Assessment of Nautical Chart Datum Via Statistical Method in Micro and Meso Tidal Regime: A Viable Option against Balay Harmonic Method
In the realm of maritime navigation, the Chart Datum (CD) plays a crucial role in providing a consistent reference for depths and heights on nautical charts. Traditionally, the method used in Brazil for CD determination has been based on a tidal datum approach, aligning with international hydrographic practices [1].
However, a new statistical method has emerged as an alternative, offering a simpler and potentially more accurate approach. This method, unlike the traditional one, does not necessarily rely on astronomical tidal components or harmonics. Instead, it focuses on the empirical analysis of tidal data distributions [2].
The study covered various tidal characteristics and meteorological influences throughout Brazilian territory, with tide stations at Pecem Port, Recife Port, Shipyard Naval Base (EBN), Itajai Port, and the Southern Nautical Signaling Service (SSN-5) being selected [1]. The Harmonic Method and the Statistical Method were applied to the tide datasets for the calculation of the CD, allowing for a comparison of the results obtained from the simulations [1].
The Harmonic Method, as used by Brazil's Directorate of Hydrography and Navigation (DHN), determines CD by summing significant components of the astronomical tide. However, it does not consider the effect of noise and meteorological tide, causing deviations in places with great influence of those components [1]. On the other hand, the Statistical Method calculates CD by considering both the astronomical and meteorological components [1].
The statistical method aided the analysis with comparative graphs of the observed tide, predicted tide, and the non-tidal residual, in addition to histograms and graphs of the PDF calculated for each location and period [1]. The CDF curve was also used to identify the accumulated density for each value of CD, and choosing probabilities of 0.5%, 1% and 5% as Safe Chart Datum (SCD) for comparison with the Balay Method CD [1].
The tidal datum approach used in Brazil is generally safer for navigation because it incorporates known tidal constituents, ensuring that the Chart Datum represents a water level that is not likely to be exceeded by ordinary tidal variations. This reduces the risk of vessels grounding, as depths referenced to CD are typically conservative [2].
However, statistical methods may yield a Chart Datum that is easier to calculate from available data. But if not properly chosen, they may underestimate extreme low water levels, potentially compromising navigational safety. If the statistical threshold is set too high (e.g., median tide level), the Chart Datum might overestimate water depths [2].
The implementation of the statistical method and process automation was made in a Python environment [3]. Calculating the Lowest Astronomical Tide (LAT) from a harmonic forecast could allow the identification of a more accurate CD [3]. The large difference observed between the calculated BCD and SCD of 0.5% for the 6-month sample is indicative of the ability to more safely identify the CD using the statistical method, especially when the influence of the non-astronomical tide is higher than the astronomical tide of a location [3].
In conclusion, while statistical methods can be simpler and may enhance accuracy in certain environments by fitting observed data patterns neatly, the tidal datum method currently used in Brazil is preferred for safety and reliability as it accounts for tidal dynamics and extreme conditions more robustly [2].
References: 1. Study on the determination of CD using a statistical method 2. Comparison of tidal datum versus statistical datum methods for Chart Datum determination 3. Automation of the statistical method for CD determination in a Python environment
- The study on the determination of CD using a statistical method [1] highlights its potential for providing a simpler approach compared to traditional hydrographic survey methods, while still maintaining accuracy.
- In the realm of environmental-science and climate-change research, the data-and-cloud-computing-based statistical method for CD determination could significantly contribute to the safety of navigation, if properly implemented, by providing a more adaptable approach to various geographical conditions.
- The implementation of the statistical method for CD determination in a Python environment [3] underscores the role of technology in advancing maritime navigation, yet it emphasizes the importance of careful consideration to ensure the method does not compromise navigational safety due to underestimation of extreme low water levels.
