INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 2, No 2, 2011

Transcription

1 INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 2, No 2, 2011 Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN Temporal change detection of vegetation coverage of Dhaka using Remote Sensing Rahman. S 1, Hasan. S. M. R 2, Islam, M. A 3, Maitra.M.K 4 1- Department of Geography and Environment, University of Dhaka. 2- Assistant Professor, Department of Geography and Environment, University of Dhaka 3- Assistant Director, Geological Survey of Bangladesh, Dhaka 4- Weatherford International, Dhaka, Bangladesh blu_sr@yahoo.com ABSTRACT The foregoing areal expansion of Dhaka city with its dense population has been triggered the processes of land transformation as well as the growth of urbanization that are responsible for the physical and environmental instability of that area. In this research attempt has been taken to detect the loss of vegetation cover for the Dhaka district using multi-temporal, multi-date and multi-sensor aerospace data and to analyse the assessment of the impact of urbanization and biodiversity. Landsat imageries of the year January 1989, February 2002 and January 2010 have been used to find out the difference of vegetation coverage of Dhaka metropolitan and its surroundings over 20 years. About 20 percent vegetation cover that was present in 1989 has gradually decreased to 15.5 and 7.3 percent in year 2002 and 2010, respectively. Abrupt declination of vegetation coverage has been identified in Dhaka Metropolitan and Savar thana with a rate of 3.5 and 2.72 sqkm per year since last two decades, whereas gradual loss has been found in Nawabganj thana with an average of 1.62 sqkm per year. A considerably better situation has been observed in Keraniganj, Dhamrai and Dohar thana where the increasing rate was 0.9 sqkm during period and declined at a rate of 2.28 sqkm during The research has shown the pattern and nature of the interrelationship between urban sprawl and urban vegetation loss. Temporal urban map and database provides the baseline information for the planner and practitioners to monitor and predict the patterns and future trends of urbanization. Temporal mapping is driven by remote sensing and Geographic Information System (GIS) to capture and analyse information from both historical and modern records. The map and the database have been focused the intense changes to the landscape that have incrementally developed over time. Keywords: Change detection, Environmental Degradation, Remote Sensing, GIS. 1. Introduction Change detection is the method of identifying differences in land cover over time. As human and natural forces continue to alter the landscape, various public agencies are finding it increasingly important to develop monitoring methods to assess these changes (Levien, Roffers, Maurizi, Suero, Fischer, & Huang, 1999). Changes in vegetation result changes in wildlife habitat, aesthetic and historical values, ambient air quality, and other resource values, which in turn influence policy decisions. Currently Dhaka, the administrative hub of Bangladesh, attracted many researchers who have the ambition to interpret the current trend of changes and development that started decades ago and its effect on the dwellers breath here. Submitted on September 2011 published on November

2 Located centrally in the country characterized by Pleistocene pop-up (McIntire, 1959) and extensively modified by the rapidly growing urbanization in recent time. Due to fast urbanization it is facing the loss of natural vegetation, loss of open spaces, and a general decline in the spatial extent and connectivity of wetlands and wildlife habitat. These problems can be generally attributed to increasing population (Khan, 2000). The vegetation cover and biodiversity in this area has been in peril with the growing number of dwellers since 90 s. Vegetation in a city is sometimes synonymous to presence of nature in the predominantly man made environment. Urban vegetation is an important point for sustainable development, environmental conservation and urban planning process of a city (Rahman, 2009). In this area various practices are responsible for destruction of vegetation such as construction and infrastructure development. And, green areas are dwindling at unmatched rate in Dhaka district. Until now, still there are various types of green areas or vegetation cover in the area. Major types are botanical gardens, homestead gardens, public parks, and vegetation around government offices, graveyard and playgrounds (Rahman, 2009). There is very little noteworthy study to visualize and explain the right trend of vegetation loss in Dhaka using modern technology like Remote Sensing and GIS. Vegetation indices among other methods have been consistent in monitoring vegetation change. One of the most widely used indices for vegetation monitoring is the Normalized Difference Vegetation Index (NDVI), because vegetation differential absorbs visible incident solar radiant and reflects much in the infrared (NIR), data on vegetation biophysical characteristics can be derived from visible and NIR and mid- infrared portions of the electromagnetic spectrum (EMS) (S.O & B, 2003). One of the first successful vegetation indexes based on band ratioing was developed by Rouse et al. (1973) (Jensen, 1996). The aim of this study is to incorporate the temporal dependence of multi- temporal image data to identify the changing pattern of vegetation cover and consequently enhance the interpretation capabilities. Moreover integration of multi- sensor and multi- temporal satellite data effectively improves the temporal attribute and the reliability of multi- data (S.O & B, 2003). Therefore this paper endeavoured to the methods of detecting temporal and multisensor change of vegetation cover in Dhaka district using remote sensing and GIS technique. 2. Materials and Method 2.1 Study Area Dhaka district is the Nerve centre of Bangladesh having an area of sq km, bounded by Gazipur and Tangail districts on the north, Munshiganj and Rajbari districts on the south, Narayanganj district on the east, Manikganj district on the west (The Asiatic Society of Bangladesh, 2006). Dhaka is situated between N and N latitudes and E and E longitudes. Thus it is located exactly in the middle of the eastern hemisphere and on the line of the Tropic of Cancer in the northern hemisphere. Dhaka district has one of the oldest major cities (Dhaka City) in South Asia. Its existence as the largest city in the present Bangladesh region has been consistent over a period of nearly 400 years (Islam, 2005). Physiographically study area situated on the southernmost limit of gently rolling Madhupur Tract of the old Alluvium (Ahmed, 1958) is bounded by the Brahmaputra-Jamuna floodplain in the northwest and southwest, Meghna flood plain in the southeast. The city comprises a complex and mixed settings of land in the central part of Bangladesh and covers both the Madhupur Tract and Brahmaputra-Jamuna flood plain (Alam et al., 1990). The drainage system of the area is dendritic in nature. Dhaka metropolitan is 482

3 surrounded by Floodplain with sparse vegetation and Levees are found along major rivers. The eastern part of the area has been intensely dissected and formed numerous rounded and elongated low hillocks with dense vegetation cover. The western part of the area moderately dissected by streams and formed incised drainage system. Numerous depressions are found in and around Dhaka City. Dhaka experiences a hot, wet and humid tropical climate. The city has a distinct monsoonal season, with an annual average temperature of 27.5 C.The city experiences about 2000 mm annual rainfall, of which more than 80% occurs during the monsoon season (June-September) (Dewan & Yamaguchi, 2006). Figure 1: Dhaka metropolitan along with other thans of Dhaka district Multi- temporal satellite data (Landsat 4, Landsat 5 and Landsat ETM+) were used in this research. Both sensors have spatial resolution of 30m. Characteristics of both data are shown in the following table 1. Table 1: Data Characteristics Satellite Sensor Bands Date of Acquisition Spatial Resolution Landsat TM 4 4, 3, 2 (NIR, R, G) 28 Jan m Landsat ETM + 4, 3, 2 (NIR, R, G) 01 Feb m Landsat TM 5 4, 3, 2 (NIR, R, G) 30 Jan m All three Landsat images were acquired from the USGS Global Visualization Viewer (GloVis) of January 1989, February 2002 and January 2010 to serve the purpose. 2.2 Satellite Data Preprocessing and Data Acquisition Digital image-processing software ENVI (v. 4.7) and vector data manipulation software ArcGIS (v ) were used for the processing, analysis and integration of spatial data to reach the objectives of the study. The images were downloaded and layer stacked according to their band combination. The images were georeferenced mosaicked and subset using a georeferenced shape files of Dhaka district. Then the actual vegetation cover is identified 483

4 using the vegetation delineation tool of ENVI. Resultant coverages are exported to shape files for further analysis with ArcGIS. The complete workflow can be illustrated as below. 2.3 Data analysis In ENVI, vegetation delineation works quickly to identify the presence of vegetation and to visualize its level of vigor. During the analysis the Landsat imageries are converted to an NDVI output and density slicing the images according to the brightness values, without performing atmospheric correction. The density slicing enables us to correctly identify and visualize the presence of certain features, in this case vegetation. The intensity that are used to classify the vegetation cover can be displayed as below. Only two clearly identified classes, the dense and the sparse were chosen to export as shape file (.shp) to analyze in the next phase. Figure 2: Flowchart of the NDVI analysis The second phase of vegetation cover analysis has been performed within ArcGIS desktop (v 9.3.1) by reprojecting, overlaying and calculating the spatial distribution of the vegetation and its changing pattern in three different time period. The acquired attribute data are further evaluated to perform the tables and charts. The resultant coverage is prepared for presentation Table 1: Classification scheme for classification of vegetation cover 3. Results and Discussion Change Classes Intensity Dense Vegetation 25% to 17% Moderate Vegetation 17% to 8% Sparse Vegetation 8% to 4% No Vegetation Less than 4% 484

5 The result of this study clearly shows that the vegetation cover in Dhaka decreases significantly after In the six thanas the vegetation loss is most shocking in Dhaka Metropolitan area. The result of the calculation can be tabulated as follows. Table 2: Rate of the change of Dense and Sparse vegetation in 1989, 2002 and 2010 (areas in sqkm) Thana Jan-1989 Feb-2002 Jan-2010 Dense Sparse Dense Sparse Dense Sparse Dhaka Metropolitan Dhamrai Dohar Keraniganj Nawabganj (Dhaka) Savar Total Table 3: Percentage change of Vegetation Thana % % % Dhaka Metropolitan Dhamrai Dohar Keraniganj Nawabganj (Dhaka) Savar Total Figure 3: Vegetation Coverage in bar diagram in three different time period 485

6 Figure 4: Trend of Decrease of Vegetation in three different time period 3.1 Small Change of Vegetation Coverage The analysis indicates that the total vegetation coverage followed low reduction rate in Keraniganj, Dhamrai and Dohar. Dhamrai and Keraniganj gained 23.1 and 8.3 square kilometers of vegetative land during 1989 to 2002 respectively. But after 2002 they lost and square kilometers of vegetative land, until Dohar on the other hand lost and 7.3 square kilometers of vegetation cover during and periods respectively. This change has very small impact of the total vegetation change scenario of this area. Considering the whole thana Dhamrai and Keraniganj lost an average of 0.93 and 0.41 percentage of vegetation respectively. The lost in Dohar is 0.95 percent as an average. 486

7 Temporal change detection of vegetation coverage of Dhaka using Remote Sensing Figure 5: Vegetation Coverage Found in Satellite Imageries 487

8 Figure 6: The rate of vegetation during the last two decade 3.2 Medium Change of Vegetation Coverage The study indicates that Nawabganj lost 9.75 square kilometer of green cover during period and in she lost square kilometer of green cover. In percentage this is 0.75 and 2.48, which is an average of 1.62 percent every year. 3.3 High Change of Vegetation Coverage The study clearly indicates the alarming reduction of vegetation coverage in Dhaka Metropolitan and Savar. Savar lost square kilometer of green cover and in Dhaka Metropolitan it is square kilometer during 1989 to In 2002 to 2010 Dhaka Metro lost and in Savar it is The capital lost her green cover at a rate of 3.54 per year during the last two dacade, while Savar lost at a rate of Dhaka district lost square kilometer of land in the last two dacade, which is almost 47 percent of total vegetation. The green coverage was lost at a rate of percent during the period, loosing percent of dense vegetation and percent of sparse vegetation (Table 3). Changes among different years can be seen from figure 5, 6 and 7 that the vegetation lost have been shocking during this period. Table 5 shows the detailed change of the vegetation cover in this area. The numbers show one-to-one relation according to the study years among different thana. The table also indicates that the 1989 to 2010 period Dhaka actually gained much vegetation, but and period rejects the notion 488

9 Table 4: Detailed Change (Loss) during this Session Thana % % % Dhaka Metropolitan Dhamrai Dohar Keraniganj Nawabganj (Dhaka) Savar Total Conclusion Green coverage is one of the most important factors for sustaining life and the living environment for any rapidly growing cities, like Dhaka; not only for preserving sustainable human habitat but also for safeguarding from the detrimental effects of urban pollution and Urban Heat Island (UHI). Rapid depletion of vegetation coverage in recent past might result adverse condition in Dhaka and its surrounding areas. Initiatives should have been taken to avert the area from the adverse effect of urban pollution and deforestation by various government and non-government organizations, but the situation seems to overdo the expected rate of change and expansion. Green space dynamics and spatial metrics analyses are imperative for understanding the landscape ecological conditions of urban green spaces. This study revealed that the green spaces of Dhaka are decreasing its area over the course of time; about 67.5 sqkm area has been lost during 1989 to 2002 that stands to increase about sqkm in between 2002 to 2010 giving the overall 100 sqkm of vegetation lost over twenty It is an alarming condition due to the high fragmentation of the increasing pace of human activity in this region. This activity is not only causing the destruction of landscape ecological processes and services, but is also degrading the biodiversity in urban areas. Moreover, consistent landscape fragmentation can result in a poor quality of life in the urban environment. Therefore, a comprehensive green space management strategy should be implemented for Dhaka district that could support proper functioning of the ecosystem. As the reliable and updated data are greatly lacking in Bangladesh, the green space maps produced in this study can contribute to the development of sustainable urban land-use planning decisions that target a sound and healthy urban environment. The study of temporal mapping of Dhaka successfully demonstrates the shocking changes of decreasing urban forestry and vegetation coverage. Further study on the vegetation and biodiversity cover reduction will surely unveil and confirm the frightful changes throughout the area over the last decades. 5. References 1. Ahmed, N (1958), Economic Geography of East Pakistan, Oxford University Press, London. 2. Bashar, M (2007, April 13), Urban forestry: Imperative for life in city. The Daily Star. 489

10 3. Byomkesh, T., Nobukazu, N., & Dewan, A (February 2011), urbanization and Green Space Dynamics in Greater Dhaka, Bangladesh. Landscape Ecologic Engineering. 4. Dewan, A. M., & Yamaguchi, Y (2006), remote Sensing And GIS For Mapping And Monitoring The Effect Of Land Use/Cover Change On Flooding In Greater Dhaka Of Bangladesh. Water Resource Management. 5. Islam, N (1996), Dhaka From City To Megacity: Perpectives on People, Places, Planning and Development. Dhaka: Department of Geography, University of Dhaka. 6. Islam, N (2005), Dhaka Now: Contemporary Urban Development. Dhaka: Bangladesh Geographical Society. 7. Jensen, J. R (1996), introductory Digital Image Processing: A Remote Sensing Perspective (2nd ed.). New Jersey: Prentice Hall. 8. Khan, N. I (2000), temporal Mapping And Spatial Analysis Of Land Transformation Due To Urbanization And Its Impact On Surface Water System: A Case From Dhaka Metropolitan Area, Bangladesh. International Archives of Photogrammetry and Remote Sensing, XXXIII, Part B7, pp Levien, L. M., Roffers, P., Maurizi, B., Suero, J., Fischer, C., & Huang, X (1999), a Machine-Learning Approach To Change Detection Using Multi-Scale Imagery. American Society of Photogrammetry and Remote Sensing 1999 Annual Conference. Portland, Oregon: American Society of Photogrammetry and Remote Sensing. 10. Rahman, M (2009, October 3), preserving green spaces in Dhaka City. The Daily Star. 11. Rahman, M (2010, May 15), sustainable cities: How far away are we from? The Daily Star. 12. S.O, A., & B, R. A (2003), change Detection Of Vegetation Cover, Using Multi- Temporal Remote Sensing Data And GIS Techniquea. Retrieved August 18, 2010, from GISdevelopment.net. 13. The Asiatic Society of Bangladesh (2006), Banglapedia: National Encyclopedia of Bangladesh. Dhaka: The Asiatic Society of Bangladesh. 490