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Poster Session
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Remote Sensing application of current field on sea water surface layer and water quality on harbor and bay
Extraction of Surface Current Field Information
- Extraction of current field information through
Water surface Roughness
The water surface information is implified in the signal Lsg as :
Lsg = E0 * cosqs * EXP [-t/cosqs] * gv (q2 , js ;
qr , jr)---(2)
Where E0 is the solar radiation on top of the atmosphere, q the sunlight incident angle, EXP [-t/cosqs] - the transmission function of atmosphere path, gv bi-directional reflectance function; where (qs js)- solar zenith and azimuth angle (qr js)
view angle. If the atmospheric columns are uniform above the area of acquistion , E0 * cosqs * EXP [- t cosqs] can be taken as constant, so signal Lsg is mainly dependant by term gv, which is related to sea status only gv can be consideredas a function of sea surface horizontalmovement. If the wind is strong there are foams,then gv in (2) can be represented as:
gf =
(1-Cf) * gv ; (qs js) + Cf * rf /p---------------------(3)
Where Cf-foaming coverage index
Cf = foaming covering area / total sea area
When the wind velocity u£9m/s , Cf = (1.2*10-5) * u3.3, r f the spectral reflectance of the foams. To decide term Lsg, we must first select the bands, TM3, TM4 and TM5, which penetrate not much into the water, so that water depth change related term Lw does not contribute much. Then atmospheric correction processing is conducted to remove the contribution of Lhg (atmospheric diffusion) and Lp (sky paricles), such that L@Lsg. If gv and other parameters are known, the reflectance of water surface can be calculated . Due to that sea status varies a lot from time to time and from space to space,so function gv is a variable . It is very difficult to optain its parameters. Therefore fuzzy matmatics is used to establish the function between remote sensing signal and current fields. By formula (2) of the fuzzy relation between remote sensing image bightness value and texture density, and current the fuzzy member ship function mc(L) described as:
where L- pixel brightness values, L@ Lsg . The texture characteristics in the image is related with current velocity, so the relaton between them can also be described by fuzzy membership
where T- testure density of the image. In shallow water sea area , the water velocity is often effected by the terrain under water. If terrain rises suddently, the water velocity will increase to maintain the continuity of water flow. So the terrain changes under sea water result in corresponding roughness change of surface water that is texture feature change. Combining (4) and (5), the current change of the whole sea area is me(E)
me (E)= mc U mc (T)-------------------(6)
where U - fuzzy "OR" operation. The meaning is that the current velocity represented by pixel brightness and texture density can be obtained by the method of taking the big value.
The flood rise field picture was made for the sea area around Miao Island with the above mentioned procedures (see Fig. 2)
The points of same brightness values and texture densities are connected by dolted lines, showing roughly equal current velocities . The velocities are differentiated by the values of brightness and texture densities. When the satelite passes over, the current velcoities and directions were measured synchronously. The field measured points are used as control points. The field information from the remote sensing current velocity picture can be marked . Actural mesurement is not strictly synchronous , because it was only 10-30 sec. for the satelite to be above the area, so it is only quasi-sychronous, sea status and sun elevation angle should not change a lot, while points of measurement are only a few . Nevertheless, the correlation of the drawn current field picture with in-site measurements is still up to 80%
The directions at currents shown on the remote sensing image are very explicit. They can be directly determined by interpreting current lines and directions, clustering, diffusing of textures.
- Extraction of the feature information of water
dynamics>
Different current systems and water patches often result from wind field on the sea surface, water temperatures, water densities and kinds of suspended matters. Their interaction is shown as water color changes and texture interleaves. For example, different water colors or the merging, paralleling, clustering, diffusing, and so on. of water dynamics. water dynamics feature information is mainly represented by term Lw of
about the band combination. Lw is back
diffusion spectral signal of water bodies, is the combined result of
back diffusion energy signals of suspended matters and diolved
substances in water. In order to display the features of current systems
and water patches of different nature, it is necessary to remove (Lhg +
Lp) in (1) from the particle radiation interference to increase the
contrast in term Lw. TM3-5 are selected . They do not penetrate a lot,
but represent a lot of water body information . The signal energy
contributed by water particles and depths should be decreased , so that
color contrast can be increased for suspended matters in different
current systems or dissolved substance. After image stretching, the
current system information is very audio -visual. One example is Fig.2,
the image picture for Miao Islands during flood rise. We can see the
Eastward flood rise a North eastward alongshore currents in winter . We
vcan also see these two currents merge at south of Dengzhou waterway.
This is the dynamic condition of the formation of Dengzhou shallow .
Synthesizing the features of flood rise and outgoing tide current
fields, and water dynamic, the relationship picture of the relative
water exchanges in the harbors is shown as fig.3
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