Navigation and Chartwork Flashcards

When plotting a position fix using two position lines on a nautical chart, the most reliable fix is obtained when the angle of intersection between the position lines is closest to which value?

Answer: 90 degrees

A 90-degree angle of intersection between two position lines produces the most reliable fix because it minimises the area of uncertainty. As the angle decreases from 90 degrees, the 'cocked hat' or area of possible position increases, reducing accuracy.

A vessel's GPS shows a position of 51°30.0'N 001°20.0'W. The chart datum is WGS84. What correction, if any, should be applied to plot this position on the chart?

Answer: No correction is needed as both use the same datum

When the GPS datum (WGS84) matches the chart datum (WGS84), no correction is required. Corrections are only necessary when the chart uses a different horizontal datum, such as OSGB36 or European 1950, in which case the datum shift noted on the chart must be applied.

A vessel is steering 045°(T) at 12 knots in a current setting 135°(T) at 3 knots. What is the approximate course made good?

Answer: 055°(T)

The current is setting roughly at right angles to the vessel's heading from the starboard quarter. Using the triangle of velocities, the current pushes the vessel to starboard of its heading, resulting in a course made good of approximately 055°(T). The exact value depends on vector resolution.

Which publication issued by the UKHO provides information on tidal streams for UK waters, including rates and directions at hourly intervals relative to High Water at a standard port?

Answer: Admiralty Tidal Stream Atlas

The Admiralty Tidal Stream Atlas provides chartlets showing tidal stream direction and rate for each hour before and after High Water at the relevant standard port. These are essential for passage planning in UK waters where tidal streams can be significant.

On an ECDIS, what does the term 'safety contour' refer to?

Answer: The depth contour selected by the mariner below which the seabed is highlighted as dangerous

The safety contour on an ECDIS is the depth contour chosen by the navigator to define the boundary between safe and unsafe water for that particular vessel. Areas shallower than the safety contour are highlighted, typically in a darker blue, to alert the navigator.

When using radar to obtain a position fix, a combination of range and bearing to a single charted object provides a fix. Why is a range generally considered more accurate than a bearing from radar?

Answer: The radar beam width makes bearing discrimination less precise than range measurement

Radar determines range by precisely measuring the time delay of the returned pulse, which is inherently accurate. Bearings, however, are affected by the horizontal beam width of the radar antenna (typically 1-2 degrees), which spreads the echo and makes it difficult to determine the exact bearing to a target's centre.