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সর্ব-শেষ হাল-নাগাদ: ৬ জানুয়ারি ২০২১

জিওডেটিক পরিসেবা

 

GEODETIC DETACHMENT (GEODESY)

 

Geodesy is the part of geosciences and engineering sciences, including navigation and geometrics. It plays a pivotal role in construction of the national economy, geoscience research, and in the process of social informatization. Advancement of modern science and technology has allowed geodesy to undergo an epoch-making transformation, to break the temporal and spatial limitations of traditional classical geodesy, and to enter a new stage of development of modern geodesy. In the geodesy, Geometrics surveying and mapping mainly study all forms of the Earth’s surface; therefore the shape and size of the Earth and its gravity field are studied and measured, and a unified coordinate system should be established to show the exact geometric positions of an arbitrary point on the Earth’s surface. Hence, Geodetic measurements usually need to be carried out before topographic mapping. Geodesy provides instant, dynamic, and quantitative spatial geometric and physical information with high accuracy and resolution. It serves as an important means of studying geodynamics phenomena such as the Earth’s rotation, Gravity, Tides, Movement of the Earth’s crust, and changes of sea surface.

 

Geodetic Detachment is an important branch of Survey of Bangladesh (SOB). It has been providing the geodetic services to the government and non-government organizations from the very beginning and contributing to nation building activities.  

 

 

RESPONSIBILITIES OF GEODETIC DETACHMENT

 

  • To maintain the National Horizontal and Vertical datum at GULSHAN, DHAKA, the Tidal Observatory at RANGADIA, CHITTAGONG and six Permanent GNSS Station at Dhaka, Chattogram, Rajshahi, Khulna, Maulavibazar & Rangpur.
  • To establish Geodetic Control Point [Horizontal (by Traverse, Triangulation and Global Navigation Satellite System) & vertical (by precise leveling)] throughout the country and maintain them.
  • ​To carryout tidal observation, determination and maintaining of Mean Sea Level (MSL).
  • Preservation of Geodetic Control Point data & tidal data and provide them to various users as per requirement.
  • To carryout departmental and extra-departmental ground control survey work as per requirement.
  • To update the Geodetic Control Points by re- survey if there is any major change.
  • Preparation of Geoid Model & Geoid Map of Bangladesh.

 

 

GEODETIC CONTROL POINTS OF BANGLADESH

 

Geodetic control networks play the most crucial role in the attainable accuracy and reliability of surveying mapping, navigation and engineering applications. Geodetic Detachment of SOB has established the Geodetic Control Network over the country by Global Positioning System (GPS) and Levelling technology.

 

 

The list of Geodetic Control Points of SOB:

  • 3D Control Points (Latitude, Longitude, Height above M.S.L)

843

  • 2D Points (Latitude, Longitude)

270

  • 1D Points (Height above M.S.L)

1483

  • Total Horizontal Control Points

1113

  • Total Vertical Control Points

2326

  • Total Geodetic Control Points

2596

 
 

 

ONLINE GEODETIC DATA SERVICES:

 

 

 

 

Total Geodetic Control Points area

  

 

 

Part of Geodetic Control Points area

 

 

 

TECHNIQUES OF GEODETIC SURVEY

  • Triangulation by Transit
  • Trilateration by EDM  [ Electronic Distance Measuring Instrument]
  • Traversing by Total Station   [ Transit & EDM ]
  • Levelling by Level
  • Baseline measurement by GPS

 

EQUIPMENTS USED FOR GEODETIC SURVEYING

  • GNSS Receiver
  • RTK GNSS Receiver
  • Handheld GPS Receiver
  • Total Station
  • Digital Theodolite
  • Analogue Theodolite
  • Digital Level
  • Analogue Level
  • Mobile Mapper
  • Data Center (Server)

 

 

DETERMINATION OF HORIZONTAL DATUM OF BANGLADESH

 

Although a geodetic control point in Gulshan had been used as a temporary horizontal datum of Bangladesh before 1990, the accuracy of its coordinates had not examined and the coordinates referred to Everest 1830. Because GPS refers to WGS 84, it is better to give not only Everest 1830 coordinates but also WGS 84 coordinates to the control points in Bangladesh. Consequently, the geodetic horizontal datum of Bangladesh was established in Glushan by repairing of the old control point monument and the coordinates on WGS 84 were determined by global GPS observations refers to four IGS observation stations; Wettzell(Germany), Hartebeesthoek (South Africa), Yaragadee (Australia) and Tsukuba (Japan) shown in Figure.

 

These four stations are operating not only continuous GPS observation stations for International GPS Society (IGS) but also Very Long Baseline Interferometry (VLBI) site or Satellite Laser Ranging (SLR) site. VLBI and SLR arc the most precise global geodetic observation technique which can determine the coordinates in few milimetres accuracy and arc operating to maintain the global geodetic reference system. This means that the coordinates of these stations are maintained most precisely. The GPS observation which connects the datum of Bangladesh to the stations was carried out for seven days, from 17 September to 23 September in 1994. The data was processed by Geodetic Department, GSI, Japan, using GAMIT baseline analysis software developed by Massachusetts Institute of Technology and University of California, and GLOBK network adjustment software developed by Massachusetts Institute of Technology, University of California and Harvard University.

 

Establishment of National Horizontal Datum

 

On the other hand, the most reliable coordinates on Everest 1830 was examined using GPS data for first order control point surveying. The GPS network included 24 old existing monuments which had coordinates on Everest 1830. The network was adjusted by free network adjustment technique using 19 stations out of the 24 old stations to examine the reliability of the old coordinates. As a result, the coordinates of this adjustment was adopted as the “Old” Everest 1830 coordinates.

 

Foundation Stone