Bathymetry and water column correction
LANDSAT 8 OLIP 
at Mulroy Bay, Ireland
Image courtesy of the U.S. Geological Survey
 
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

Work done july 2018
GSD 15 m
 
 
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WHAT CODING SCHEME?

where N is the number of wavebands in the spectral image

If the raw image is 16 bits,

then a 16_to_8 bits scaled spectral image is first written in channels 1_to_N.
 

  • U8_Image_mSE in channel_N+1
    • This Special Effects mask that has to be prepared manually.
  • U8_Image_WZ in channel_N+2
    • This image_WZ makes a note of which waveband with longest wavelength exhibited bottom detection in excess of its threshold Lm value.For example, if bottom detection starts with band 3 and the BOA radiance in this band is higher than Lm 3, then a value of 3 for that pixel is written in Image_WZ.
  • U8_Image_WR in channel_N+3
    • This image_WR makes a note of the radius of the circular smoothing window. For example, a value of 3 means a 7*7 circular smoothing window.
  • U8_Image_Z250 in channel_N+4

    • Shallow depths are coded in decimeters from 1 to 250

    • Depths in excess of 250 are coded 253.

    • Deep waters are coded 254.
       

    • No_Data areas are coded 255.
  • U8_Image_Z in channel_N+5
    • This image is for screen display purpose only, in the case when depths exceed 25 meters.
    • Depth in shallow water area is written in decimeters down to a certain MAXimum. Beyond this maximum, depth is written as MAX+(depth-MAX/10).
    • Image_Z20 : MAX is 200 decimeters. Beyond 20 m, depth is coded as 200+(depth-20), like 137 stands for 13.7 m and 204 stands for 24 m. Maximum is 210 for 30 meters.Use thepctZ20 color pallet provided.
    • Image_Z10 : in very clear waters, the depth range may exceed 30 m. In such case, MAX is 100 decimeters. Beyond 10 m, depth is coded as 100+(depth-10), like 37 stands for 3.7 m and 137 stands for 37 m. Maximum is 210 for 110 meters.Use thepctZ10 color pallet provided.
    • Land area is delineated with high precision using the NIR band (XS3 of SPOT XS or TM4 of Landsat TM). The green-vegetated land area is interpreted into Normalized Difference Vegetation Index (NDVI) and coded into shades of green; Perpendicular Vegetation Index (PVI) also available upon request. The red-vegetated land area is coded into shades of brown. The non-vegetated land area is coded in shades of gray.
    • Non-shallow water area is masked and coded separately: clouds and shadows, breaking waves, etc.
  • U8_Image_B in channel_N+6
    • It is coded for easy visual inspection on screen. An average bottom brightness, i. e. a shade of gray, is computed for every pixel which exhibits bottom detection by at least two bands.
    • For non-shallow water area, the coding scheme described above for imageZ applies.
    • Shallow bottom brightness is scaled from 0 for black bottoms to 200 for the brightest type of shallow bottom that exists in the area studied. Pixels for which computed bottom reflectance exceeds 200 are saturated at 201.
    • If the depth has been computed using only the last band exhibiting bottom detection and through the arbitrary choice LBref[1] or LBref[2] of a prevailing uniform bottom reflectance (the 1-band case), then corresponding pixels in image_B are coded at the value of LBref.
    • PctB : the image_B file may be screen-displayed using the pctB pseudocolor table provided.
  • U8_Image_BSC in channel_N+7
    • This channel is mostly a visual display and control of how good -or bad- the spectral bottom signature might be.
    • This output channel is mostly a display of the ratio LB green/LB blue.
    • If a Red waveband exhibits bottom detection and the LB redexceeds (LB blue+ LB green)/2, then the ratio LBS red/(LB blue+LB green)/2 may be written instead.
    • Underestimated depths tend to yield a low LB green/ LB blueratio == show as a dense-blue artifact in image_BSC
    • Overestimated depths tend to yield a high LB green/ LB blueratio == show as a dense-green artifact in image_BSC.
  • U8_Spectral_Image_X in channel_N+12+1 to channel_N+12+N
    • Spectral linearized radiances are multiplied by 40 in order to use the whole 8-bits range of 0_to_255
  • U8_Spectral_Image_Deglinted in channel_N+12+N+1 to channel_N+12+N+N
    • Spectral radiances are scaled, smoothed, deglinted, normalized as BOA radiances, then saved for a reference to just prior to modeling.
 
  • U8_Spectral_Image_LBS in channel_N+12+N+N+1 to channel_N+12+N+N+N
    • Image_LBS is a "water column corrected " spectral view of the scene. It intends to map the spectral reflectance of the whole area, whether dry land or shallow water, "like if there were no water", so as to allow for thematic multispectral classification of the shallow bottom using the same suite of classifiers as typically used for land cover typing.
    • These radiances are BOA radiances, i. e. after removal of the atmospheric path radiance
    • Scaling of image_LBS
      • Bottom reflectance in each of the images that are stored in image_LBS is scaled from 0 for a black bottom to 250 for the brightest type of shallow water bottom that exists in the area studied.
      • Pixels for which reflectance exceeds 251 are saturated at 251.
      • These images may be screen-displayed individually as shades of gray,
      • Or any three of them may be screen-displayed as a RGB color composite.
  • Information content of image_LBS
    • Depth dependence : because the attenuation of the sun light through water is strongly wavelength-dependent, the information content of image_LBS decreases gradually as the water depth increases.
    • In very shallow waters , all bands exhibit bottom detection and each of them in image_LBS carries its own contribution to the spectral signature of the shallow bottom.
    • In deeper waters , some bands at longer wavelength in the red part of the spectrum are extinct, and may only be allocated a bottom reflectance which is computed to be the average of spectral reflectances for other bands.
    • Average brightness : this process may finally end with only two bands exhibiting bottom detection with suitable color separation.
       
  • 16S_Image_ZG channel_N+12+N+N+N+1
    • A 16-bits-Signed channel: depths are coded in centimeters. Non-marine areas are coded at -1.


 




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