Abstract

The conventional definition of levelling rolling around the process of height difference determination. Thus, when the height of one point is known, then, heights of the other points can be calculated. Based on the height difference technique or method adopted, levelling can be classified into so many types such as ordinary, precise, trigonometric, barometric, tacheometric, GPS, etc.

To determine the reduced level or height of points, it is necessary to define a datum upon which heights of points are referred. The most important reference datums are the Geoid that can be represented by the mean sea level and the Ellipsoid which defines the mathematical surface of the earth. 

The mean sea level that physically represents the base of orthometric heights is rising due to climate change. This rise is caused primarily by global warming which added water from melting land-based ice sheets and glaciers and the expansion of seawater as it warms. This future continuous rise leads to changes in ground reduced levels based on the mean sea level.

This research work tried to discuss methods of height determination that can be in direct contact with objects or at a distance. In addition to the reference datums adopted which may often either be geoid or ellipsoid.   

The work concluded with that, geodetic heights based on ellipsoid as a reference datum could recently be easily determined through GPS and it may be transferred to orthometric heights since accurate measurement for geoidal model is available. Direct methods for height determination are suitable for a limited number of points where indirect ones could be used to collect data for large areas. Where the accuracy of the latter depends largely on the resolution and the number of ground control points used.   

Since the mean sea level is continuing to change; it is not coinciding with the geoid where the world geodetic datum (WGS84) ellipsoid is now the dominant geodetic model.

Digital level is now replacing optical one where trigonometric levelling is now directly carried out with total station.