Tides and currents are giving hydrography an extra (some say challenging) aspect, which has to be accounted for. If you are measuring ‘absolute’ water depths, as do surveyors for nautical charting or those involved in dredging or wreck clearing, you have to determine a vertical reference level to reference the measured depths. The height of the tide above this reference at the time of the measurements needs to be determined. Close to shore this can be done using existing tide gauges or erecting extra ones yourselves. Farther offshore one way of solving how to collect the necessary tidal data is to lay your own tide gauges. Often it is necessary to lay quite a few, not only to cover the whole survey area but also just to insure against the risk of malfunctioning or tide gauges being carried away, a feat easily achieved by trawling fishermen or passing ships. Every surveyor can tell you ‘nice’ stories about this.
Modern technology allows us to tackle the (once) time-consuming and laborious reduction of measured depths to a vertical datum. There are at least two directions from which the reduction for tides is approached. The computing power of present ‘modal’ computers makes it possible to run mathematical tide models, which can be considered to suppress the necessity always to have (offshore) tide gauges laid in a survey area. Especially tidal models that take meteorological conditions into account have proven to be good substitutes for tide gauges in the survey area. However, Lidar survey techniques can encompass large survey areas, thereby introducing the complexity of more tide zones and a possible necessity of extra tide gauges.
The other direction from which efficiency in tidal reduction is approaching the hydrographer is the use of the vertical component of several GNSS techniques, such as Real Time Kinematic GPS. Once the difference between the vertical reference level and the ellipsoid is known, knowledge of tidal heights ceases to be necessary for tidal reduction. While the use of RTK for tidal reduction is mainly restricted to approximately 20 kilometres from a base station, more recent GNSS techniques are also becoming applicable to offshore areas and the results will improve when Galileo becomes operational. It is and will continue to be interesting to see the developments in these techniques, each of which will find its specific areas of application. Yes, even with (or thanks to) modern techniques, the tide and currents remain a challenging aspect of our profession...
And this is to address only one aspect concerning the tide and currents. There are many more; for example, ocean currents that affect offshore construction work. Forecasting ocean currents, important for operational planning of this work, has been improved by a combination of satellite remote sensing and in situ measurements. We will keep you informed on developments regarding techniques involved in tides and currents affecting our profession.
Our regional correspondent in Latin America, Jorge Pereira Libor, has made a drastic change to the course of his life and will be leaving the hydrographic scene. He has therefore asked to be relieved of his task as regional correspondent. It is with reluctance that we let him go; not only was he an active regional correspondent but he also supplied us with his own interesting features. Jorge, thank you very much for all the work you did for Hydro international - we wish you good luck in your new life.

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