The challenges of mapping the submerged vegetation in the coastal zone using airborne remote sensing.
The mapping of the submerged vegetation using optical remote sensing from airborne sensors is a bit more difficult than e.g. the mapping of terrestrial vegetation as it has been performed in a number of years.The water and its constituents limits the usable spectral region
In the mapping of terrestrial vegetation by scanners but also near- infrared aerial photography, the characteristic high reflectance of vegetation in the near-infrared region is utilized to discriminate between vegetation and other targets. Variation in the reflectance properties in both the visible and the near-infrared is used to discriminate between the various vegetation types.
In the mapping of features on the bottom of lakes and the sea, the effective usable spectral region is practically limited to the spectral region below 700 nm and for depth more than a two meters even below 600 nm. See also >
The upwelling signal to the sensor is low and disturbed
Due to the absorption by the water the radiance
levels are low compared to the situation on land. Where both the coastline and
the water are to be mapped, there is furthermore a very huge dynamic range
between the reflected radiance from a sandy beach to the low radiance levels
of deep water. This set high requirements to acquisition and post-processing
at the same time to maintain both an acceptable dynamic range and good
discrimination of values in the low end of the signal.
Sampling of information regarding the target types seen in the images is a necessary element for training of the classification of the acquired images. The sampling of this information is, compared to the terrestrial situation, both difficult and expensive since boat and diver assistance is required.
A special problem in this relation is the selection of sampling points, since it is difficult to assess the variations to expect and know where to find the characteristic types and densities of the vegetation.
The geographical orientation is difficult in the images