HowTo SPECTRAL K
K[green]/K[red]=0.277: try other values for K[green]/K[red] like 0.25 or 0.35
make your decision on what seems to be the best visual fit of the BPL with BPL pixels
Which value for Knir?
towards a quasi-absolute calibration
- Jerlov's data are provided in the range 400 to 700 nanometers, by steps of 25 nm,
- for clear Oceanic Waters and Coastal Waters, free of heavy suspended load
- KWLi and KWLj are estimated from Jerlov's data using the ratio KWLi/KWLj=0.277
- observed in the image for wavelengths 550nm and 650 nm
- this specifies an intermediate curve among Jerlov's curves: type ~C1
- from the complete set of ratios Ki/Kj observed in the image
- and one of KWLi=0.247 m-1 or KWLj=0.893 m-1
- Knir the NIR band is not covered by Jerlov's data
- default: by default, Knir is set to the value which appears in the -KK argument.
- -CP: if a calibration pixel, which is valid for the NIR band is provided in the -CP argument,
- "valid" is to mean: "with a significant bottom contrast Ls-Lsw"
- then 4SM derives a value for Knir that is consistent with the complete series of Ki/ Kj ratios observed in the image among all bands.
- look in tarawa-subset_m0.cal for a suitable set of values
- -CPf0159.0/100.0/015.0 may be selected
- this yields Knir =4.506 m-1
- this value shall be used by 4SM instead of the value provided in the commandline
- the pixel selected yields a depth of 0.70 m in the calibration
- yes, try it: enable a -CP... argument, and see for yourself in the calibration plot
- See that this entails a value of K1/K2=0.3376 and a bad fit of the BPL model
- adjust the values in -CPf159.0/100.0/015.0, so as to get a good fit of the BPL in the Green/Red pair
- we need to disable the existing argument and make a new one
- we can
- either reduce the Red value
- or increase the Green value
- or both
- while aiming at a satisfactory fit with a slope K1/K2=0.30
- see that -CPf161.0/098.0/015.0 yields
- a good fit of all three BPL models
- K1/K2=0.3016 with water type ~OIII+0.22
- enable that value
CoefZ in the -Z argument
all K values in the visible range are divided by CoefZ prior to any computation.
- If CoefZ!=1.0 , like 1.387 or 1.0001 or 0.9999 or 0.8736...,
- then 4SM applies it to all computed depths in all circumstances
- this is how to accomodate a seatruth-derived depth correcting factor
- This not compatible with a -CP... argument though
- therefore if a -CP... argument is enabled, 4SM makes CoefZ=1.0000000
- so that the following may take place
- If CoefZ==1.000000000
- and a calibration pixel, which is valid for the NIR band, is provided in the -CP argument,
- then a new CoefZ is computed (as CoefZ=4.288/4.42=0.9701 in our case)
- and applied to all computed depths: this is noted in the calibration diagram.
- To the extent that K850 would appear to be stable at ~=4.4 m-1 whatever the water type
- still to be confirmed,
- the combined use of the -CP argument and actualized CoefZ
- amounts to a quasi-absolute_calibration of spectral K
- which may be controlled through the Knir value provided in the -KK argument.
- This is fairly heavily dependent though on the deep water radiances adopted through the -deglint argument: for example
- if Lsw 3= 9 ==> K 1=0.2549 K 2=0.8932 CoefZ=0.930 Z=12.2 m at pixel 469/562
- if Lsw 3=10 ==> K 1=0.2554 K 2=0.8933 CoefZ=0.971 Z=12.6 m at pixel 469/562
- if Lsw 3=11 ==> K 1=0.2559 K 2=0.8934 CoefZ=1.020 Z=13.2 m at pixel 469/562