Bathymetry and water column correction
at Lee Stocking Island, Bahamas
Image courtesy of the U.S. Geological Survey
7671*7841 30 m pixel size, UTM zone 18

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
scene LC80120442016035LGN00, February 4th 2016
Work done september 2016

As I don't have seatruth data, apart from the above speaker_harris_fig_01.png,
I used results obtained on lsiOLI_20140129 as a seatruth DTM
The result is astounding,
knowing that part of the fuzziness is caused by slight misregistration of the PAN band.
No smoothing applied

February 4th 2016    vs   Jan 29th 2014
This is a test of the robustness of 4SM

ZR-0.50m - ZC
February 4th 2016   -   Jan 29th 2014
Three specific sources of discrepancies must be considered
  • Cloud shadows: they cause over-estimation of retrieved depths. I could not find a way to avoid it.
  • Misregistration: the 30 m re-sampled PAN band is misregistrated by one pixel in both row and line. This causes fuzziness, and overestimated depths on occasion.
  • Variation of water optical properties: they cause local over -or under- estimation of retrieved depth
The results are much more robust than I expected!
I intend to produce
a Combined Depth raster
 out of all these results.

Data and Deglinting

TOA TCC: raw image
logaritmmic enhancement

BOA TCC deglinted image
logaritmmic enhancement

Winter is the time of choice
  • Clear atmosphere, hardly any glint
  • Homogeneous waters, particularly Blue bands (most unusual), from Exuma Sound to Tongue Of The Ocean and to open ocean
  • No local patch of discoloured waters,
    • although the PAN band comes to near extinction between waypoints B2 and B3, where depth is 5-7 m

Top notch optical calibration
16U data are scaled to allow for comfortable screen display

Calibration diagram for the whole scene
for bands Blue, Green, Red and NIR

Calibration diagram for the whole scene
for bands Blue, Green, Red and NIR

Raw calibration
The optical calibration in 4SM
assumes that BPL pixels represent
spectrally neutral bottom reflectances
Not quite true for bottom type_10,
which is brightest ooid sands

LsM reflectances for best fit


Optimized calibration
Optimization of the optical calibration
enforce spectrally neutral bottom reflectances
for bottom type_10, which is brightest ooid sands

See that optimization achieves it
This involves fine-tuning of LsM reflectances

Optical calibration of the NIR band is excellent
2KNIR~=6 m-1  seems to fit OLI data

Blue vs Red
When the atmosphere is thin,
the Vegetation Line assumption allows
for straightforward estimation of La = Lsw - Lw
in the Blue range

Ready for modeling
No Smoothing

BOA TCC: water column corrected

Retrieved depth
see legend


Bottom typing

SAM classified image
see legend for SAM

Average bottom brightness


Créer un site
Créer un site