Practical chart work

Understanding Nautical Charts

Understanding Nautical Charts 1

A position, no matter whether it is given in latitude and longitude, expressed as a range and bearing from a landmark/object or as ten paces towards the palm tree due East of Black Jakes skeleton, is of very little value to us unless it can be laid off or applied as a visible representation to a map or chart
Just as true as X marks the spot for Black Jakes treasure X marks the spot for our position on a nautical chart or map

A nautical chart is similar to a map however where as a map is a physical representation of a portion of the earth’s surface a chart has additional functionality in that it is an aid to navigation both by sea and by air  A chart provides valuable information to the navigator allowing him or her to avoid hazards and dangers anticipate difficulties and develop a safe passage plan  It acts as a work sheet enabling the navigator to plan, execute, calculate progress and continuously monitor the passage It provides a temporary record of the passage and subsequent events, changes of course, speed and distance made good etc.

As charts are the single, most important tool we will be using I believe that it’s important that you become comfortable handling and reading charts as soon as possible In fact  let’s take a quick look  right now! So open up practice chart N05041 and let us take a look

NOTE: A chart is always printed so it is read North up So if you look at the chart and the text and symbols are the correct way up North is at the top and South at the bottom East is to the right and West to the left

Nautical Chart example The first thing we notice is the chart is in colour This indicates that the chart is metric and that all depths and heights are in meters The older and now very less common fathom charts were all white with black line contours, text and graphics, the depths and heights were always given in fathoms and feet on these older charts

What the colours mean

Yellow Colour indicates the position of Dry Land Populated areas such as  towns or villages are labelled with their common names such as SOUTHEND ON SEA  as you can see in the left bottom quarter of the chart These populated areas shown as darker shades of yellow You will also see that some topographical and important geographical information are included, in particular conspicuous features such as churches, airports, towers ,and high ground which can be useful in determining our position

Green Colour indicates the area of the shore line that dries out at low tide between the highest and lowest tide These areas also contain information on the nature and appearance of the coast marshes, sand dunes, seabirds, obstructions etc In the green areas you will see numbers that have been underlined These are heights that the particular area will be above the water line at the lowest tide and known as drying heights The green areas are the only areas you will see these underlined numbers 21

Contour lines are lines that join points of equal depth The distance separating the contours combined with the corresponding colours gives a very good interpretation of what the sea bed is like

 Tight and narrow contours indicate a steeply shelving reef, bank or trough Widely spaced contours indicate a more gentle undulating nature to the sea bed

Blue Colours indicate areas of the sea or ocean which is above a certain depth contour sometimes still called a fathom mark by old salts  If we look at the top of Gun-fleet Shoals in the centre of the chart in the top quarter we can see that the green area that occasionally dries is marked by the contour 0, this is known as the drying contour

Outside of this there are three more contours each representing a different depth and filled with different shades of blue The black line of the second contour here shows a small figure 2 on the contour line that indicates that every depth in the darker blue area is 2 meter or less
The lighter mid  blue area has a contour line which is marked with a small figure 5 which indicates that all depths  in the mid blue area between the two contours is 5 metres or less As we follow the shoals up to the top we can see that the lightest blue area has a contour marked with a small figure 10 which indicates that everywhere in the light blue area is 10 metres or less On this particular chart the contour are 2, 5 and 10 however they will vary on different scale charts Depths within these contour lines are in meters shown as a number and a decimal of ie 15 - 46 - 92

White Areas: indicates areas of sea that are where the depths are greater than those shown in any contour line Depths within these contour lines are in meters shown as a number and a decimal of 201

Chart Datum: Is a datum point, base line, or benchmark if you like from which all depths of the sea bed are measured and from which all drying heights of land or objects that are regularly covered and uncovered by sea are measured
Chart datum is approximately the level of lowest astronomical tide Or in other words the lowest tidal level we are ever likely to experience This to our advantage as it means that there is usually more water under us than we think It is possible for the water level to drop below Chart Datum on very rare occasions when a high barometric pressure can suppress the tide

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