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Projections

The nautical chart is a 2-dimensional representation of a 3-dimensional world. And although this results in various distortions, as long as two requirements are met we can use this image for navigational purposes.
  1. The angles between three objects in the chart should be the same as the angles between the real objects which they represent.
  2. A straight course should appear as a straight line in the chart.
To fulfil these demands a nautical chart requires parallels and meridians that are both straight and parallel. Moreover, the meridians will need to be perpendicular to the parallels.
A well known method to create such a chart is called the Mercator projection after Gerard “Mercator” Kremer, a Flemish scholar who studied in 's Hertogenbosch (the Netherlands) and Leuven (now Belgium) and who invented his famous projection in 1569.

The Mercator chart was designed for sailors and can be constructed by wrapping a cylinder around the planet so that it touches the equator. On this cylinder the surface of the earth is projected and finally the cylinder is cut open to yield the 2-dimensional chart.
But where the meridians converge on the globe they run parallel in the projection (see chart below), indicating the distortion. Look, for example, at a high parallel. The length of such a parallel on the globe is much smaller than the equator. Yet, on the chart they have exactly the same length creating a distortion which gets bigger nearer to the poles. The figure below shows the construction of the Mercator projection. From this it is clear that only the vertical scales should be used for measuring distances.
Mercator Mercator deformation and distortion
Click on the world map on the right to see the distortions of a Mercator projection. Each navy coloured circle/ellipse has a radius of 500 km.
Vertical scale of the nautical chart The vertical scale depicted on the right demonstrates the distortion. The two little navy coloured markers have precisely the same size, the upper one measures only 0.64 degrees (= 38.4 nm) while the other measures 1.00 degrees (= 60 nm). So, distances (in degrees or in miles and minutes) should not only be read on the vertical scale, but also at approximately the same height.

The horizontal scale is only valid for one latitude in the chart and can therefore only be used for the coordinates (a point, but not a line). If you divide the surface of the earth in eight pieces, and lift one out and project it, you end up with the figure below. The result is that both A-A' and B-B' are now as long as the bottom of the chart and are “too long”.
But there are of course other projections in use by sailors. An important one is the Stereographic projection, which is constructed by projecting on a flat plane instead of a cylinder. On this chart parallels appear as slightly curved and also the meridians converge at high latitudes. So, strictly speaking, a straight course will not appear as a straight line in the chart, but the parallels remain perpendicular to the meridians. Most often, distortions are scarcely noticed when this projection is used to chart a small area. Like the Mercator projection, the vertical scale represents a meridian and should be used for measuring distances.

Another projection is the Gnomeric projection on which the meridians are again converging. But most importantly, the parallels are arcs of a circle while great circles appear as straight lines. On a sphere the shortest route between A and B is not a straight line but an arc (part of a great circle). Though this is also true when you – for example – cross a little bay, we use for simplification a loxodrome (a handy straight line on your Mercator chart which does not reflect your shortest route). On a Gnomeric chart this same loxodrome is an arc, while your shortest route (a great circle) ends up as a straight line. Hence, the gnomeric projection is particularly useful when sailing great circles (like when you dabble in circumnavigation) and is beyond the scope of a coastal navigation course.

Organization of the chart

Information in the chart

Coordinates and positions

A pair of nautical dividers (single handed dividers) is used to obtain precise coordinates from the chart. This device enables you to take the distance between that particular position and the closest grid line. You then place the dividers on the scale with one end on this grid line, leaving the other end precisely at your coordinate. Do this twice to get both latitude and longitude at the scale on the edge of the chart.
Below are some examples.
Danger mark Danger mark
32° 06,3' N  ,  25° 07,3' E
Sailing schools in Athens Navigation chart, coordinates
Fish farm - Marine farm Fish farm
32° 04,4' N  ,  24° 54,7' E
Anchorage Anchorage
31° 46,0' N  ,  25° 04,0' E
Church Church
31° 48,4' N  ,  25° 25,0' E
Windmill Windmill
32° 01,0' N  ,  24° 57,8' E
Castle Castle
32° 14,2' N  ,  25° 29,6' E
Water tower Water tower
31° 54,9' N  ,  24° 54,8' E
Radio mast Radio mast
31° 54,8' N  ,  25° 10,0' E
Beacon green Beacon green
31° 52,0' N  ,  24° 44,3' E
Plotting a position in the chart is done by reversing this method.

Some chart symbols come with a little line and circle precise location of chart symbol indicating the precise location, like the “Radio mast”, otherwise the center of the symbol is the precise location.

Another possible notation of 33° 28,5' E is 33° 28′ 30" E, which however doesn't easily allow for more precision like 33° 28,500' E does. Also note that in most countries a comma - and not a dot - is used as the decimal separator. So instead of 33° 28.500' E, the consensus notation for mariners is 33° 28,500' E.

Distances

Chart and Distances To measure the distance between, for instance, these two oil rigs, we will again need our dividers. Remember, we can only use the vertical scale.

We first take a convenient distance like 10' (10 nautical miles) on the vertical scale using the middle latitude. Then we start walking with the dividers from the southern oil rig to northern one. Finally, we adjust the dividers to measure the small remaining part at its own height, i.e. its own latitude.
The image shows that the total distance is 37 nautical miles.

Courses

   © sailingissues.com   
So, now we can measure distances and both plot and read out positions, but we also need directions. For example we need to find the course from safe-water buoy A to safe-water buoy B. To accomplish this we may use parallel rules as shown in this chart below:
Nautical course - parallel rulers

First you line this instrument up with the two buoys. Then follows the intriguing part in moving the device to the compass rose without losing its alignment. Finally, when one of the rules is aligned with the heart of the compass card, you can read course AB. In this example: 170°.
Besides the parallel rules there are other types of instruments available, notably the Breton plotter - also known as a Portland Course Plotter - which features an adjustable rose.

Selection of chart symbols

Danger line Danger line in general
Wreck, least depth unknown. Caution : on many charts, this symbol is used for wrecks of unknown least depth, but considered to be covered by more than 20 meters of water. The wrecks thus represented are then potentially dangerous to vessels with a draught greater than 20 meters. Note : this symbol is also used for all wrecks in water over 200 metres deep. Wreck, least depth unknown but usually deeper than 20 metres
Visible wreck Visible wreck
Wreck of which the mast(s) only are visible at Chart Datum Wreck of which the mast(s) only are visible at Chart Datum
Wreck; Obstruction: least depth known obtained by sounding only       Obstruction, least depth known obtained by sounding only Wreck, least depth known obtained by sounding only
Wreck; Obstruction: least depth known, swept by wire drag or diver       Obstruction, least depth known, swept by wire drag or diver Wreck, least depth known, swept by wire drag or diver
Rock which covers and uncovers, height above Chart Datum   or   Rock which covers and uncovers, height above Chart Datum Rock which covers and uncovers, height above Chart Datum
Rock awash at the level of Chart Datum Rock awash at the level of Chart Datum
Underwater rock of unknown depth, dangerous to surface navigation Underwater rock of unknown depth, dangerous to surface navigation
Underwater rock of known depth, dangerous to surface navigation   or   Underwater rock of known depth, dangerous to surface navigation Underwater rock of known depth, dangerous to surface navigation
Remains of a wreck, or other foul area, non-dangerous to navigation but to be avoided by vessels anchoring, trawling etc. Remains of a wreck, or other foul area, non-dangerous to navigation but to be avoided by vessels anchoring, trawling etc.
Depth unknown, but considered to have a safe clearance to the depth shown Depth unknown, but considered to have a safe clearance to the depth shown
Sounding of doubtful depth     Existence doubtful     Reported, but not confirmed Sounding of doubtful depth; Existence doubtful; Reported, but not confirmed
Position approximate     Position doubtful Position approximate; Position doubtful
Wind turbines   or   Wind turbine Wind turbine
Chimney Chimney
Tower       Radio tower Tower; radio/television tower
Monument Monument
Marina Marina - boat harbour
Mosque, minaret Mosque, minaret
Silo Silo
Tanks Tanks
PlaceholdersCupola, Church, Hotel, Chimney Placeholder examples: Church (Ch)   Tower (Tr)   Hotel   Cupola (Cu)   Chimney (Chy).
CAPITALS indicate that the landmark is conspicious.
Quarrie or mine Quarrie, mine
Major light, minor light Major light; minor light
More on lights in chapter 9
Limit of safety zone around offshore installation Limit of safety zone around offshore installation
Position of tabulated tidal stream data with designation 'A'     Position of tabulated tidal levels data with designation 'a' Position of tabulated tidal stream data with designation “A”;   Tidal levels data “a”
Green or black buoys (symbols filled black) Green or black buoys (symbols filled black): G = Green ; B = Black
Single coloured buoys other than green and black Single coloured buoys other than green and black: Y = Yellow ; R = Red
Multiple colours in horizontal bands, the colour sequence is from top to bottom Multiple colours in horizontal bands, the colour sequence is from top to bottom
Multiple colours in vertical or diagonal stripes, the darker colour is given first. Multiple colours in vertical or diagonal stripes, the darker colour is given first. W = White
More on buoys in chapter 9
Lighted marks on multicoloured charts, GPS displays and chart plotters. Lighted marks on multicoloured charts, GPS displays and chart plotters. A yellow coloured lobe indicates a White light! Also note that beacons (here the rightmost symbol with the green light) has an upright G, instead of an oblique G
 

 

Wrecks

 

Glossary

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2 February 2014
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