Keywords: RADARSAT, SPOT, Oyster Bed Detection, Oyster Bed Counting.
Abstract: The authors attempted to detect oyster beds in whole areas of Hiroshima Bay
using high resolution satellite images like RADARSAT FINE-mode SAR image and SPOT
HRV panchromatic image. An improved approach in which oyster beds are detected as a
pixel-cluster in order to reduce noise effect was applied to both of RADARSAT-FINE and
SPOT panchromatic images. The results of oyster bed detection and counting the numbers of
oyster beds were slightly better for SPOT than for RADARSAT although the difference
between the two was small. The result of this study suggested the promising application of
both of RADARSAT and SPOT images for oyster bed monitoring in wide inner bay areas,
especially for RADARSAT images due to it's all weather characteristics.
1. Introduction
Hiroshima Bay is one of the most suitable inner bay areas for cultured oyster farm and more
than 70 percent of cultured oyster of Japan are produced in Hiroshima Bay. Oyster is sensitive to
environmental conditions like water temperature and water quality and easily gets a severe
damage by an environmental degradation, for example by a red tide. Therefore, it is quite
important to keep the oyster farms in appropriate environmental conditions. Monitoring of exact
locations and numbers of oyster beds is the basic issue for the management of oyster farms and
now current high resolution satellite sensors are considered to have a great potential for the
monitoring of oyster beds.
The location of oyster beds are possibly recognized with high resolution sensor data since the
oyster bed is made by MOUSOU bamboo with the size of 10m in width and 20m in length. In
addition, oyster beds are usually standing in a row with five or six carriages. The authors have
attempted to detect oyster beds using Landsat TM image with 30m spatial resolution (Suga et al,
1985), RADARSAT-FINE SAR image with 6.25m resolution (Suga et al, 1997), and SPOT
HRV panchromatic image with 10m resolution (Suga et al, 1999). In these studies, a
pixel-wise thresholding approach was employed, however, this approach was quite sensitive to
noise, especially to speckle noise in SAR images. In this report an improved method for
detecting oyster beds with both of RADARSAT-FINE SAR and SPOT HRV panchromatic
images is presented and the results of oyster bed detection and counting are evaluated by the
comparison with the official report of oyster bed counting.
2. Test Data and Test Site
The RADARSAT FINE mode data were processed from signal data and the amplitude images
with 6.25 meters resolution were generated. The SPOT HRV panchromatic data were resampled
into the image data with 6.25 meters resolution same as that of RADARSAT images. The
amplitude-stretched image of RADARSAT-FINE SAR (Feb. 1st, 1999) and the
contrast-stretched image of SPOT HRV panchromatic (Jan. 31, 1999) around Hiroshima Bay are
shown in Figure 1. The enlarged images around a test site of RADATSAT and that of SPOT
HRV panchromatic are shown in Figure 2. The test site is located at the left upper part of
original images shown in Figure 1 and the test site is indicated as bright rectangular area in
Figure 2.
Fig.1. Amplitude-stretched image of RADARSAT-FINE SAR on Feb. 1st, 1999 (left) and
contrast-stretched image of SPOT HRV panchromatic on Jan. 31, 1999 (right) around Hiroshima
Bay.
(RADARSAT Image : © CSA & RADATSAT International 1999. SPOT Image: © CNES 1999.)
Fig.2. Enlarged images around a test site of RADARSAT-FINE SAR (left) and SPOT HRV
panchromatic (right). The test site is indicated as bright rectangle in the center of the images.
3. Method for Oyster Bed Detection
We attempted to apply the smoothing window corresponding to one oyster bed and to detect an
oyster bed as a cluster composed by several pixels by thresholding. The reason of this approach
is that a pixel-wise thresholding approach is quite sensitive to noise and a cluster-based
detection approach is expected to be more robust due to noise reduction effect by the smoothing
window. For relatively high resolution image like RADARSAT-FINE and SPOT panchromatic
images, it is possible to form a cluster for an oyster bed because the size of one oyster bed is
larger than the pixel size of the images. Actual size of the bed is 10 by 20 meters, so for the
image data with 6.25 meters pixel size 3 by 3 window is considered to be an appropriate size.
However, for RADARSAT data bigger window sizes of 5 by 5, 7 by 7 and 11 by 11 were
attempted and finally 7 by 7 window size was selected as the best one.
