The Scale of a Chart
Since there is a chart for every ocean, every coastline and every port in the world, it follows that there must be charts of differing scales.
On an ocean chart, for example, there would be little need for great detail since, for the most part, the area covers vast open spaces. On an inshore chart, by contrast, fine detail of every meter of the coastline is required so that vessels can navigate safely through the many shoals and hazards.
There are three major scales for naval charts, and each covers a specific area of navigation:
(1) Small-scale charts cover large stretches of ocean and carry only limited detail of the coastline.
(2) Medium-scale charts carry sufficient detail along a stretch of coastline to enable a ship to be successfully navigated offshore.
(3) Large-scale charts carry fine detail of virtually every metre of estuaries, ports and rivers, enabling ships unfamiliar with the area to make a safe entry.
The would-be navigator setting off on a passage can select from the chart catalogue the charts required to cover all aspects of navigation and safety en route without cluttering up the ship with unnecessary folios.
If making an ocean crossing, for example, the navigator will select the small-scale chart to cover the ocean area, two or three medium-scale charts to cover landfall and any coastal passages involved, and large-scale charts of departure and arrival ports or any other places that may be visited, either for interest or for shelter.
A gradual conversion to metric charts is taking place in many parts of the world. It is now possible to purchase both fathom and metric charts, sometimes of the same area, often of the same coastline. Needless to say it is vitally important to the navigator to know which chart is being used. This information is carried in the chart title and should be checked when the chart is purchased.
The Chart Title
Every chart carries a title with a wealth of information of use to the navigator. Apart from the obvious description of the area covered by the chart, other factors relating to the scale, the soundings, tidal anomalies, military areas, special areas, specific dangers and any other factors which will assist in safe navigation are listed in the chart title.
Latitude and Longitude
Mercator charts are covered by a ‘grid’ pattern indicating lines of latitude and longitude which match those on the Earth’s globe. The lines which run vertically up and down the chart are meridians of longitude, and those which run horizontally across it are parallels of latitude.
The scale across the top and bottom of the chart is the longitude scale and is marked with degrees and minutes of are (60 minutes = 1 degree). Longitude commences at 00° on the Greenwich meridian (London), and runs east and west 180° to the opposite side of the world. Thus the graduations on the top and bottom of the chart will be in degrees and minutes of longitude east or west of Greenwich and marked accordingly.
On either side of the chart are the graduations for the latitude scale.
Latitude commences on the equator and is measured north and south of the poles. The graduations are the same as longitude, but the two are dissimilar in scale and must never be confused. Latitude at the equator is 00° and runs only 90° to the north and south poles.
Distances at sea are always measured in nautical miles, and thus the distance scale is the same on every chart regardless of whether it is graduated in metres or fathoms. Any distance scale which may be incorporated in the title–or anywhere on the chart for that matter–can be ignored, since it is related to land miles or kilometres. The latitude scale on either side of the chart (never the longitude scale at top or bottom) is used to measure distance, thus:
One minute of latitude = one nautical mile.
The depth of water indicated by figures all across the seaward area of the chart may be in fathoms or metres (according to the chart title). Close inshore these depths may be represented in smaller denominations, either as fathoms and feet or as decimals of a metre. This is probably the most confusing and dangerous aspect in the conversion to metric charts and the navigator must be aware of it.
The depths are indicated by a normal-sized sounding figure with a smaller figure below and to the right of it. They are read off thus:
Fathoms: 52 (5 fathoms 2 feet = 32 feet) [1 fathom = 6 feet]
Metres: 52 (5.2 metres = 17 feet approx.)
The danger of confusing the two systems is obvious from this example. The marked depths, known traditionally as soundings, are reduced to the lowest mean low tides experienced in the area. Thus it is safe to say that with only rare exceptions, the soundings on the chart indicate the least water over the sea bottom that will be experienced in a normal tidal cycle. Or, to put it more practically, there will rarely be less water than that indicated by the soundings.
This is the term given to the level of soundings on the chart. It is the basis on which all tide tables are calculated. Thus, to find the depth of water over the sea bottom at any time, the height of the tide, from the tide tables, is added to the sounding on the chart.
Modern charts have a system of colour shading to give greater emphasis to certain depths. This is particularly the case with inshore soundings. Shallow waters are usually indicated by deep blue colouring (green if they are dry at low tide), which fades out to paler blue as the water becomes deeper. The land is often shaded yellow or some similar contrasting colour and certain danger areas, or areas of military or commercial use, may show pale mauve shading.
Navigation lights, which are the navigator’s signposts, as it were, are indicated by a red ‘flash’. While there are other shore lights which can be used for navigation, those indicated by the red flash are the only completely reliable navigation beacons.
Although mountains are not used widely for navigation unless they are very prominent or have some particularly obvious characteristic, their presence may be indicated by the usual contour lines used on normal survey maps.
Offshore, the undulations of the seabed are indicated by contour lines, often similar to those of mountains. Another system, however, uses a coded contour line to indicate the variations in depth. The most common indication is a continuous contour line forming the line of soundings, into which the figure of the depth is inserted at intervals.
Towns, Villages etc.
Since streets, houses, shops and the general make-up of towns and villages are of no interest to the navigator, they are rarely marked on the chart except in basic form. This is particularly the case when they are inshore from the coastline. However, any prominent object which can be seen from seaward and used for navigation is pinpointed and marked very distinctly.
Typical of the sort of objects which fall into this category are church steeples, factory chimneys, water towers and radio towers. Indeed, any object anywhere which can be seen clearly from seaward will be marked on the chart.
Details on a Chart
One of the biggest problems in constructing a chart is to include all the detail necessary to enable it to be used for safe navigation. The motorist has road signs all along the route to supplement the road map. But there are no signs to assist the ship navigator and all the information needed must be carried on the chart. To avoid completely cluttering the chart, the wealth of detail relating to both sea and coastline is abbreviated or indicated by symbols.
Abbreviations used on a chart are usually in the form of a single capital letter. The letter M, for example, when printed on a stretch of open water, indicates the nature of the sea bottom–mud. If one letter is not sufficient, then two or perhaps three may be used, thus: Tr-tower; Whf-wharf; Bn-beacon. While abbreviations are often fairly obvious, the same cannot always be said of symbols. A dangerous rock, for example, is illustrated by a small cross, a lighthouse by a star. These symbols are all listed in detail in publication ‘Chart 5011’ (Symbols and Abbreviations Used on Admiralty Charts), but since there is always the possibility that a copy of this may not be kept on board, the wise navigator, will have committed to memory the more common symbols so that they can be quickly identified on the chart.
Radar is a major navigation tool and has made life much easier for the ship navigator. Electronic navigation has come into its own in the years since World War II and charts have been kept in line with this development. One of the weaknesses of radar is its tendency to show up different types of objects with different emphasis on the screen.
Thus a high cliff will show up clearly while a low scrubby shoreline may not show up at all.
Charts have been altered to help overcome this problem by emphasising areas which are ‘radar prominent’. These areas of coastline are printed more heavily on the chart and thus stand out against the rest of the coastline, much as they would when viewed on the radar screen.
Outside the normal symbols and abbreviations there may be need to convey some special information about an object marked on the chart. In this case a more expansive abbreviation than a single letter may be used. Most of these are self-evident.
An object which stands out well from its surrounding environment, for example, will be marked with the abbreviation (conspic). An area in which a light cannot be seen will be zoned and marked (obscd) indicating that the light is obscured in this zone. Similar abbreviations may be used for a whole variety of objects where there is some special navigational feature about them.
The Compass Rose
Since most navigation on a chart involves the use of the compass, a reproduction of a compass card is printed at strategic points across the face of every chart. These are termed compass roses and their positioning is such that there is always one close to hand no matter where on the chart the navigator is working. They are graduated in three-figure notations from 00° to 360° and contain details of the variations in force in that area.
Correcting the Chart
Like all maps, charts can quickly become outdated by changes to the coastline or to navigational objects. Removal of a prominent shore object such as a radio tower or chimney, for example, or changes in the light shown from a lighthouse due to maintenance are but two factors which could cause confusion to the offshore navigator.
Such changes are published weekly in a sort of news sheet produced by the navy and called Notices to Mariners. From the details in these notices, the navigator can correct the chart and keep it up to date with events which may affect navigation. Permanent changes are made on the chart in purple ink, temporary changes in pencil.
Whenever a change is made on the chart, the number of Notices to Mariners is entered in the margin at the bottom of the chart. In this way, a glance at the listed numbers will indicate the extent to which the chart has been corrected.
Also known as ‘pilots’, these are publications put out either by the navy or the local authorities. While they are not a part of the chart, they are directly associated with it in that they supplement the information given on the chart. As the navigator progresses along a coastline, the chart gives a ‘moving picture’, as it were, of the area. Sailing Directions can then be thought of as the soundtrack to that ‘movie’.
This publication goes into all the finer points of the coastal scene in detail which cannot be shown on the chart due to space limitations. It is therefore vital that an up-to-date issue of Sailing Directions is carried on any coastal passage. Corrections and changes to Sailing Directions are also found in Notices to Mariners.
These are double-edged rules which are used to transfer courses and bearings across the chart. They are essential for navigation and can be obtained in two principal forms–sliding rules (sometimes known as ‘clackety-clacks’) which shuffle across the surface of the chart, or roller rules which are, as their name denotes, fitted with small rollers to enable them to be rolled parallel across the chart.
By placing one edge of the parallel rules against a line or a compass bearing, the line can be transferred to any part of the chart simply by moving the rules gently across the surface. Patented types of rules for laying off courses are also available and these can be quite useful if one becomes adjusted to using them.
These are used for measuring distances, as described earlier, and are as essential as the parallel rules. The ‘one-handed’ variety, are the easiest to use and make a good span to cover long jumps along the course line.
Electronic chart plotters offer a full navigation system interfaced with GPS to give the ship’s position on a chart projected electronically onto a computer screen. Chart folios are stored, usually in CD-ROM format, so that the user can zoom in, typically from a Continental or global scale to an individual berth.
Electronic chart plotters
The electronic chart plotter started life as a computer screen depicting a caricature of a paper chart. Now the device has truly come of age, offering full-quality chart coverage with a zoom facility to take the user from global scales down to the local quay. In some cases, fine-tuning tools can make the view even clearer, such as soundings that can be ‘layered’ on and off the display.
An inbuilt computer either interfaces with GPS or contains its own GPS facility, so that the ship’s position is shown on the chart. Screens are now available which can switch from chart to radar, and some can even layer one image onto the other. It goes without saying that with such a system, inputting a waypoint is extremely simple, sometimes requiring no more than positioning a cursor on the spot and clicking the computer mouse. Waypoints can generally be linked as routes by dragging the cursor from one point on the planned passage to the next, clicking them in as you go. A line is left joining one waypoint with the next, so you can see at a glance whether the chosen route is safely clear of dangers.