| The true color product is formed by a combination of red, green, and blue
channel data from the MODIS instrument. It can be produced at 1km, 500m,
and 250m spatial resolution, selected based on the scale of the sector.
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. |
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