A 2200*1600 4-bands Landsat ETM of Rangiroa atoll,
Tuamotu archipelago, French Polynesia

The 4SM license cost for this image is 330 Euros
This atoll is 80 km long. 

The hydrologic properties are clearly NOT homogeneous in this huge atoll


True color composite

True color composite, histeq enhancement

ETM_2, smoothed
most of the deep lagoon is optically deep 
in the Green band ...


ETM_1, smoothed

... but not in the Blue band


The hydrologic properties are clearly 
NOT homogeneous,

as illustrated by inconsistant dark bottoms for lagoon floor.
We know that the lagoon floor in the atolls is draped
with very bright fine grained coral debris

The computed depth
can only be very faulty for the lagoon floor

Swell features at sea surface badly affect the results
Optical water type OII+.15 for the clearest waters
for wavelengths at 477, 577, 661 and 835 nm

definitively not clear oceanic water,
in spite of fairly good communications with open ocean
basically the same water type as for the SPOT XS image:

==> we seem to get confirmation here
that ETM Green band must be set ~=577 nm

(instead of mid-waveband at 560 nm)

==> bottom detection by green band is less for Landsat ETM sensor at ~577 nm than for SPOT XS sensor at 550 nm. This is sad.


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