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This value, when used in the computation for the Chart Sounding, gives the correct result. We have now computed some kind of ‘pseudo-tide’ value. It gave us the ability to compare our RTK Tide with the conventional tide correction value and to generate statistics on the variation between the two. Notice that the computed RTK Tide no longer equals the conventional tide correction! In all of our previous examples, Tc = Tk. In this case we have the same values as the last example, except that we are going to have to use D = 0, since we don’t have any input for the value. (Don’t shoot me, I’m only the messenger.) Figure 5 shows the known values. They either don’t have the means to automatically measure the draft or they haven’t taken the time to manually correct for the draft. Having been on hundreds of different survey vessels, I can honestly state that less than 50 per cent of them ever apply any kind of dynamic draft correction.
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As long as we can measure the dynamic draft value, the computed RTK Tide should equal the conventional tide correction (Tk = Tc) Using the formula to determine our RTK Tide correction gives the following:Īs the vessel squats and the water level changes, we still get the correct Chart Sounding. Since the vessel has ‘dropped’ 0.2, the values for A and RD will also drop by the same amounts. The tide remains the same as in the previous example. We will assume we have a method to determine this change in the vessel’s dynamic draft. Due to ‘squat and settlement’, the vessel is sitting 0.2 lower than its normal static position. In this example (Figure 4) the vessel is underway. It would be possible to carry out a statistical comparison between the Tk and Tc values for each sounding to see exactly how the two values compare. In this case, the computed RTK Tide (Tk) equals the conventional tide correction (Tc) and we can obtain the chart sounding in real time without any delay. Using the formula to determine our RTK Tide correction gives the following: We would also expect that A (Height of RTK Antenna above the Ellipsoid Reference) and RD (raw depth from the echosounder) would also rise by the same amount. In the next example (Figure 3), our vessel is still sitting statically at the dock but the tide has risen another 2.0. We may be safe using it as a constant for a local survey about a pier, but we wouldn’t want to rely on a single separation value used over a large area. The height of the Chart Datum above the Ellipsoid Reference can change over a short distance, especially in areas of complex tides. It is important to note that this value may change over a large survey area. So at this location, the Chart Datum is located 1.5 above the Ellipsoid Reference. In order to determine the separation of the Chart Datum above the Ellipsoid Reference, we can just transpose our observed values into formula (3), using the conventional tide value as our RTK tide value. Using our conventional formula (1), we get the Chart Sounding:
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In this first example, there is no draft correction as the vessel is in its ‘static’ condition. In Figure 2 I have provided some values to each of our parameters. Note also that the Dynamic Draft measurement has cancelled itself out of the formula and is not necessary to compute the chart sounding! (More about that later on.) The key to getting this technique to work is to accurately determine SEP (the height of the Chart Datum above the Ellipsoid Reference). We have everything we need to determine the Chart Sounding in real time without having to resort to a conventional tide reading. Substituting this into our formula for the chart soundings results in: Solving for the RTK Tide component gives: From conventional hydrographic surveying, we can define a simplified Chart Sounding (CS) from our example as: For the following examples, we have calibrated our echosounder to the ‘static water line’ and our sounder is outputting depths based on this level. Looking at Figure 1, let’s define the following terms:įor simplicity, I am ignoring the heave-pitch-roll of the vessel and am assuming the vessel is in a pleasantly static, upright, condition. There are currently several district offices of the US Army Corps of Engineers using this technique in place of conventional tide measurements.The main purpose of this article is to explain some of the basics behind the technique (without getting overly technical) and to point out some of the benefits and drawbacks to the method. One of the fastest emerging trends in hydrographic surveying is the use of the vertical component of RTK GPS to determine real-time water level corrections.