Bangladesh is a low lying country with vast flood plain. The elevation of the land surface on the southern part is not much above Mean Sea Level (MSL). As we all know, sea level is rising day by day mainly due to the effect of climate change. Hence, it is an urgent need to know the height information over the coastal areas of Bangladesh. Survey of Bangladesh (SOB), The National Mapping Organization (NMO), has almost carried out precision leveling (both First and Second Order) to survey height information throughout the country. Before 1991, the then height information was referenced to MSL of the Arabian Sea located in India. Then SOB entered into a new era by establishing a new geodetic network under a project initiative with the cooperation of Japan International Cooperation Agency (JICA). It covered a two-third area of Bangladesh excluding Chittagong Hill Tracts and coastal areas. Under this initiative, SOB established a permanent Tidal Observation
Station at Rangadia, Chittagong in 1993 and in 1994 determined a tentative MSL of the Bay of Bengal.
SOB also established a National Vertical Datum at Gulshan, Dhaka in 1994. With reference to Vertical Datum, a national vertical control network was established. Around 2300 kilometers 1st Order leveling survey was carried out determining MSL height of 536 Benchmarks (BMs). Later during 2001-2004 the leveling network was extended to Chittagong Hill Tracts and the coastal areas through 1500 kilometers of 1st Order leveling line and 313 BMs were connected. For densification of vertical control points network additional 207 BMs stretched over some 3500 kilometers, were connected by exercising 2nd Order leveling during 2002-2004. Another 150 BMs were connected by carrying out 2nd Order leveling over 1600 kilometers during 2004-2008.
The 1st order leveling was carried out from the vertical datum located at Gulshan, Dhaka and the 2nd order leveling was carried out from the 1st order benchmarks.
River Cross Leveling
To carry out leveling in the coastal areas, SOB has to cross many wide rivers with special method of leveling which is a big challenge. The mentionable wide rivers are The Padma (Mawa, Munsiganj), Rajganj (Lebukhali, Patuakhali), Arial Khan (Muladi, Barisal), Dharmaganj (Hijla, Barisal), Ilisha (Ramdaspur, Bhola), Tetulia (Dhulia, Patuakhali), Bishkhali (Patharghata, Barguna), Payra (Amtali, Barguna),
Kacha (Pirojpur), Baleswar (Sharankhola, Bagerhat) and Passur (Mongla, Bagerhat) .
Equipment Used for Leveling
Survey of Bangladesh uses the following types of equipment for 1st and 2nd order leveling:
a. Analogue level machine with invar staff; accuracy (Standard deviation for 1km. double run leveling) is ≤0.4mm. Such as Wild N3, SOKKIA B1 etc.
b. Digital level machine with invar bar-coded staff; accuracy (Standard deviation for 1km. double run leveling) is ≤0.4mm. Such as Leica NA 3003A, Leica DNA 03, Topcon DL 101, Topcon DL 101C etc.
Accuracy of Vertical Control Network
The accuracy of 1st and 2nd Order vertical control network are ±4mm√D and ±8.4mm√D respectively. Where, ‘D’ is the length of leveling line.
Statistics of Vertical Control Points as on June 2013
1st Order vertical control points (BMs) : 849 numbers
1st Order leveling line : 3,800 kms
2nd Order vertical control points ( BMs) : 1,257 numbers
2nd Order leveling line : 14,300 kms
Average horizontal spacing of vertical control points (BMs) : 5 kms.
(Along leveling route)
At present, SOB is implementing a new project titled “Improvement of Digital Mapping System of Survey of Bangladesh” for densification of geodetic control points (GCPs) to augment preparation of a new base map for Bangladesh. Under this project 900 BMs were connected through 2nd order leveling covering 9,200 kilometers during 2008-2013. More 200 BMs are going to be connected by 2nd order leveling in the year 2013-2014 and 2014-2015.