| The true color product is formed by a combination of red, green, and blue
channel data from the SeaWiFS instrument. It can be produces at spatial
resolutions as high as 1.1km from the High Resolution Picture Transmission
(HRPT) captured via direct broadcast from the Navy Regional Centers. The
data are first corrected for atmospheric molecular scatter effects using a
radiative transfer model (without this correction, the product would assume
a milky-white appearance especially near scan edge, when the sensor peers
through a deeper layer of the atmosphere), and then combined to form a
Red/Green/Blue composite image. Coastlines and other annotations are then
added to form the final product hosted on this website. An important
consideration is that the model used to remove the atmospheric molecular
scatter is not perfect, and sometimes over or under corrects the data. For
this reason one must proceed with caution when interpreting the true color
product, especially near the scan edges where correction terms are largest.
The intent of the true color product, as stated above, is to depict the
surface/atmosphere in a way familiar to our own eyes. As such, oceans
appear as shades of blue (depending on water clarity and depth), clouds and
snow appear as shades of white, land surfaces assume earth tones, and
vegetation canopies are shades of green (depending on density and
chlorophyll content). Sometimes shallow water or water with high
chlorophyll content (called " blooms ") will appear are shades of
green. These are not false signals, but rather, represent an example of
how true color imagery actually differs from what our eyes see (our eyes
filter the light differently than what is done in the image product).
Sunglint, which is the mirror reflection of the solar disk off the surface,
is a feature that should be noted when interpreting true color imagery.
Sunglint occurs typically only over water bodies, and results in brighter
values in the imagery. Because these brighter values may be mistaken for
high aerosol content, an attempt to flag the region of possible sunglint is
done in the current true color product. The region enclosed by the
red/yellow lines in the figure above describes the region of possible
sunglint. The plot legend indicates that the red-side of the line denotes
the potential glint region (this two- color line is used since some of the
focus sectors may not be large enough to contain both lines). In the case
where a sector is completely within the glint zone, no indication is
available. For this reason it is suggested that the user always refer
first to the larger region products to orient themselves with respect to
the glint zone. It should also be noted that the current glint zone was
specified at a fixed value, when infact the spread of the glint zone is a
function of surface windspeed (expands with higher winds). For this reason
the glint delineator should only be regarded as a first-order approximation
of the sunglint zone--the actual glint zone may be smaller or larger
depending on local sea state. |
)
)
)
)
)
