Bangladesh Transverse Mercator (BTM) Projection

In this post I'll try to give a very brief overview of Bangladesh Transverse Mercator (BTM) projection system and necessary parameters of BTM, along with the projection file (BTM.prj) that can be used directly into ArcGIS 9.x or later versions.

Geographic Information System (GIS) users are quite familiar of Map Projection, which is the method to represent the three dimensional earth surface into a two dimensional plane. Universal Mercator Projection (UTM) is a widely used projection system now-a-days in which the earth (between 80°S and 84°N latitude) is divided into sixty zones, each a six-degree band of longitude, and a secant transverse Mercator projection is used in each zone. For example, zone 1 covers longitude 180° to 174° W and then zone numbering increases eastward to zone 60 that covers longitude 174 to 180 East. In a zone, the coordinate of a point is given by the Northing and Easting. The intersection of the equator and the UTM zone's central meridian is considered as the point of origin of each UTM zone and the central meridian of each zone is set at 500,000 meters East to avoid negative numbers.

 

Figure 1. UTM Zones (click to make it larger) Source: Wikimedia Commons

Bangladesh's longitude is from about 88º East to about 92º East. As UTM uses 6º band on longitude to divide the earth into zones, Bangladesh falls into two different UTM zones: Zone 45 N (West of 90º E) and Zone 46 N (East of 90º E). However, the parameter values that characterize a UTM zone are varies with zones. Therefore, problem arises interns of projection accuracy when someone go for mapping the area of Bangladesh that falls in both of the UTM zones (e.g., the whole Bangladesh Map).

To overcome this problem, in Flood Action Plan 19 (FAP 19) study a new projection system was evolved from the UTM, and is known as BTM projection system. Usually, the mid longitude of a Zone is taken as the central meridian in UTM. For example, if anyone choose Zone 45N, the central meridian should be 87º East, while if anyone choose UTM Zone 46, the central meridian should be 93º East. In BTM, instead of using either 87º East, or 93º East, they have taken 90º East as the central meridian. Since introduction of this new BTM system in May 1992, most of the official digital map of Bangladesh has been generated using this projection system. The necessary parameters of this BTM projection system are listed below:

Projection: Transverse_Mercator
False_Easting: 500000.000000
False_Northing: -2000000.000000
Central_Meridian: 90.000000
Scale_Factor: 0.999600
Latitude_Of_Origin: 0.000000
Linear Unit: Meter (1.000000)

Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.017453292519943299)
Prime Meridian: Greenwich (0.000000000000000000)
Datum: D_WGS_1984
Spheroid: WGS_1984
Semimajor Axis: 6378137.000000000000000000
Semiminor Axis: 6356752.314245179300000000
Inverse Flattening: 298.257223563000030000


Anyone working with ArcGIS 9.x or 10.x version can input those parameters to set BTM projection. However, I have created a projection file (*.prj) which can be used directly to set a map into BTM. Anyone can download the file (Zipped file) and use the BTM.prj file (after unzipping) to set projection to a new map or to re-project a map into BTM projection system.

Published in WRE Forum on July 01, 2012(Bangladesh Time)