• Todate, and with regard to radiometric quality of multispectral data, the only sensors worthy of shallow water work I've had access to are
  • AAHIS (courtesy of Eric Hochberg)
  • SPOT 5 (courtesy of IGN)
  • EOS ALI (courtesy of USGS)
• Quite many other existing sensor should be added to this short list, that I don't have access to
  • it is a question of "Noise Equivalent change in radiance"
     
• Things are improving over time, though, but still at present, most sensors are designed to service land based applications

Bad deglinting and high level of system noise are obvious
Linearity of the sensor's response seems questionable

• The BPL assumption allows for the estimation of the complete series of ratios Ki/Kj among visible bands
  • which in turn allows for the estimation of jerlov's intermediate water type
  • which in turn yields a complete series of diffuse attenuation coefficients 2K in units of m-1

• the error on the estimation of a suitable ratio Ki/Kj from the image entails

  • an error on the estimation of Jerlov's water type

  • an error on computed depth which increases with depth

• The Soil Line assumption: the spectral properties of the bareland areas in the image, which are at null depth, are used to elaborate a spectral model for water column corrected bottom reflectances
• The Soil Line  is defined as the straight line that joins the following two points in a biplot of Li vs Lj
  • La: the radiance of a black body
  • LsM: the radiance of a the brightest shallow substrate at null depth
• The Soil Line represents the continuous series of spectrally neutral substrates from the brightest to the darkest
• The normalisation is achieved by ensuring that the BOA radiance LsM-La is made to equal a value of 200, by application of a normalization coefficient CN=200/(LsM-La)
  • Therefore, the slope of the normalized Soil Line equals 1
• In 4SM, all radiances are normalized: Ls=CN*Ls
• In 4SM, which is a "ratio method", inverse modeling is achieved by increasing Z until the water column corrected spectral bottom signature fits the Soil Line one way or another
• Ideally, this approach assumes that the bottom exhibits a spectraly neutral signature
  • this suits any substrate whose color is a shade of grey from bright to dark,
  • but it entails substantial error for subturf, and the like: this the main source of error in any ratio method
• This error shall be of just a very few decimeters at most
  • where the overall bottom  contrast is high, i.e. at shallow depth, even over fairly dark bottoms, where all bands exhibit healthy bottom detection
• Conversely, this error can exceed several meters
  • over dark bottoms at depths which are optically deep in all but the Blue-Green wavebands
  • or if thresholds have been raised in order to counter artifacts
• Most shallow substrates shelter some sort of life, which is dependent on the chlorophyll activity and causes them to look greenish
  • this causes an under-estimation of the computed depth
  • this is why, in 4SM, the Soil Line is somewhat re-routed in order to counter this under-estimation,
  • according to our experience, this appears to suit the vast majority of substrates as soon as they depart from the bright and clean sands paradigm
• But some coral, algae or turf communities exhibit a wide variety of exotic colorations: blueish, brownish, reddish, purplish
  • To-date, it is not possible to account properly for these less frequent cases in 4SM   

• This error is most obvious is the case in our WV2 image.  It is located in the western part of the image over a field of relatively dark corals (?) which slopes from the fringing reef at ~ 1 m down to 4-5 m: where computed depths are over-estimated by up to 4 meters (red tones).
• The cause is as follows: subject to the very high thresholds applied, the Red and Yellow bands are disqualified, and modeling only uses bands 1 and 2 versus band 3: -Lm/1/1/1/10/10/255/255/255
• Then, why disqualify the Red and Yellow bands at that location, so far away of the area affected by "pollution"? For 4SM to yield more acceptable depths, the practioner needs to release the thresholds and run a special processing of the erroneous area: -Lm/1./1/1/1.5/2/255/255/255

The use of the Panchro band
along with other WV2 bands
should be extremely beneficial
in order to alleviate this problem