A time series of Landsat 8 images
at San Lorenzo Channel, Baja California

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
Collaboration with 
Fabio Favoretto, Ph.D Student, Coralline Algae Ecology
Grupo Interdisciplinario de Ciencia Ambiental, Universidad Autonoma de Baja California Sur
Carretera al sur km 5.5  | La Paz
Using pan sharpened images in this study
Pan sharpening  using Rstudio with Brovey method

work done october-november 2016
Procedure/flowchart for operating  4SM.7.00

go to Combined Depth vs Fabio's Sonar seatruth depths

Combined Depth vs Fabio's DTM
at Landsat 8 pan-sharpened 15 m 
march 4th 2017

  • I count all points with both Combined_Z4SM<=50m and ZDTM<=50m
  • histograms account for all points: N=187,044
  • all depths are scaled into rounded decimeters for regression or histogram
  • some bins have less than nbMin points
Red outliers
  • depth bins with less than nbMin are excluded from regression ==>N decreases accordingly
nbMin=0  ==> N=187,044
nbMin=2  ==> N=147,620
nbMin=10==> N=  85,840
format: XUTM YUTM ZDTM Z4SM ...

ASCII textfile 3MB from GoogleDrive at

ZRecorded-ZComputed in cm
  • Blue tones: CombinedDepth is deeper
  • Red tones:  ZDTM                is deeper
Tide height

Combined Depth and profile_black

DTM and profile_black
Please note isobaths abutting to the shoreline!

Fabio: please 

send me a shapefile of all depth points you used to produce this DTM, so that I can map them here

Regression CZ vs ZDTM
for all points which have both CZ and ZDTM:

Histograms CZ and ZDTM
for all points which have both CZ and ZDTM:

Regression CZ vs ZDTM
for all points which have both CZ and ZDTM
for all bins with at least nbMin=10 points:

CZ and DTM along profile_black
operational diffuse 2K

This fuzzy regression does not prove much in this complex and variable hydrologic environment: we would need a LIDAR or MBES DTM 

Still, in the 10-20 m depth range, 
it demonstrates two sensitive things==>
 operational diffuse 2K

retrieved depths in meters only need a tide correction
  • using Jerlov's data at WLblue and WLgreen,
  • along with the observed ratio Kblue/Kgreen ,
  • to interpolate 2Kblue and 2Kgreen
  • yields a slope=1 in the above regression
  • no need for field data for the optical calibration of the model
the PAN solution is a good operational option

About the Soil Line assumption

Spectral water column corrected
bottom reflectance is the main unknown
for all SDB methods
Therefore, all methods resort
to some assumption in this regard
  • empirical model: by using field depths for calibration of a multi-linear regression
  • 4SM simplified RTE: by using the Soil Line assumption
  • analytical RTE: by using a LUT of all possible end-member bottom substrates submitted to all possible variations of illumination/attenuation parameters over the whole shallow depth range
In the end, all methods yield
a more/less biased depth result:
there is no escaping that

  About the Soil Line assumption

In this study case:
(please help me ensure I have it right!)
  • 5-10 m range ZDTM<ZC: most retrieved depths are over-estimated:
    • this means that bottom substrates are actually less green than assumed by default in 4SM
  • 10-20 m range ZDTM>CZ: most retrieved depths are under-estimated:
    • this means that bottom substrates are actually greener than assumed by default in 4SM
    • a good case for reddish substrates
Therefore also,
computing a seatruth linear regression
with a slope=1
becomes tricky, to say the least
By default in 4SM>>==>>
  • By default since 2016, I use knowledge acquired over the Bahamas, and indulge into extending its application to all images/sites over the whole Panchro depth range, by tweaking the Soil Line in 4SM code.
  • Therefore, by default in 4SM code, all bottom substrates, from the brightest to the darkest, are treated as if they actually exhibited  spectral signatures similar as those observed in the Bahamas over the RED depth range: turtlegrass everywhere in variable abundance over very bright coral/oolithic sands.
  • This of course is over-simplistic, and I shall in the future have to diversify that according to depths results obtained within the RED range of bottom detection (0-~10 m) on an image-wise  or site-wise basis.
>>==>>What we see here
  • 0-10 m: retrieved depths are over-estimated
  • 10-20 m: retrieved depths are under-estimated
  • though tide correction should somehow alleviate the "dramatic" tone of this pronouncement
  • so, for now, I tend to infer that these two depth ranges harbour different families of bottom substrates:
    • 0-10 m:  bottoms there would be less "green" than Bahamas's turtle grass bottoms
    • 10-20 m: bottoms there would be "greener" than Bahamas's turtle grass bottoms. Note that by being "greener", we can expect them to also be "redder"
GSD 15m; Co-registration; DTM
  • PANsharpening, CombinedDepth and Seatruth all rest on a perfect co-registration among images and with the DTM
  • Further development of 4SM code shall need to ascertain that perfect co-registration is achieved (my prefered solution, can be trickythough), at professional level,
    • or the practioner shall be left with their petty COTS image pre-processing packages
Time series
small ROI
  • Fabio's ROI is limited to San Lorenzo Channel
  • If I were to limit this study case to Fabio's ROI, that would be a lost case, or I would have to take unreasonable risks
extended ROI
  • Instead, I insisted that I would take a broader view
  • so that I, as an informed practioner, would get a feeling of the prevailing hydrologic conditions in this area so as to inform the Soil Line assumption and the Brightest Line assumption for this scene
Clearest waters
  • In 4SM, the practioner gets an estimate of the optical properties of the clearest waters over the (extended) ROI, but can also wander in search of evidence of locally less clear waters (see La Parguera, Puerto Rico, Landsat 8).
Clearest waters
  • This means that less clear waters over the ROI are not accounted for
  • and therefore that the 4SM results are affected accordingly?
Fabio, if I may
Your DTM is going to be a weak link in this story.
Just think of bathymetric lines abutting the shoreline.

Fabio, if I may
So, maybe Andy would not mind contributing a more realistic DTM from your very "fish finder" measured depths.
Sorry, I can't offer to do it myself.
When I say realistic, I mean: limited to areas which are adequately/densely informed.

I suppose you would just provide us with list of
          XUTM YUTM YourMeasuredDepth
as an ASCII textfile, mentioning
          Depth Datum (tide correction),  
          UTM Datum and Zone.
And I could also use a shapefile of this data, as. mentioned above


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