RYA Day Skipper Navigation and Chartwork — Questions and Answers
Question 1: On an Admiralty chart, what do the letters 'M' and 'S' indicate when describing the seabed?
- Metal and Steel
- Mud and Sand (Correct answer)
- Magnetic and Standard
- Marks and Signals
Correct 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.
Question 2: What is the difference between 'variation' and 'deviation' in compass navigation?
- They are the same thing
- Variation is the difference between true and magnetic north; deviation is the error caused by the vessel's own magnetic field (Correct answer)
- Variation only applies to GPS
- Deviation is the difference between true and magnetic north
Correct 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.
Question 3: When plotting a position on a chart using latitude and longitude, which coordinate is read first?
- Longitude
- Latitude (Correct answer)
- It doesn't matter
- Bearing
Correct 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).
Question 4: What is a 'cocked hat' in chartwork?
- A type of navigational buoy
- The triangle formed when three position lines do not intersect at a single point (Correct answer)
- A weather pattern
- A type of anchor
Correct 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.
Question 5: How is distance typically measured on a nautical chart?
- Using the longitude scale
- Using the latitude scale at the side of the chart, where one minute of latitude equals one nautical mile (Correct answer)
- Using a separate ruler
- Using the compass rose
Correct 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.
Question 6: What does a chart datum of 'LAT' mean?
- Latitude reference point
- Lowest Astronomical Tide — the lowest level the tide is predicted to reach under normal conditions (Correct answer)
- Local Authority Territory
- Latest Arrival Time
Correct 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.
RYA Day Skipper Navigation and Chartwork
On an Admiralty chart, what do the letters 'M' and 'S' indicate when describing the seabed?