Chartwork Tutorial: Datums, Depths & Heights

Summary

In this video, the presenter explains how depths and heights are defined on a nautical chart, focusing on chart datum and height datum. The video covers the importance of understanding tidal data, including mean sea level, spring and neap tides, and how these affect depth measurements and vertical clearance. The presenter also explains how to calculate the actual depth of water and height of obstructions like bridges, factoring in tide levels and different datums. Useful diagrams and resources are provided on the website for a deeper understanding.

Takeaways

• 😀 Chart datum is the reference point for soundings on a nautical chart, typically set at mean lower low water (MLLW).
• 😀 Different chart datums are used depending on the area, such as lowest astronomical tide (LAT) or mean sea level (MSL).
• 😀 Charted depths on a nautical chart are measured from chart datum down to the seabed, with occasional references to drying heights for areas exposed at low tide.
• 😀 Tides oscillate, and understanding tidal definitions like mean sea level, neap tides, and spring tides is crucial for calculating safe water depths.
• 😀 Neap tides result in the lowest high waters, and spring tides lead to the highest high waters, each with corresponding tidal ranges.
• 😀 The lowest astronomical tide (LAT) is often used as the chart datum for charts in tidal areas, providing a consistent reference for depths.
• 😀 When determining actual water depths, you must add the height of the tide to the charted depth to find the observed depth at any given time.
• 😀 When dealing with drying heights, the height of the tide is subtracted from the drying height to calculate the water level above the seabed.
• 😀 For vertical clearance, you need to consider both the height datum (for obstacles like bridges) and the ship's air draft to ensure safe passage.
• 😀 The difference between chart datum (used for depths) and height datum (used for elevations) becomes crucial when determining clearances, such as under bridges.
• 😀 It's essential to ensure that the chart and tide table use the same datum for accurate navigation and clearance calculations.

Q & A

What is chart datum, and why is it important on nautical charts?

Chart datum is a reference point used on nautical charts to measure depths. It provides a consistent starting point for all depth measurements, ensuring uniformity across the chart. Common examples include Mean Lower Low Water (MLLW) and Lowest Astronomical Tide (LAT). The choice of chart datum is critical for navigation and ensuring safe vessel clearance.

How do tidal datums like Mean Sea Level (MSL) and Mean High Water (MHW) differ from chart datum?

Tidal datums like MSL and MHW describe average tide levels at specific times, while chart datum refers to a fixed reference point used for depth measurements. For example, MSL is the average sea level over a long period, and MHW represents the average of high tide levels over a set period. These tidal datums are different from chart datum, which may be based on the lowest expected tide.

Why are there different tidal datums for different locations on nautical charts?

Different locations experience varying tidal ranges and effects due to geographic and astronomical factors. Thus, different datums (e.g., Mean Lower Low Water or Lowest Astronomical Tide) are chosen for consistency in a given area. These datums help ensure reliable navigation data across different regions.

What are 'drying heights,' and how do they affect charted depths?

Drying heights indicate areas that might become exposed above the water at low tide. In these regions, charted depths are not reliable for navigation because they may be subject to tidal changes. When depths are measured above chart datum, a drying height is given instead.

How does the tide affect the actual depth of water in a location?

The tide's height adds or subtracts from the charted depth to give the actual water depth. For instance, during high tide, the water level rises, increasing the depth compared to charted measurements. Similarly, at low tide, the depth decreases. Accurate tidal height information is necessary for calculating the true depth at any given moment.

Why is the Lowest Astronomical Tide (LAT) commonly used as a chart datum?

LAT is often used as a chart datum because it represents the lowest point the tide is expected to reach, providing a conservative and consistent reference. This ensures that the water depth is never underestimated and allows for safer navigation, as any tide above LAT increases the depth.

What is the role of mean high water springs (MHWS) in calculating vertical clearance under obstacles?

Mean High Water Springs (MHWS) is often used as a height datum for calculating the clearance under obstacles like bridges or lighthouses. Since the highest tide levels are more critical for clearance, MHWS is chosen to ensure safety when the tide is at its highest.

How do you calculate the vertical clearance under a bridge when considering both charted heights and tide levels?

To calculate vertical clearance, you add the tide height to the height of the mast (air draft) of the vessel. Then, compare the total clearance to the height of the bridge, which is measured from a height datum such as Mean High Water Springs (MHWS). If the vessel's total height exceeds the clearance, it indicates a potential obstruction.

What is the significance of using the same datum for both the chart and the tide table?

Using the same datum for both the chart and the tide table ensures consistency when calculating actual depths and clearance. If the datums differ, it could lead to errors in navigation and clearance calculations, potentially leading to unsafe conditions.

Why is it important to understand both charted depths and the effects of tide when navigating a vessel?

Understanding both charted depths and the effects of tide is vital for determining whether there is enough water beneath the vessel's keel. Tidal variations can significantly alter water levels, and without accounting for these changes, a vessel could run aground or collide with submerged obstacles.