Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries

Equalize a mosaic

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In order to process a mosaic of shallow water images, whether airborne or space borne,

it is required that


  • DARK TARGET: the deep water radiance Lsw be precisely balanced across the mosaicking cut line
  • BRIGHT TARGET: bright targets of similar nature on land be precisely balanced
  • 4SM has a module to do this.
    • The raw data channels must be supplemented with an additional 8-bits channel where each pixel is labeled with its flight/image number (a value in the range 1 to 255) or a value of 0 for a No-Data pixel.

BGR Color Composite of CASI mosaic at Mahone Bay, Nova Scottia, Canada
courtesy of HDI
BGR are 490, 575 and 625 nanometers, histogram equalization enhancement

Upper view is Raw 16-bits data
Lower view is Equalized 8-bits-scaled data

Upper view is Raw 16-bits data
Lower view is Equalized 8-bits-scaled data

Flightline N° 4 is the second from top, very bright

It can be seen that the equalization process is badly limited by

1) variable illumination conditions: incident light increased by a factor of ~two from start to end of data collection in certain bands
2) variable spectral composition of incident light through the whole morning here: rising sunlight does not have the same spectrum as noon sunlight
3) possibly variable spectral composition of sky glint and sunglint, as a consequence of the above
4) possibly also by variable tide height(?)
5) this is further complicated by the "race track" flight plan here, which causes a "random time lap" between collection of two adjacent flight lines


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