Collin's Normalized Difference Ratio

4SM's NDR

NDR=(Green-Purple)/(Green+Purple)

• NDR : using water column corrected bottom signatures in units of reflectance,
  • this ratio is seen to increase linearly in the range 0.3-0.8
  • as an expression of the increase in living coral colonies at Moorea, French Polynesia
• "the NDR green-purple is likely to embody a robust proxy for detecting coral pigments in live corals that exhibit higher reflectance in the purple band and a lower reflectance in the green band"
• Live Coral Index: this allows Collin et al. to produce a map of the density of living coral colonies down to ~2.5 m at Moorea

• NDR: using normalized water column corrected bottom signatures in units of DNs, it is to be expected that this ratio
  • equals 0 where Purple=Green over arguably clean sands
  • increases linearly as the Living Coral Index increases
  • decreases linearly (?) as the Leaf Area Index or biomass of vegetation increases

• It is to be expected that a similar relationship could be developped for  the density of shallow vegetation in order to map the Leaf Area Index or biomass of shallow vegetation

water column correction and mapping NDR

• in order to achieve water column correction of bottom signature in units of reflectance, the authors need to
  • calibrate the data in units of reflectance
  • apply the dark pixel correction in order to obtain BOA data
  • use existing depth sounding points to calibrate Stumpf's m0 and m1 parameters for Stumpf's method
  • compute a DTM of shallow area using Stumpf's method
  • apply the DTM depth, and spectral K values observed by Maritorena at Moorea, in order to produce spectral water column corrected shallow biottom signatures, and then map the NDR

water column correction and mapping NDR

• in order to achieve water column correction of bottom signature in units of DNs, the 4SM method needs to
  • apply the dark pixel correction in order to obtain BOA data
  • use the image itself to calibrate the optical model
  • compute water depth, water column corrected bottom signature, and NDR "on the fly" in one single 4SM run
• During the inversion process for the current pixel,
  • if the current pixel does not exhibit good bottom detection in the yellow band
  • once a depth is obtained
  • a NDR is immediately computed
  • from that last bit of information, the SoilsLine model is adjusted accordingly ,
  • so that a new depth is computed, which is more realistic than the first one

Z4SM
is used to compute bottom signature
LB=Lw+(L-Lw)*exp(2K*Z4SM)

ZDTM
is used to compute bottom signature
LB=Lw+(L-Lw)*exp(2K*(ZDTM+Htide))

profile_black

• Z4SM and ZDTM are shown in the profile plot

• it is seen that the SoilsLine model used to achieve water column correction is the cause of an error on estimated depth

• this can cause Z4SM to be offset by up to several meters for specific bottom types where only bands 1, 2 and 3 are used for modeling

• by using Z4SM, water column corrected bottom signatures are seen to be
  • badly affected by this offset in the Red and Yellow bands which have high  K values ,
  • much less affected in the Blue-Green range because K values are much lower

• Z4SM and ZDTM are shown in the profile plot

• by using ZDTM in place of Z4SM, true water column corrected bottom signatures are computed

• this kind of plot is extremely precious, as it can be used when it comes to

  • identifying training sites for bottom typing

  • specifying the rules for assigning a pixel to a particular bottom type

• note the Purplish signature along most of section A

  • this gives credit to assigning a coverage of live coral colonies that increases in density as the depth increases, as was evidenced by the NDR image

obtained by applying Z4SM,
modeling by only bands 1, 2, 3

-Lm/1/1/1/255/255/255/255/255

• in the 0-10 m depth range, bands 1, 2 and 3 are not enough:
  • green coverage is clearly overstated: depths over-estimated
  • purple coverage is clearly understated: depths are over-estimated
  • therefore we want to make good use of the Red and Yellow bands
• in the 10-30 m depth range, purple coverge is clearly overstated : as seen in section A of profile_black, depths are under-estimated
  • I need to find a way to alleviate that: ==> see below

NDR image 
obtained by applying ZDTM+Htide,
 

This is a "seatruth" NDR image

NDR image 
obtained by applying Z4SM
using bands 1, 2, 3, 4 and 5

• the pollution artifact shows up badly in purple tones in the central area, which originate in underestimated depths

• apart from polluted areas, this image is quite encouraging: it shows that in the absence of pollution,  such NDR image over the whole shallow depth range should attract some interest

Over time, it shall be known
that this NDR image  can be produced
without the need of any existing field data

NDR image 
obtained by applying ZDTR+Htide

• coral patches seem to be mostly dead coral colonies
• the moats and gully are covered in dense vegetation, and maybe also the central deep lagoon

Now, it would make sense
that someone comes up
with seatruth observations
on the nature of shallow bottoms
at Waimanalo Bay

• evidence in the radiance data on the day of imaging
  • radiance in the Red and Yellow bands is boosted by the pollution; some sort of glow radiated by backscatter in the water column
  • plume of suspended sediment can be ruled out
  • can we exclude fluorescence?
• long term effects on the shallow bottom
  • seaweeds: sandy bottoms in the areas affected seem to have turned "green": this is evidenced by the NDR image in the sandy areas. This even is observed in the gully and moats. If this can be confirmed, a seaweed cover has developped, quite dense and healthy in the gully and moats
  • dead corals: coral colonies seem to have died: this is evidenced by the NDR image of the oval coral patches
• cause of this situation
  • red tide? how about some sort of "red tide" that boosts the radiance, mostly in the Red and Yellow bands, with negligeable impact on other bands
  • this seems to happen just where Waimanalo stream pours out into the bay