Navigation and Chartwork Flashcards

On an Admiralty chart, what do the letters 'M' and 'S' indicate when describing the seabed?

Answer: Mud and Sand

On Admiralty charts, 'M' denotes Mud and 'S' denotes Sand as bottom characteristics. These abbreviations help mariners understand the nature of the seabed for anchoring.

What is the difference between 'variation' and 'deviation' in compass navigation?

Answer: Variation is the difference between true and magnetic north; deviation is the error caused by the vessel's own magnetic field

Variation is the angular difference between true north and magnetic north at a given location (caused by the Earth's magnetic field). Deviation is an additional error caused by magnetic influences aboard the vessel.

When plotting a position on a chart using latitude and longitude, which coordinate is read first?

Answer: Latitude

By convention, latitude (measured in degrees north or south of the equator) is always stated first, followed by longitude (measured east or west of the prime meridian).

What is a 'cocked hat' in chartwork?

Answer: The triangle formed when three position lines do not intersect at a single point

A cocked hat is the small triangle formed on a chart when three position lines (from bearings or other observations) do not meet at a single point, due to small errors. Your position is assumed to be within this triangle.

How is distance typically measured on a nautical chart?

Answer: Using the latitude scale at the side of the chart, where one minute of latitude equals one nautical mile

On a nautical chart, one minute of latitude equals one nautical mile. Distance is measured using dividers against the latitude scale on the sides of the chart, at the same latitude as the measurement.

What does a chart datum of 'LAT' mean?

Answer: Lowest Astronomical Tide — the lowest level the tide is predicted to reach under normal conditions

LAT (Lowest Astronomical Tide) is the lowest tide level that can be predicted under average meteorological conditions. Charted depths are measured below this level, so actual depth is usually greater.