Determination of Water Depth at Nanwan from Formosat-2 Multispectral Image
Date Issued
2009
Date
2009
Author(s)
Liu, Chung-Chen
Abstract
The bathymetry is important and necessary for oceanographer. The common method for measuring water depth uses ship-board echo sounder. For remote area that is not easily accessible by surveying boats and there is no in situ data. The bathymetry at costal area is affected by sediment and collapse and the percentage of change is large. The utility rate at costal area is higher than deep area, so it’s more important to get the bathymetry safe and convenience at costal area.e use the Formosat-2 multispectral image to calculate the bathymetry at Nanwan area, the bay at the southern tip of Taiwan. The radiance is exponential decay when the light penetrates into the water and reflects with bottom materials. The upward radiance is received by satellite and the radiance is relation to water depth. The wave amplitude is higher in costal region and it increases the surface reflectance towards the satellite. Because near infrared band has almost no reflection from the water body, and wave-induced reflectance is the same for all bands, any increase of near infrared radiance may be used to estimate the wave-contribution in blue and green band radiance. The water absorption is higher at green band than blue band so green light decays faster than blue light. The difference in radiance ratio of blue band to green band represents the changes in depth. A change in bottom albedo affects both bands similarly, but changes in depth affect the high absorption band more. Accordingly, the change in ratio because of depth is much greater than that caused by change in bottom albedo. The difference in attenuation coefficient between blue and green band is a constant with different water. We use the ratio to reduce effect on bottom albedo and then calculate depth.ost of the depth data calculated with three bands are shallower than in situ data. The reason is that most of the radiance received by satellite is radiated from shallowest part in a pixel. The application of this method can easily be extended to regions without in situ depth data. This method is suitable for the regions where depth is shallower than 30 m and water is clear, otherwise the attenuation of light varies with type and concentration of suspension material.
Subjects
ship-board echo sounder
Formosat-2
multispectral image
bottom albedo
water reflectance
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-98-R95241103-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):e9d099f6d9643c402c9629f4fbdc3020