National Grid System

MAP PROJECTIONS AND CO-ORDINATES

Sri Lanka extends about 2° in longitude and 4° in latitude. The map projection adopted for Sri Lanka is the Transverse-Mercator projection with the central meridian through Pidurutalagala. The scale along the central meridian is multiplied by a factor 0.99992384 so that the scale error along the central meridian is equal. The result is that there is no great scale variation across the country.

The scale error at the central meridian is 1:13,000 and at the eastern extremity of the island which is a little over 1° from the central meridian the scale error is 1:10,000. The geographical co-ordinates of all stations of geodetic control network were computed, using the Everest figure of the earth and Clarke's formulae. The rectangular co-ordinates of all principal, secondary and minor points were next computed using the Transverse-Mercator projection.

SHEET INDEXING SYSTEM

For numbering the one inch to one mile maps the island was first divided by lettered rectangles having dimensions 68 miles East - West and 44 miles North - South. These are called AA , A,B,C,….. Q. Each of these rectangles was divided into 25 sheets called '1 mile sheet squares' which are therefore 13.6 miles by 8.8 miles, numbered consecutively from the NW corner of the rectangle. Each '1 mile sheet square' is divided into 64 small sheets, sometimes called 'field sheet squares' which are 1.7 miles x 1.1 miles and are numbered consecutively from the NW corner of. the '1 mile sheet square'. Thus a field sheet having a number like J2/58 means that it is 'field sheet square' 58 in '1 mile sheet square' 2 in rectangle J. This system was placed quite arbitrarily to the origin of co-ordinates which was Pidurutalagala

This system has been replaced by the Metric sheet system, . In this system, there are 92 1/50,000 sheets each covering an area 25 km x 40 km and these are divided into 25 1/10,000 sheets, each 5 km x 8 km

SHEET INDEXING SYSTEM

The transverse mercator projection on Everest ellipsoid (1830) with the following parameters is used to compute new grid co-ordinate system.

  • Central meridian E 80° 46’ 18.16710”
  • Latitude of Origin N 7° 00’ 1.69750”
  • Scale factor 0.9999238418
  • False Northing 500000m
  • False Easting 500000m
  • Pidurutalagala trigonometrical station is used as the latitude of origin and central meridian.