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EARSeL: 2nd Workshop on Remote Sensing of the Coastal Zone Porto, Portugal, 9-11 June 2005 |
SESSION COASTAL ZONE MANAGEMENT |
Pham Thi Thanh Hien, Martin Béland, Ferdinand Bonn, Kalifa Goïta
Centre d’Applications et de Recherche en Télédétection (CARTEL), Université de Sherbrooke
2500 Boul. de l’Université, Sherbrooke, QC, Canada, J1K 2R1
Thi.Thanh.Hien.Pham@USherbrooke.ca, Martin.Beland@USherbrooke.ca, Ferdinand.Bonn / Kalifa.Goita@USherbrooke.ca
Pham Van Cu
Center for Remote Sensing and Geomatics (VTGEO), Institute of Geology
Vietnamese Academy of Science and Technology, 340 Bach Dang, Hanoi, Vietnam
phamvancu@hn.vnn.vn
Shrimp culture is a sector of aquaculture which has a high potential for poverty alleviation and rural development in Vietnam. However, shrimp culture experiences in neighbouring countries and other areas of Vietnam clearly indicate that some of the changes induced by the shrimp farms on the environment have potential negative impacts, one of which is wetland deterioration. Despite the rapid growth of aquaculture and the evolution of the consciousness about its environmental hazards, few studies have considered this matter in an objective manner.
The submitted communication describes the use of two change detection methodologies in the assessment of mangrove forest alterations caused by aquaculture development in two neighbouring districts of the Red River Delta, Vietnam, between 1986/1988, 1992/1994 and 2001. Images from the Landsat TM and ETM+ sensors were used in this study. The coastal territory in the Red River Delta being extremely parcelled, appropriate image processing methodology is required, especially for TM/ETM+ images of 30 m resolution. Thus this study aims to (1) compare two hybrid change detection methods applied on TM/ETM+ images, and (2) elucidate deforestation’s causes in two neighbouring districts through different trends of mangrove alterations.
For both methodologies, image comparability was assured with a radiometric correction done by first transforming digital numbers to at-sensor reflectance and then applying a relative normalization. The first method, applied to the district of Giao Thuy, consists of a validation of changes identified by post-classification comparison according to rules defined in a feature selection matrix. These rules allow the selection of the Tasselled Cap feature most suitable in detecting the presence of change between given land-cover classes. This proposed approach aims at minimizing commission errors in the change detection process, which constitutes the weak point of post-classification methods.
The second method, which is applied to the district of Tien Hai, is based on a mask of change/no-change generated by the total magnitude of change in three Tasseled Cap dimensions. Images are classified under the mask and then correlated to produce the land cover change maps.
Finally, we have evaluated the accuracy for each method.
In the Giao Thuy district, analysis of the results shows that 63 % of mangrove areas appearing in 1986 had been replaced by shrimp ponds in 2001. Between 1986 and 1992, 440 ha of mangrove were destroyed, however, between 1992 and 2001 the extent of the mangrove increased by 441 ha. This recovery is attributed to reforestation projects carried out by the Red Cross Organization since 1997, and to natural regeneration promoted by conservation efforts through the establishment of the Xuan Thuy Natural Reserve in 1995.
In the Tien Hai district, mangrove forest areas sustained important decrease (867 ha) between 1988 and 1994. The district lost approximately 70 % of its mangroves. Most of them (790 ha) were replaced by aquaculture ponds. For the period between 1994 and 2001, even though some mangroves were replaced by aquaculture (62 ha), the forest gained 360 ha in area over humid soils and sea water as a result of reforestation programs by local government and international organizations.
Last Update: 2005-03-17