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Basic Chart Work.
Is electronics enough....
The new ship here is fitted according to the reported increase of knowledge among mankind. Namely, she is cumbered end to end, with bells and trumpets and clock and wires, and it has been told to me, can call voices out of the air of the waters to control the ship while her crew sleep. But sleep thou lightly. It has not yet been told to me that the Sea has ceased to be the Sea.
From 'Sea to Sea and Other Sketches' by Rudyard Kipling. |  |
A chart is a map of an area of ocean/coast with details to assist the mariner navigate safely, and to determine his position.
Chart Details.
A chart is given a name and number. e.g. Bustard Head to North Reef, AUS 819. Be familiar with the ranges of latitude and longitude on this chart. Identify the main grid lines. Have a concept of what one nautical mile represents. When buying a chart, make sure it's an up to date version.
Scale. Charts are drawn to a range of scales depending on the area and degree of detail to be represented. e.g. AUS 819 Bustard Head to North Reef, 1:150000 scale. (medium scale). Whitsunday Passage AUS 5169a 1:100000 scale. Approaches to Port of Gladstone AUS 242 1:37500. (large scale). Be familiar with reading the distance scales on various charts. It is important to note the physical difference in scale sizes on small, medium, and large scale charts. e.g. minor graduations may be in 0.5, 0.2, or 0.1 minute of angle (nautical mile).
See also | Chart Scales | | Scales.|
Units of Measurement. All soundings and heights will be in metres/tenths on up to date charts. It is important to check this, as older charts may be in fathoms/feet. Understand the difference in notation between the two.
Special notes and cautions. Special notes regarding buoys and beacons for a particular port, or areas of restricted navigation.
Reliability diagram. Details an overview of the type, areas, and accuracy of depth surveys.
Chart Tidal Datum. This reference point is the lowest astronomical tide recorded over a long period, and is noted in the Official Tide Tables as LAT or the lowest astronomical tide. The complement is HAT or highest astronomical tide. Mean tide levels relative to chart datum will be detailed.
N.B. Soundings given on the chart are the depth of water below the tidal datum, and all tide heights are above the tidal datum.
Coastal Lights.Details will be provided of the light characteristics, height above MHWS tide level, viewing distance, and sector angle. Height of coastal lights is noted in lower case m. Effective viewing distance is noted in upper case M. and is a nominal range at 5 m above sea level and dependent on light intensity and good visibility. Sectored lights are described as the arc as seen from seaward.
Channel Entrance Leads.Details of channel markers and approach courses for port entry leads. Note that these are true bearings unless otherwise noted.
Contours are lines on the chart joining points of similar depth, and are useful to navigators in avoiding or seeking coastal currents, estimating distance offshore, or following a channel along a coastline.
Soundings give depths below chart tidal datum, and indicate the least depth to be found at any state of tide.
Drying heights are above chart tidal datum, and are underlined.
Heights of land based objects are given as above MHWS tidal level.
Tidal flows. Indicators are given for flood and ebb tide directions in open water by directional arrows. Offshore ocean currents are denoted by a wavy arrow. Some charts have detailed tables of tidal flows e.g. AUS 833.
Compass rose. One or more depending on the chart scale, detailing magnetic variation.
Compass readings should always be given in three figure notation clockwise from 0 deg. North. Note the difference in details of magnetic variation notes on more recent charts to older charts. Note also that variation has been decreasing in recent years in the East Australian area.
Satellite Derived Positions. GPS positions are standardised to World Geodetic Datun 1984 (WGD 1984). Corrections will be noted on the chart if the chart is to the Aust. Geodetic Datum 1966. Differences between a GPS derived position and an accurate visual fix on a chart to the AGD 1966 datum could be in the order of 200 metres in Australian waters. GPS positions can be transferred directly to the chart if it is to the World Geodetic Datum 1984 or AGD 1996. Refer to the note on the Gladstone Boating Safety Chart. For more info on Geodetic Datums, see | Chart Datums | or | Datums and Coordinates. |
Corrections. Notices are issued fortnightly as required to major users or can be obtained at chart agents or on the internet, and brief details should be attached at the lower left corner of the chart by the responsible person.
See also | Australian Hydrographic Service |
To determine a position on a chart the mariner needs reference coordinates. For purposes of navigation, the Earth sphere is divided into 'vertical' divisions by meridians of longitude, which are imaginary circles running from true north pole to true south pole, and into 'horizontal' divisions by equally spaced parallels of latitude, which are imaginary circles running parallel to the equator, north and south, and perpendicular to the earth's polar axis. All meridians and the Equator are known as Great Circles. All parallels other than the Equator are known as small circles.
The basic unit of circular measurement is the degree of angle. 0 deg. longitude is called the Prime meridian, and is aligned with Greenwich in the UK, and 0 deg. latitude is at any point on the Equator. Positions of longitude are expressed as either deg. East or deg. West (of Greenwich). Positions of latitude are expressed as either deg. North or deg. South (of the Equator). Consecutive meridians (0 - 180) or parallels (0 - 90) on the chart represent one degree of angular measurement, symbol °. Meridians are numbered 0 - 180 E and 0 - 180 W, and parallels are numbered 0 - 90 N and 0 - 90 S. Each degree is further divided equally into 60 parts called minutes, symbol ‘, and for normal chart navigation, each minute is subdivided to two decimal places. The Earth surface is therefore effectively divided into a grid by which any point can be allocated coordinates. e.g. VMRG base is 23 deg.50.265' S and 151deg.15.137' E. to WGS 84. Check your GPS with the standard marker at the boat ramp at VMRG base. | Lat.& Long. Diag. | This is two dimensional or geographic position plotting. Another common system uses Polar Coordinates as in a radar screen, where a position is plotted relative to its distance from a central 'pole' and at an angle from a 0 deg. polar axis.
Navigation charts in general use are based on variations of the Mercator Projection, by which the spheroid surface of the Earth is developed on a cylindrical surface and subsequently on plane surfaces. On this projection, all meridians are parallel instead of converging at the poles, and parallels are farther apart towards the poles. Because of this, in latitudes greater than 60 deg., the charts suffer from noticeable distortion, and true distance is not to scale. Other types of chart projections are used in the extreme and polar latitudes, e.g. Gnomonic projection. See also | History of Cartography. | Derivation of the Projection. | Transverse Mercator Projection. |
It is important to understand that latitude is a north - south angular notation, measured along meridians, ( great circles), and that the vertical edge scale of a Mercator chart represents angular measurement of latitude in degrees and minutes. It is also important to remember that one minute of latitude represents one nautical mile on charts, i.e. one nautical mile on any great circle.
Longitude is an east - west notation measured along parallels ( small circles). The horizontal edge scale of a Mercator chart represents angular measurement of longitude in degrees and minutes, but one degree of longitude does not represent one nautical mile ( except along the equator). This is because a Mercator chart is distorted in order to make meridians appear parallel, instead of converging at the poles.
The longitude ( horizontal ) scale should never be used to measure distance.
One minute angular measurement of latitude = one nautical mile or 1.852 kilometres.
Note that this is the International Nautical Mile, not to be confused with the British Standard Nautical Mile which is less.
A speed of one nautical mile per hour = one knot.
One cable's length is one tenth of a nautical mile. The format is Degree, minute, min/10 (cable).
Note that the angular subdivision of seconds is no longer used.
Speed = Distance / Time. Knots = Nautical Miles / Time.
From this angular data, other interesting facts may be calculated.
1 day = 24 x 60 x 60 = 86400 sec.
Time for Earth to rotate through 1 min. of arc = 86400 / 360 / 60 = 4 secs.
and through 1 deg. of arc = 4 minutes. ( 60 x 4 = 240 secs.)
and through 15 deg. of arc = 1 hour. ( 360 / 24 = 15 )
Equatorial surface speed of Earth = 1 nm / 4 secs = 1.852 / 4 x 60 x 60 = 1666.8 km / hr.
Equatorial circumference of Earth = 1666.8 x 24 = 40003 km. or 360 x 60 = 21600 nautical miles.
Also, from the above, the displayed accuracy of a GPS unit will be 185.2 metre to one dec. place.
18.52 metre to two dec. place, and 1.852 metre to three dec. place.
Plotting a Position. On chart AUS 819, plot these positions. 23 deg. 38.6' S by 151 deg. 37.4' E.
23 deg. 12.9’ S by 151 deg. 51.2’ E. 23 deg. 58.5’ S by 152 deg. 10.6’ E. These would be typically given by a vessel in distress or from one's own GPS. Note that the grid lines on this chart are at 15 minute intervals. First identify the general positions on the chart.
Plotting Latitude. Take the drawing compass and on the latitude scale (vertical), place the point on the nearest numbered grid division to that required, i.e. 30', and set to 38.6'. Place the compass on the intersection of 23 deg. 30' S and 151 deg. 30' E and lightly mark the position of 23 deg 38.6' S. Using the square protractor to maintain accuracy, mark this latitude in the general alignment with 151 deg. 37.4' E.
Plotting Longitude. Take the drawing compass and on the longitude scale, place the point on the nearest numbered grid division to that required, i.e. 30', and set the to 37.4'. Place the compass on the intersection of 23 deg. 30' S and 151 deg. 30' E and lightly mark the position of 151 deg. 37.4' S. Using the square protractor to maintain accuracy, extend this longitude to intersect with the latitude line previously marked.
The latitude scale is an expanding scale, and it is advisable to always measure distance on a part of the scale adjacent to the work area to maintain accuracy.
Plotting a Simple Course to steer. (allowing for compass error only)
Plot a course to steer from the first example above, to the entrance to Fitzroy Reef lagoon.
By convention the start point is called the origin, and the finish point is called the destination. The intended track or rhumb line would be a line joining these two points and would be the shortest distance.
1. Place the parallel rule across the two points and step the track to the nearest compass rose on the chart.
2. Accurately note the reading as E (or W) of 0 deg. as applicable. This is the true bearing.
3. Apply compass variation as noted on the compass rose. This is the magnetic bearing.
4. Apply deviation for your vessel for that magnetic course. This is the compass course. i.e. the course to steer.
Draw a line between the points as the course to steer line, marking the direction with one arrow head and the compass course e.g. 053 C.
Repeat the exercise using the square protractor instead of the parallel rule to read the true bearing.
This simple plot would be adequate for short passages in good visibility. On extended voyages, with poor visibility, further allowances would be made for set and leeway, due to currents and wind.
It is important to understand the difference between true, magnetic, and compass, and to use the terms appropriately.
Course Plotting Terminology.
It is important to be familiar with the conventional terms and understand their meanings.
| Origin/Destination. | The starting point and desired finishing point of a passage. |
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| Intended Track. | The true bearing or shortest direction between origin and destination. |
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| Track. | The path the vessel actually travels over the ground. Loosely called COG. |
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| Magnetic bearing. | The true bearing with correction for compass variation only. |
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| Course. | The compass direction in which the vessel is to be steered. True bearing with correction for compass error & drift. |
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| Sw. | Speed of the vessel through the water. A known constant. |
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| Sb. | Speed of the vessel over the bottom. (SOG). |
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| Set. | The direction in which ocean and tidal currents move, causing offset from the intended track. |
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| Leeway. | The effect on the intended track of a vessel due to wind/waves. |
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| Drift. | The distance offset from the intended track due to the combined effect of set and leeway. |
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| Rate. | The speed of the drift in knots. |
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| Dead Reckoned Position. ( D.R.) | A vessel position obtained by dead reckoning, based on the direction of the intended track, and the known speed of the vessel through the water, Sw. |
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| Estimated Position. | A vessel position by dead reckoning, with a correction for drift. |
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| Actual Position. | A vessel position determined by a bearing fix, or by GPS. |
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| Waypoint. | A destination on a chart defined accurately by L. & L. coordinates. |
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We deal with two types of speed. Sb, speed over the bottom, and Sw, speed through the water.
Set is the direction of a current having an effect on the intended track and Sb of a vessel. Set has direction and rate, and will have the same effect on any vessel, be it a supertanker or a dingy.
Leeway is the angle a vessel is offset to the intended track by wind, and consequently wave action. More of a problem with yachts, and one which can best be gauged by experience of the skipper on a particular vessel. Correction for leeway is always applied to the true course. Set and leeway will determine the speed over the bottom Sb, and the track made good.
Drift is a combination of the effects of set and leeway, and is the distance a vessel is offset from the intended track.
Allowance for set and leeway must always be applied to the true course. In sailing vessels, it is preferable to treat set and leeway separately, as set is considered to affect the cog and Sb of the vessel, whilst leeway is considered to affect only the cog. In motor vessels, the effects of set and leeway are usually combined. In a seaway, allowance for drift will always need to be taken into account.
Rate is the speed of the drift - Dist/Time - in knots.
Intended track is a line which may be drawn on a chart between two points and along which it is desired the vessel shall pass. It is the shortest distance between the two points, taking the least time and using least fuel. Track made good is the actual path traced by the vessel over the seabed. Both these lines would be marked with double arrow heads, but are not normally drawn on the chart.
A course is the direction by compass in which a vessel has to be driven through the water to reach its destination or waypoint. A line drawn on the chart showing the direction in which a vessel has to be steered is a Course line, and is indicated by a single arrowhead in the direction of travel. Due to set and leeway, the track or track made good will be quite different from the course steered. With accurate navigation, intended track and track made good may be similar.
Sb always refers to track / track made good and Sw always refers to course steered.
Sb is obtained by measuring the time of a passage between two known positions. e.g. the origin and a position fix.
Sw is usually obtained by experimentation or from an on board speed log, and for calculation purposes, is a constant figure for all sea conditions.
Courses (i.e. compass course), are the foundation of the navigators work, and must always be plotted. A course or a series of courses when plotted on a chart, becomes a sea plot. A sea plot is started from an accurately known position. Each course line should have the compass course written above it, and be identified by a single arrow head. The sea plot is an accurate record of courses steered and distances run through the water (Sw). It is not a record of the vessels track. It is important to understand this difference. Positions on a sea plot are dead reckoned positions (or estimated positions if correction has been made for drift), and are marked by a small triangle with the time written alongside. This symbol should also be used to mark any significant course event.
During the voyage, say every hour, an accurate fix is obtained, either by observation or GPS, and the point noted by a small circle and the time. This is an actual position, and allows the track and speed over ground to be calculated. If drawn, the track line is denoted by two arrow heads. The successive sea plot is commenced from this observed position, making any necessary course correction for expected drift.
It is important to keep a clear distinction between course / dr or estimated position and track / actual position.
Speed, Time, Distance. Speed = Distance / Time. Time = Distance / Speed. Distance = Speed x Time.
When set can be gauged from chart information, i.e. the direction and rate, its effect can be demonstrated graphically by a speed diagram. Draw a line AB to scale to represent Sw. e.g. 8 k. From B, draw a line BC to scale representing estimated set, i.e. speed and direction. Draw a perpendicular CD to AB. AD now represents the vessel speed Sb, CD represents the component of set that is causing drift or offset from the intended track, and DB the component determining Sb. As the diagram is to a scale of one hour, CD and DB can also be read as distances, and the rate of drift determined. The angles x, between BA and CA are the course correction angles.
Conversely, when a position fix is obtained, a diagram can be constructed to determine the actual drift being experienced. From a diagram, it is easy to see that the more abeam the set and wind, the greater the drift and lesser the effect on SOG. As set and wind is encountered more ahead or astern, the lesser the drift and greater the effect on SOG. In a seaway, the d.r. position will never be the same as the actual position. With accurate assesment of set and leeway, the estimated position will be nearer to the actual position.
The above is illustrated in | Fig. 1. |, which has been drawn with two examples of set. From this, a distance diagram could also be constructed showing the estimated drift for a passage without course correction.
Refering to | Fig.2. | the effect of a set of 2 knot at 270° on various courses for a vessel with Sw of 10 knots, can be appreciated.
Consider the following.
Plot a voyage from the fairway beacon Gladstone to Lady Musgrave Island. Your vessel has a known speed of 8 knots. i.e. Sw.
Determine the intended track line from the chart and compass rose - 90 deg. T.
Determine the distance and thus estimated passage time. 48.5 M. 6.00 hours.
Obtain tide details from the tables, and tidal flow direction from the chart. Assume a flooding tide is indicated during the passage, creating a set at approx. 290°, and at a rate of 2 knots. The weather report has indicated a moderate s.e. swell which will give wind and sea on the fwd. stbd. quarter.
The resultant of these influences will cause a drift from the track in a n.w. direction, and have an effect on Sb.
At the beginning of the passage, the navigator would make an estimate or calculation of the expected drift, make a correction for drift (set and leeway) to the true bearing and determine a compass course to steer. This would allow him to plot an estimated position for one hour steaming, with the aim of maintaining the intended track. A position fix at the first hour would then give the actual position. The drift and Sb could be measured from the latitude scale, and the eta revised. A new compass course from the actual position to the destination would be determined, making a correction to the new true bearing for expected drift.
The navigator would draw the consecutive compass courses on the chart with a single arrow head and the compass course written above, and mark each estimated position for one hour steaming with a small triangle and time, and each actual position with a small circle and time. The complete exercise for the voyage is called the sea plot.
It is good practice to draw the compass course line only on the chart, to maintain a tidy appearance, and to construct other diagrams on a separate sheet. A typical sea plot for a voyage from A - B might appear as such - | Sea Plot. |
Always endeavour to use the correct nautical terms when navigating. Remember that the term 'course' refers only to the direction being steered by compass.
Notwithstanding the value of GPS, recreational mariners should not underestimate the importance of a sound understanding of basic chart work. See GPS Accuracy.
 Return to practical boating.
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