# DATUM for BANGLADESH

A datum defines the reference spheroid (or ellipsoid) used to describe a portion of the earth. In Bangladesh, the Everest spheroid has historically been used in all projections. However, there are various Everest datums and there is a great deal of confusion about which to use. To date it has not been possible to completely clarify the confusion, but an attempt is made here in this regard.

The most commonly use spheroid and the one given in all manuals and textbooks for Bangladesh is the Everest (1830). The British Ministry of Defense, Directorate of Military Survey Plans lists five different Everest spheroids. However, in its table of grids they only refer to the reference spheroid as E”, or Everest, not specifying anyone of the five. Most people have assumed that the Indian IIB grid (used on most of the Bangladesh SOB maps) datum is Everest (1830). Some reliable sources have indicated that the actual datum is Everest (Malaya RKT). The Everest (Malaya RKT) is about 5 m longer on both semi-major and semi-minor axis than the Everest (1830).

In addition there is the confusion about the Indian yard. The Indian yard is defined as 0.914395233 meters, slightly shorter than the international yard (0.9144 exactly). This small change makes a difference when considering the radius of the earth, which is over six million meters.

An Everest spheroid which takes some of these discrepancies into consideration is now being used by Finn map, F AP24 and SOB (but not on all projects, apparently). F AP 24 has called this the Modified Everest Modified. For simplicity this report will refer to it as Everest (Bangladesh).

The difference in semi-major axis lengths between the various Everest spheroids is less the 30 meters. Although there is no linear conversion some tests have indicated that the offset in coordinates is about half the difference of major-axis length in one direction and negligible in the other direction. (example: the difference in co-ordinates between using a spheroid with an axis length of 6,377,307 and one with a length of 6,377,297 is about 4.5 m in one direction and 0.15 m in the other.)

The WGS84 datum has become important in recent years because it is the basic datum for the GP WGS84 provides a best-fit spheroid for the entire earth. It is important to remember that the geographic co-ordinates (latitude and longitude) of any given point are different for each datum. The parameters for the different datums used in Bangladesh are as follow:

__Everest 1830 __

Semi-major axi: 6,377,276.345 m

Semi-minor axi: 6,356,075.413 m

I/f (inverse flattening): 300.801700000

e2 (eccentricity squared): 0.006637847

Source: Most textbooks and manuals

__Everest (Bangladesh, or Modified Everest Modified) __

Semi-major axi: 6,377,298.524 m

Semi-minor axi: 6,356,097.518 m

I/f (inverse flattening): 300.8017

e2 (eccentricity squared): 0.006637847

Source: SOB, F AP24, Finn map

__Everest (Malaya RKT, or 1948, or Modified) __

Semi-major axi: 6,377,304.063 m

Semi-minor axi: 6,356,103.039 m

I/f (inverse flattening): 300.8017

e2 (eccentricity squared): 0.00663784663

Source: British Military survey (may be datum for Indian IIB)

__ __

__WGS84 __

Semi-major axi: 6,378,137.000 m

Semi-minor axi: 6,356,752.314 m

I/f (inverse flattening): 298.257223563

e2 (eccentricity squared): 0.006694380

Source: Most textbooks and manuals

Practically the Everest (1830) is still the most commonly used datum in Bangladesh. However, a number of organizations which have recently investigated the datum and projection problem in detail are using the Everest (Bangladesh). The Everest (Malaya RKT) is not used today, but it appears to be the datum used in preparation of the Indian Zone IIB and IIIB maps.

**Datum Shifts **

Most GPS data is output in the WGS 84 datum, and most of the final output in Bangladesh is in one of the Everest datums. A number of programs require datum shift constants to make the conversion from WGS 84 to the datum being used. However, here too there is some confusion as there are a number of different constants being recommended. The table below gives four sets of constants that are recommended for Bangladesh.

Ref: **Water Resources Planning Organization**