Evaluating GPS for datum transfer in Hydrography

In hydrographic surveying, vertical datum is an important aspect for sounding as it is used as a reference to reduce for the depth of water at any instant. The recorded soundings made in hydrographic surveys must be corrected for any departure from true depth, such as transducer draft and reduction to the vertical datum. The tide reductions were derived from data recorded by tide gauge mounted on piers close to the project area, so the relationship between the water level and tidal datum has to be established and realized. If no tidal datum is available at the project area, a new datum must be established, whilst, if there exist a datum nearby, the datum can be transferred to the project area. To accomplish datum transfer, simultaneous tide observations at project area and known datum were conducted, conventional leveling was carried out and static Global Positioning System observations carried out for three days. Comparison of height differences between the three methods reveals that tide gauge readings produce the most precise result, followed by leveling and GPS. However, for the 5km separation, all three methods produced accuracies within the acceptable limit of ±10 cm as specified by the IHB (1998). GPS observation of within 30 minutes is sufficient to achieve the above limit whence, GPS can be used for datum transfer and with the shorter period of observation, saving in time, cost and labor.