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 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. The dimensions of the ellipsoid vary according to the one selected by each country. The dimensions of the semi-major and minor axes for Sri Lanka are 6377276.345 meters and 6356075.413 meters respectively which is the Everest 1830 of the Earth used in the Survey of India.

PRESENT NETWORK

Horizontal Control Network

Control network is a series of well-spaced and interconnected markers in the ground which have accurately determined positions. Covering the entire country Sri Lanka, old triangulation network was established using triangulation and trilateration techniques by taking 200,000m, 200,000 m as false northing and false easting of the Transverse Mercator Projection for the coordinate system, which is mostly named as Kandawala coordinate system. Consequently, it was decided to upgrade the control network by using Global Navigation Satellite System (GNSS) observations. GNSS survey technology has opened a new era of National Geodetic Reference Frame service, so that each land survey can be carried out without expecting dense physical control stations. Hence, all the GNSS observations are in WGS84 coordinate system and those coordinates are required to transform in SLD99 for all surveying and mapping work and related executes, since all the survey tasks are done based on the said coordinate system, SLD99. Moreover, RTK (Real-Time Kinematic) GNSS survey can determine horizontal point coordinates within a few tens of seconds per point, and SLCORSnet (Sri Lanka Continuously Operating Reference Station Network) service can eliminate conventional monument stations.

Vertical Control Network

Vertical network is a series of Bench Marks connecting the Mean Sea Level. The earliest level recorded was 1000 miles single leveling which was started in 1865 and completed in 44 years. Old level lines were formed with no network since leveling was only done as the need for it arose. Between 1904 and 1909 the standard of leveling was improved and more attention was given to construction of permanent Benchmarks. In 1909 more staff was deployed and leveling operations were undertaken systematically. As a part, new network was tied down to the mean sea level determinations made by the Great Trigonometry Survey of India at Colombo, Galle and Trincomalee between 1884 and 1895.

In 1925 whole leveling procedure was reviewed and decided to start afresh with modern instruments and methods of precision. The leveling was done with precise levels as well as invar staves. The great care was adopted in leveling procedures in order to achieve results of the highest accuracy. The geodetic leveling network comprises of 59 FBM, 5 SBM, and 4000 km of double-leveling forming 27 circuits. The primary leveling network covers the entire country and compares favorably with leveling of high accuracy in other countries of the world. From then on, leveling has been extended by secondary, tertiary and minor leveling to provide height control for all development projects in Sri Lanka. In this process 6 FBM constructed in 1924, 31 FBM in 1925, 22 FBM in 1926 and principle network was completed by 1928.