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Topography


Topography configuration of a land surface including its relief and contours, the distribution of mountains and valleys, the patterns of rivers, and all other features, natural and artificial, that produce the landscape. Although Bangladesh is a small country, it has considerable topographic diversity. It has three distinctive features: (i) a broad alluvial plain subject to frequent flooding, (ii) a slightly elevated relatively older plain, and (iii) a small hill region drained by flashy rivers. On the south, a highly irregular deltaic coastline of about 600 km fissured by many estuarine rivers and channels flowing into the bay of bengal. The alluvial plain is part of the larger plain of Bengal, which is sometimes called the Lower Gangetic Plain. Elevations of the plains are less than 10m above the sea level; elevation furthers decline to a near sea level in the coastal south.

The hilly areas of the southeastern region of Chittagong, the northeastern hills of Sylhet and highlands in the north and northwest are of low elevations. The Chittagong Hills constitute the only significant hill system in the country. It rises steeply to narrow ridgelines (average 36m wide), with elevation ranges between 600 and 900m above mean sea level. In between the hilly ridges lie the valleys that generally run north to south. West of the Chittagong hills is a narrow, wet coastal plain lying parallel to the shoreline.

Topographic map The map showing surface features including relief, vegetation cover, water bodies or topography by means of contour lines, plastic shading, hatching, or other graphic devices. A large-scale topographic map even includes the locations of hats (market-places), petrol-pumps, education institutes, rural narrow roads, etc.

The Survey of Bangladesh is the sole authority to survey, prepare and update the topographic maps of the country. Depending upon scales there are four national topographic map series. These are 1:50,000 maps of large-scale format, 1:250,000 map of small-scale format covering Bangladesh, 1:500,000 map of small-scale format, and 1:1,000,000 map of small-scale format. The common scale of national topographic mapping throughout the Commonwealth is 1:50,000. In Bangladesh, this scale has been adapted by conversion of one inch to one mile or 1: 63,360 maps of old Survey of India. Similarly, 1:250,000 scale maps have been converted from old maps on one inch to 4 miles. The two other forms of small-scale maps (1:500,000 and 1:1,000,000) are classified as geographical maps. The basic national topographic map series of 1:50,000 scale are revised in 10 years. In revision of topographic maps of 1:50,000 scale series aerial photographs are rectified on the basis of ground control points fixed by traverse survey. [Mohd Shamsul Alam]