What follows is a bit old
since the ScreenCastify tool is so/most practical.
Still worth reading though,
as you want to capture the spiritof 4SM's collection of tools.

Calibration under mBPL_3_to_20
for the whole lagoon
Now we want to cover the whole lagoon: 
both mSE=3 and mSE=4
  • change for...............-Extract/v/mBPL3/mask_4
    • this shall only select shallow pixels under mSE>=3 and mSE<=20
    • in order to exclude shallow pixels under mSE<3
  • run the script
    • see in X[1-2]vsX[3]
    • that -LsM/180.7/169.2/156.3 now is much too low
  • change for ...............-extract/v/mBPL3/mask_4
  • change for.........-B/tclNe/Bmin0/LBref170_100/cLM1.15
  • run the script
    • see that -LsM/203.4/192.1/178.5_cLM=1.150 would suit better
    • find it on your screen
  • disable     ...........@LsM/180.7/169.2/156.3_cLM=0.95
  • add.......................-LsM/203.4/192.1/178.5_cLM=1.15
  • don't forget to change back for cLM1.00...........-B/tclNe/Bmin0/LBref170_100/cLM1.00
What follows now provide more insight
into what we have just done

Now we are in for quite a surprise
  • We ran a new extraction of calibration data over the whole lagoon
    • A new calibration file tarawa-subset_m0.cal was extracted
    • it overwrites the previous file tarawa-subset_m0.cal
    • now the BPL pixels account for all shallow pixels in the lagoon area,
      • whether under mSE=3 or mSE=4 all the way to mSE=mSOIL-1=20
    • that is: pixels with mSE<3  are excluded.
  • The result is a new calibration diagram for the whole lagoon
    Very shallow and very bright BPL pixels
are located at the outlet of the hoas
which bring cristal-clear waters
from the outer reef
inside the lagoon
   Yellow/Red tones denote BPL pixels
for the pairs 13 and 23.
Blue tone denotes BPL pixels for the pair 12
First calibration
under mask 4 only

Second calibration
under masks 3 and 4

Bulging BPL pixels
Sure you want to see where in the image those bulging BPL pixels are located
This is done by enabling .../MapBPL/... in the -Extract/... argument
It is quite obvious that very shallow BPL pixels (0-0.5 m)
exhibit a distinctly brighter bottom signature: 
 In order to avoid saturation upon modeling, we had to increase LsM.
We did this by way of CoefLM in -B... argument
Syntax B
For this reason, in order to avoid excessive saturation of the computed bottom reflectance
  • we used a CoefLM=1.15  in the -B... argument
  •  -B/tclNe5.00/LBref170_100/Bmin0/cLM1.15 
    • this "slides" the LsM point up the Soil Line
      • (or down if CoefLM<1.00)
  • a new -LsM... argument is offered on your terminal
  • search for NewLsM in your terminal:
    • we enabled -LsM/203.4/192.1/178.5_cLM=1.15
    • then returned to -B/tclne_2.0/LBref170_100/1/cLM1.00
  • this new -LsM... argument now accounts for the very bright bottoms that just showed up at some very shallow locations under mSE=3
The calibration now accounts for this new maximum bottom refelctance

before increase of LsM
  • -LsM/179.6/173.6/174.2
  • -B/tclNe5.00/LBref170_100/Bmin0/cLM1.15

after increase of LsM
  • -LsM/202.3/197.3/199.1_CoefLM=1.15
  • -B/tclNe5.00/LBref170_100/Bmin0/cLM1.00

NewLsM and CoefLM
  • See that, if CoefLM is given a value >1 or <1,
    • a NewLsM is computed and applied:
    • it is obtained by sliding LsM up or down along the Soil Line
    • its value is shown in the Postscript display,
  • If found satisfactory, it may be used/enabled
    • to replace/override the existing -LsM... argument in the command line.
Why locally Brighter bottoms?
  • This very shallow extent of pure white bottoms is a very gently sloping lime mudflat
    • calcium carbonate precipitation occurs: waters there can be milky at times
    • Halimeda sands/muds are ubiquitous in this lagoon
    • finer grained entails higher reflectance
    • locally cleaner muds entails higher reflectance
    • etc
Why locally clearer waters?
  • This is easier to explain:
    • waters over the reef flat are cristal clear oligotrophic ocean waters
  • These cristal clear  oceanic waters intrude into the lagoon
    • through "hoas" across the reef rim:
    • nice swimming in those streams!
  • It is really necessary that the practitioner undertakes to gain understanding of the optics of the shallow water scene.
  • A very nice calibration has been obtained,
    • particularly thanks to the availability of a NIR band

Modeling stratified waters in 4SM
using -CP/... arguments
Very shallow
We want to fine-tune the optical model in the 0-0.50 m depth range
  • remove any @CP... arguments
  • run the script
  • on your terminal, look for the offer of  calibration pixels at 0.25 and 0.50 m
  • add the following to your command Line_10, just below the -M... argument
    • @CP/193.96/157.85/065.64_0.25m
    • @CP/185.04/130.29/027.71_0.50m
0.25 m
  • enable................-CP/193.96/157.85/065.64_0.25m
  • run the script for 0.25 m
    • see that........-CP/193.96/157.85/065.64_0.25m .....Green.....is too low
    • see that........-CP/193.96/157.85/065.64_0.25m .....NIR........is too high
  • disable.........@CP/193.96/157.85/065.64_0.25m
  • add.................-CP/196.96/157.85/058.64_0.25m
  • run the script
    • K1/K2=0.175: Jerlov water type becomes OI
      • water is as clear as can possibly get!
    • BPL fit is much improved in the 0-0.4 m depth range
0.50 m
  • enable....................-CP/185.04/130.29/027.71_0.50m
  • run the script for 0.50 m
    • see that no change is needed
    • deeper than ~1.0 m, BPL model is now parallel to BPL pixels 
    • K1/K2=0.2589: Jerlov water type OII+0.75 (i.e. almost OIII)
0.75 m
this is not needed anymore
  • Oceanic I: a very thin layer of extremely clear waters is present at very shallow depth (0-0.5 m)
    • Oceanic I water type of Jerlov
    • over the brightest bottoms in the area masked at 3 (orange in mask mSE),
    • where the bottom substrates are ~15% brighter than anywhere else in the scene.
  • Oceanic III: waters inside the lagoon exhibit an OIII Jerlov water type 

So, what's this fuss all about?
  • Not much indeed for this SPOT image
  • Worth trying anyway to show off how well we  master the situation!
  • Might be useful with some complex cases:
search your session screen display:
  • using a series of -CP... arguments, one can model "stratified" waters
  • here with two -CP... arguments, we have 
    • OI    water type , from 0 to 0.30 m
    • OIII water type , deeper than 0.5 m
  • this is presented here to show that 4SM has this capability
  • actually of not much interest here, 
    • as we are fiddling within the first 50 centimeters of the water column
    • and this refinement seems apply only in the vicinity of a few hoa intlets 
Straight BPL
no -CP/... argument
Curved BPL
two -CP/... arguments


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