Geodesy deals with the precise size, shape and gravitational field of the earth. The actual physical surface of the earth is very irregular. A more uniform surface is the geoid which is the equipotential surface which coincides with mean sea level (MSL). This too is irregular in shape in the sense that it cannot be represented by a finite mathematical formula, due to variations of gravity. In dealing with precise dimensions and the shape of the earth it is necessary to adopt a reference surface on which mathematical computations can be carried out and variations of the geoid from this reference surface could be indicated by elevations and depressions, termed Geoidal undulations above and below the reference surface. The mathematical surface which best fits the geoid is an ellipsoid of rotation. The dimensions of the ellipsoid vary according to the one selected by each country and for Sri Lanka the dimensions of the semi-major and minor axes are 6377276.345 meters and 6356075.413 meters respectively which is the Everest 1830 of the Earth used in the Survey of India. Observations of angles and distances for surveying are carried out on the earth's physical surface. They have first to be reduced to the ellipsoid before computations commence. Computations are done using the dimensions of the ellipsoid adopted and appropriate formula. The first control network or principle triangulation of Sri Lanka was carried on Everest 1830.
The principal triangulation of Ceylon began in 1857 with the measurement of the Negombo (Kandawala to Halgastota) baseline. The triangulation observations were made on pillars at Kandawala and Halgastota (See photographs below, at present known as Landesi Kuluna) with 8-inch and 13-inch vernier theodolotes, and were completed in 1885. This triangulation was subsequently connected to the Indian Triangulation in 1887 by a long narrow triangulation chain running roughly through Batticaloa, Trincomalee, Manar and Delft. The Ceylon network was recomputed with some additional observations in 1890 due to inconsistencies occurring mainly in the minor triangulation station.
Kandawala Trigonometrical Station (Broken down on 28th November 2015)
|Halgastota Trignometrical Station|
|Primary Triangulation System in Sri Lanka|
In 1993, the Sri Lankan Geodetic Survey Unit (GSU) decided to establish a new Sri Lankan horizontal geodetic control network using only Global Positioning Systems (GPS). This geodetic survey was carried out during 1996 to 1998 using Leica System 300 dual frequency GPS receivers over 1265 baselines at a typical station spacing of 20-30km. The origin base station was established at Institute of Surveying and Mapping referred to as ISMD. This control network was adjusted with least squares adjustment using Geolab v 2.6 software. This led to the horizontal datum of Sri Lanka and named it as “Sri Lanka Datum 1999 (SLD 99)”. At present all the control points and surveying are done according to SLD 99.
|Present Geodetic Control Network|
|Present Level Network|