Change detection of selected topo-features using geographic information system (gis): An investigation into the topographic map making process in Bangladesh

Abstract

Topographic maps are fundamental units of information for many types of field studies including geosciences and archeology that present a wide range of information, making them useful to both professional and recreational map users. The topographic mapping technologies in Bangladesh have seen two major transitions, the analogue mapping techniques of the British-Indian/Pakistan/Bangladesh Period (up to the 1980s) and the Bangladesh Period digital mapping techniques since the 1990s to the present times. The author had access to the topographic maps of SOB of both the period and therefore availed this unique opportunity to research the technology used for mapping in both the period and detect changes that had taken place on selected important features (river, forest and shore) of this country over last 60 years using GIS. Traditional mapping technology was studied by collecting internal documents of Survey department at various times; the publications/hand books about mapmaking in the British Indian Subcontinent and key persons and selected number of very experienced SOB Surveyors were Interview. Study of old instruments of SOB archive was also helpful in this aspect. It was found that the traditional method was very much laborious, methodical, time consuming, labor intensive and confined within the government domain. Modern mapping technology was studied by implementing various recent projects at SOB and exposure to mapping technology of various countries by the author. Modern method is more cost effective, quicker, precise and digital technology dependent. It is spreading quickly to the private and enterprise domain. Various data Sources both primary and secondary were used for change detection. Primary data used was topo- maps of SOB. Secondary data were collected from other organizations and sattelite image from web sites. All these data were processed, harmonized and desired features extracted into vector format using GIS. Then algorithm was developed to make GIS model for data analysis to detect changes. Three major rivers- Padma, Meghna and Jamuna were selected for change detection. Division wise changes are detected first where it was found that Barisal and Chittagong divisions are subjected to major changes by the Meghna river. Then the results are compared with similar studies done before by other organizations like WARPO and JICA to validate the findings. No such study involving entire Bangladesh territory has been carried out so far because of the unavailability and cost of map and satellite data at this scale. However, few studies are available that involves part of our river system. Padma river was found to retreat more than 6 kms most probably between 1940 to 1967 and flowing through a completely new channel. Jamuna river was found to be braided from 8 to 11.5km in last six decades. Its getting more wider and uniform. On average, most erosions are taking place at upstream and deposition at downstream near estuaries. Over all net changes in entire country is only 100 km sq erosion which is negligible in 60 years although commoners apparently think about huge erosion and loss of lands. In forest change detection it was found that both deforestation as well as afforestation has taken place over the ages. Most of the deforestation is found in Dhaka division. A big chunk of forest has altogether been deforested at Haluaghat. Rest of the deforestation is found in Gazipur and Modhupur forest. Some afforestation is found around the same area which is done mainly under “Community Afforestation” scheme taken by the government. A good amount of afforestation is noticeable along coastal belt of Noakhali, Vola, Potuakhali and Borguna districts. This is done mainly under the projects of Coastal belt protection schemes. It was found in the shore change detection that land mass of Kamalnagar, Ramgati and Subarnachar which was an island before but has become part of mainland now due to major shift of the Meghna River towards west. In other words, the Meghna River had two main channels into the Bay of Bengal keeping those islands in between the channels thus separating the islands from mainland. Now, the west channel is the only channel and the east channel has totally disappeared. Hatia and Sandwip islands have been eroded heavily in the north and deposited lightly on the south. Monpura has been elongated southward by deposition with some more islands deposition further down the channel. Bhola is also eroded in the north east and deposited southward and connected up to Char Fassion and beyond. A big island has also deposited in between Char Fassion and Golachipa. Some erosion is also observed at Mohesh Khali. Over all net changes in entire country is only 100 km sq erosion in sixty years which is negligible. It means that bank erosion and growth of land/char goes parallel due to meandering nature of the rivers. Erosion takes place mostly at mid and upstream of the river channels where as deposition takes place mostly at the lower part of river channels specially at the estuaries.