Question 1

On a chart, a depth is shown as 5 underlined. What does this mean?

Accepted Answer

A drying height of 5 metres above chart datum, exposed at low water. UNDERLINED FIGURES on a chart = DRYING HEIGHTS, the height of the feature above chart datum (LAT). The area dries (is above water) at low tide. It is covered when the height of tide exceeds 5m. When planning a passage, check: is the height of tide greater than the drying height? If yes → covered. If no → drying/exposed. Always check whether the tide is rising or falling when near drying areas.

Question 2

A lighthouse is shown on a chart with the notation 'Fl(2)WR.10s20m15M'. What does '20m' and '15M' mean?

Accepted Answer

20m is the light's height above MHWS in metres; 15M is its nominal range in nautical miles.. Fl(2) means a group of two flashes; WR indicates white and red sectors; 10s is the period of the complete cycle. The figure 20m is the light's elevation in metres above the chart's stated height datum, commonly MHWS on British tidal charts. The figure 15M is its nominal range in nautical miles, meaning its luminous range in a meteorological visibility of 10 nautical miles. Nominal range depends on luminous intensity but is not itself a measure of power. Geographical range depends on both the light's elevation and the observer's height of eye. A nominal range shorter than the geographical range does not imply inadequate power.

Question 3

What are depth contours on a chart and how are they useful?

Accepted Answer

Lines joining points of equal depth below chart datum, helping identify shallow areas and safe water, and supporting contour navigation.. DEPTH CONTOURS: lines joining points of equal depth below chart datum. Common contours include 5 m, 10 m, 20 m, 50 m and 100 m, depending on chart scale and coverage. Uses: 1. AVOID SHALLOW AREAS: contours help identify potentially safe water, but remaining seaward of a 10 m or 20 m contour does not guarantee adequate clearance. Consider draught, tidal height, charted hazards and required under-keel clearance. 2. CONTOUR NAVIGATION: echo sounder readings can help follow a contour or support an approximate position assessment. Account for the sounder's reference point and tidal height when comparing readings with charted depths. Depth information alone does not establish a reliable position and should be checked against other navigational information. 3. IDENTIFY FEATURES: contours help reveal shoals, banks, channels and deep-water approaches. They are depicted as lines labelled with depth values.

Chart, questions and explanations

On a chart, a depth is shown as 5 underlined. What does this mean?

A lighthouse is shown on a chart with the notation 'Fl(2)WR.10s20m15M'. What does '20m' and '15M' mean?

What are depth contours on a chart and how are they useful?

Practise this properly

Chart, questions and explanations

Questions

On a chart, a depth is shown as 5 underlined. What does this mean?

A lighthouse is shown on a chart with the notation 'Fl(2)WR.10s20m15M'. What does '20m' and '15M' mean?

What are depth contours on a chart and how are they useful?

Practise this properly