4SM at Gulf of Laganas, Greece

some seatruth

The data
Four tiles had to be mosaicked.
Data includes a 4*4 sum-aggregated, 2 m resolution,
co-registered Panchromatic band

TCC raw

PAN raw

Glint Regressions

TCC image Region of interest where to sample "glint" is very difficult to identify. There is hardly any sea-surface glint effect.	NIR2 image But a fairly strong adjacency effect MUST be corrected for in view of optical modeling.
Glint regressions based on NIR2 band 9 Note that adjacency effect can only be sampled where bottom is assumed to be optically deep Therefore, glint regressions for this scene are not very robust	Glint regressions based on NIR1 band 8

Deglinting:
removal of the adjacency effect

RED deglinted For comparison, a vertical strip has been left un-deglinted in this image Note distinct turbidity off Argassi (far north) it extends far away offshore	TCC deglinted Please note negative bottom contrast in bands 1, 2, 3 and PAN over very dark bottoms BLACK: horizontal stripes of NoData occur in the NIR1 and PAN wavebands
Pan deglinted Please note negative bottom contrast in bands 1, 2, 3 and also PAN over very dark bottoms	GREEN deglinted Please note negative bottom contrast in bands 1, 2, 3 and PAN over very dark bottoms
Deglinting along Profile GREEN Note the negative bottom contrast from 0 to ~5 km in both blue bands, and also in green band	Raw Blue_2_band with profile locator Note a slight offset of low radiances in the Blue_band_2 over a westernmost vertical strip Deep water radiances see Profile RED

Negative bottom contrast

This is where Ls<Lsw: the TOA radiance is less than the Deep water radiance, over the whole depth range.

For a blue band, this means a very low reflectance:
- this is customary for seaweed/grass beds, and for healthy vegetation on land.
For a green band, this means a near-null reflectance: this is sometimes the case for coconut grooves in atolls;
- it is very rare underwater.
At this extremely low level of reflectance, optical modeling would require top notch sensors designed for coastal work.
- this is not the case of current EO spaceborne sensors like OLI or WV2.

Homogeneous water body?

Unlike sophisticated analytical methods, 4SM normaly assumes a homogeneous water body over the whole scene and over the whole depth range.
- where this condition is not met, results can be very badly affected.
- but we have designed a way in 4SM to overcome a situation where waters get progressively clearer away from the waterline.
For this scene, we seem to observe slightly turbid waters in the RED range of the solar spectrum in the 0-5 m depth range over brightest bottoms -see red arrows.
- in the optical calibration, we shall attempt to account for a progressive clearing of waters deeper than ~5 m deep in this image,
- from a consistent OIII marine water type of Jerlov in the 0-5 m range
- to OIB at ~30 m.

Landsat 8

Landsat 8 OLI data include a .MTL.txt metadata text file with which to convert TOA radiance into units of TOA reflectance:
- REFLECTANCE_ADD_BAND
- REFLECTANCE_MULT_BAND
- SUN_ELEVATION
We have processed many Landsat 8 images of coral reef environments, and so derived an average BOA reflectance signature spectrum for coral reef sands, calibrated in the 0-1 range.
This signature is then used to scale water column corrected of any current scene relative to coral reef sand signature.

WV2

Using the .IMD metadata text file of a WV2 image, we do the same:
- absCalFactor, effectiveBandwidth, etc.
It shall be observed that the reflectance of the brightest shallow substrates in this Laganas scene is only ~0.25 that of coral reef sands,
- although the whole of Zakynthos island is mainly made of limestone.

This is a marked recent improvement in 4SM,
as it allows for in a tighter control of the Soils Line.

Using the PANchromatic band:
The "PAN solution"

Deeper than ~10 m over bright bottoms in a WV2 scene, bottom reflection only occurs in the Green and Blue bands even over bright bottoms: that's three bands.
- in 4SM, to retrieve bottom depth, Green is weighed against the average of two blue bands.
- this results in underestimation of depth over green submerged vetetation:
  - the darker the bottom, the largest the underestimation.
  - this is sometimes very nasty at shallow depths.
By adding a PAN channel, we now have four bands.
- in 4SM, to retrieve bottom depth, PAN is weighed against the average of three green-blue bands.
- this is expected to yield a better estimation of retrieved depth over green submerged vetetation.

Thanks to your seatruth dataset,
this exercise should allow us to comment on that aspect.