Depths soundings need to be related to a survey datum by correcting for the water level at the time of the sounding. Traditionally this has been done by simultaneously, but separately, measuring tidal and heave corrections during a hydrographic survey. This practice is flawed because the effects of tides and heave are not separate, and are combined into the water level at any one instant. Measuring both corrections separately and then combining at the time of post-processing can introduce additional uncertainties. Measuring a Water Level Correction (WLC) using RTK GPS is considered a better methodology. Here, measurement of a WLC, as well as separate tidal and heave corrections, is tested using a Trimble MS750 receiver. Some solid-state heave devices require a survey vessel to travel at a constant speed and direction before each run line in order to stabilise. RTK GPS does not need to stabilise thus making it a better alternative for short runlines and small survey areas such as ports, marinas and estuaries. The most important consideration is to precisely match GPS heights with soundings. A method for aligning the data within 10 milliseconds was developed and is presented here. It is demonstrated that with such a precise latency heave caused by long period swell (>5 second period), as well as short period waves (2-5 second period), can be successfully corrected.
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