.0/010.4_5.00m -CP/200.1/170.0/085.8/063.0/055.5/049.0/029.5/019.0/010.4_20.00m This amounts to assume that the water type changes progressively from OIII at 5 m to OIB at 30 m this is compatible with the common experience by scuba divers: waters tend to get clearer as one swims away from the beach!
Reference to coral reef sands -LsM/423.5/553.4/511.3/544.8/566.3/586.5/426.0/379.3/374.1 TOA in the image for coral reef sand	Reference to coral reef sands See that coral reef sands are ~3 times brighter than even the brightest shallow bottoms at Laganas.
Calibration diagram for bands 2, 3, 6 and 7 optical model applied to an average coral reef sand	Calibration diagram for bands 2, 4, 6 and 7 optical model applied to an average coral reef sand
La and Lw Then get an estimation of La: atmospheric path radiance and Lw: water volume reflectance (backscatter)	La and Lw Then get an estimation of La: atmospheric path radiance and Lw: water volume reflectance (backscatter)
Calibration diagram for bands 2, 3, 6 and 7 Land areas have been excluded so that the bidimensional diagrams represent only marine pixels over the whole depth range	Lsw: TOA spectral deep water radiance has been estimated when specifying deglinting conditions. We now need to estimate Lw: BOA spectral water volume reflectance (backscatter). So that spectral atmospheric path radiance may now be specified as path radiance La=Lsw-Lw this is the "dark pixel assumption" for a first order atmospheric correction (assuming that glint -and/or adjacency effect- has been removed efficiently). This is conveniently done using the calibration diagram by adjusting the -Lw... parameter so that the curvature of the Soil Line parallels that of bidim histogram pixels at null depth knowing that, for clear blue waters, the backscatter Lw is negligeable over the RED-NIR range Lw is low but significant over the GREEN range Lw increases dramatically over the BLUE range

Optical calibration for PAN band
PAN band is in channel 4
of my 4SM work database laganas_wvmp.pix
certainly a "not for beginners"
see PAN response for WV2
For a 4SM application,
the PAN response should be stronger in the Blue-Green range:
this would reduce the system noise,
and yield more reliable and stable results.

As the optical water type has been evaluated at discrete depth intervals over the whole depth range,
- the spectral diffuse attenuation coefficient for all of Jerlov's visible wavelengths is now specified at 1100 depth intervals for all MULTI bands.
This is sort of a LUT which describes diffuse attenuation properties of a homogeneous OIII water from 0 to 5 m, then clearing progressively to OIB from 5 to in excess of 30 m.

Using the response curve of the PAN channel of the WV2 sensor, one may now compute K_PAN for each of these 1100 depths.

The optical model now includes spectral diffuse 2K value for all WV2 bands, including PAN, at 1100 discrete depths over the 0-50 m depth range

Calibration diagram
for bands 2, 4, 6 and 7
X[2] vs X[4]: see that the model
using the PAN response curve
is quite close to yield a very good fit

Calibration diagram
for bands 2, 4, 6 and 7
with reference to average coral reef sand

March 2nd 2016 AutoCALIBRATION March 2nd 2016

AutoCAL

After considerable investigation of this scene, I realized that a few artifacts in the sea area, which had not been masked, were still interfering with the extraction of calibration data, thus preventing a clear appreciation of the calibration diagram.

I diagnosed this by inspecting the laganas_wvmp_m0.cal text file: all BPL pixels are referenced with their row/line location. Odd pixels turned out to be artifacts, i.e. objects at the sea surface which should have been masked out.
It just happened that the area under mask_4 did not have any remaining adverse artifact!

"on the fly"

The AutoCAL procedure goes all the way from

creating the database,
then importing the raw 16U data,
then getting glint regressions
then using shapefiles to create the special effects mask in channel_10,
then extracting the calibration data from the whole scene (except unwanted areas masked at 1) into the text file laganas_wvmp_m0.cal ,
then formating the script calibration_AutoCal.sh
to displaying the laganas_wvmp_AutoCal_8_6_3_2_m0.eps calibration diagram.

AutoCAL

laganas_wvmp_8_6_3_2_m0_AutoCal

This diagram represents the whole scene,
-instead of just the area masked at 4.

In the X[2_3] vs X[6] diagram,
it is quite clear that
K_blue/K_red~=K_green/K_red.

In other words K_blue/K_green~=1
in the 0-5 m depth range
of the RED band.
This denotes
a OII+0.9 water type of Jerlov.

In this AutoCAL procedure,
the ratio K_blue/K_green=0.5 by default,
just because a value is needed
for the procedure to execute.

The manual calibration that follows
now needs to apply
a realistic value for K_blue/K_green.

If the BPL assumption holds,
all BPL pixels in the X[2] vs X[3] diagram
represent
the same brightest bottom substrate.
This leads us to assume that
waters get progressively clearer as
the depth increases deeper than ~5 m.

"on the fly"

nice -19 ./4SM.5.03 -Process   -Origin/Oikonomou
-DB/laganas_wvmp/50_3S_9S_0s/9_54/8114_6968/484.615_4181.115/1_1
-Mis/Greece/Zakynthos/WV02/MultiPan/PIX/bOA/UTM_34_008/0.002_0.002/22_AUG_2014
-MakePIX/Import/scale/AutoCAL   @-Prepare/Scale
-Import/v/dTM/data*laganas_data/dbnc_0_0s_9S_0s/R8114_L6968/Origin_484.614_4181.116_0_0/chIn1,9/chOut54,62
-LS/005000/05000/05000/05000/05000/05000/05000/05000/05000
-M/@000001/@0002/@0003/00004/00005/00006/00007/00008/00009
-SCL/00050/00035/00060/00060/00080/00060/00100/00110/00100
-deglint/vRbaD/q/DegTol1.00/F1/L9/N1/FN/mDEGLINT1/GlintM70
-extract/v/rawBDH/FullBDH/NIRband8/NIRmax255/mapBPL/mSOIL21/mBPL2
-AutoCAL/print_2_3/mapBPL/MakeNewMask/Land_5_1.05_8/GetGlint/GlintM70/clouds

#First create laganas_wvmp.pix working database, then import the 16U data
#Then use masks to get glint parameters, and to create special effect mask in channel_10
#Then extract calibration data textfiles laganas_wvmp_m0.cal and laganas_wvmp_m0.bdh
#Then format calibration script calibration_AutoCal.sh
#Then run calibration_AutoCal.sh
#Then display calibration diagram laganas_wvmp_AutoCal_8_6_3_2_m0.eps
#Then format a complete command line AutoCal.txt
#Then stop

**mBPL=1 vs mBPL=2**
mBPL=1 to 20 all marine pixels are accepted -Extract/v/raw...............SOIL21/mBPL1	mBPL=2 to 20 marine pixels masked at 1 are excluded -Extract/v/raw...............SOIL21/mBPL2
Zoom on X[2] vs X[3] mBPL=1 to 20 Most confusing see how artefacts feature	Zoom on X[2] vs X[3] mBPL=2 to 20 Things get clearer! see that artifact pixels have disappeared