A 4000*2305 SPOT_1 bi-HRV mosaic of Rangiroa atoll,
Tuamotu archipelago, French Polynesia

The 4SM license cost for this image is 526 US$
This atoll is 80 km long.  (see also rangiroa_ETM)


1 - NO NEED for field data, nor for atmospheric correction
2 - this is demonstrated in this website, using a variety of hyper/multi spectral data
Requirements are
1 - homogeneous water body and atmosphere
2 - some coverage of optically deep water
3 - some coverage of dry land
Problems are
1 - the precision on estimated depth is found wanting, because the noise-equivalent change in radiance  of accessible data is too high for shallow water column correction work 
2 - radiance data should be preprocessed by the provider at level 1 in order to improve S/N ratio
3 - exponential decay: the deeper/darker the bottom, the poorer the performances
I keep digging
until suitable data
become available

As this image does not have a blue band, 4SM modeling is only possible down to 4-6 m depth range.
For deeper areas, a depth is computed using XS1 alone
and assuming that the bottom is uniformly bright (coral sand/mud).

The "One Band Case", of course is the cause of much confusion,
as darker bottoms substrates (coral growths) are mapped to be deeper than their immediate surrounding.



RGB false color composite

Grey shaded image_Z

  • Where modeling is possible (i.e. down to 4-6 m), 4SM maps in details the delicate bathymetric features of the atoll rim.

For deeper areas,
  • interesting geomorphologic features are revealed,
  • their depth may very well be "inverted", depending on the actual bottom reflectance features.

Lower: Image_Z in grey shades 
Today's Blue_Lagoon is just the heir of an older one: see how depths over the rim of the old_Blue_Lagoon is "inverted" by the One_Band case: this rim of course is expected to be shallower than inner lagoon bottom, whereas it is shown to be deeper!!


 Lower: Image_Z in grey shades
This location appears to have experienced a Blue_Lagoon_development_phase in the old days.
No inversion of depths over the rim here, though.

Lower: Image_Z in grey shades

Left: Image_Z in grey shades

An easy calibration, for XS1 and XS2 at least
Jerlov water type OII+0.36: not that clear, hey,

although this very large lagoon has a complex hydrologic structure, with much less clear water masses at some locations far away from major inputs of crystal clear oceanic waters.
Atmospheric adjacency effect
is the cause of some problems
along the densely vegetated areas.

A SPOT 1 bi-HRV image of exceptional quality,
taken on a "calm and clear day"

It carries its own radiometric limitations though.

The radiometric quality of SPOT 5 images
is much better : see also  sanaa.htm?

Deep lagoon by the "one-band case"

Deep lagoon by the "one-band case"


Bright target recovery artifact at Motu Paio

<== Bright target recovery
     is a very cunning artifact!!

LEFT    : thresholds are low, no smoothing, noise everywhere!

: thresholds have been set much higher, smoothing enabled


Sea truth lines at  BlueLagoon are in Green
  • excellent DGPS positioning by SHOM
  • "one-band case" is applicable to bright lagoon bottom

Sea truth lines at  BlueLagoon are in Green
  •  excellent DGPS positioning by SHOM
  • "one-band case" is applicable to bright lagoon bottom