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Charts are graphic representations of areas of the earth for use in navigation.
Nautical charts depict features of particular interest to the navigator.
Stereographic and orthographic projections date from the 2nd century BC.
In 1569 Gerardus Mercator published a chart using the mathematical principle, which now bears his name.
Sailing directions or pilots have existed since at least the 6th century BC. Modern pilots and sailing directions are based on extensive data collection and compilation efforts begun by Matthew Fontaine Maury beginning in 1842.
Classification of projections depends on whether the projection is centered on the equator (equatorial), a pole (polar), or some point or line between (oblique). The name of a projection indicates its type and its principal features.
For mariners the most frequently used charting system is the Mercator projection, classified as a cylindrical projection upon a plane, the cylinder tangent along the equator.
Similarly, a projection based upon a cylinder tangent along a meridian is called transverse (or inverse) Mercator or transverse (or inverse) orthomorphic.
The Mercator is the most common projection used in maritime navigation, primarily because rhumb lines plot as straight lines.
In a simple conic projection, points on the surface of the earth are transferred to a tangent cone. In the Lambert conformal projection, the cone intersects the earth (a secant cone) at two small circles. In a polyconic projection, a series of tangent cones is used.
If the origin of the projecting rays is the center of the earth, a gnomonic projection results; if it is the point opposite the plane’s point of tangency, a stereographic projection; and if at infinity (the projecting lines being parallel to each other), an orthographic projection. The gnomonic, stereographic, and orthographic are perspective projections.
In the Mercator projection the parallels can be derived mathematically as well as projected geometrically. Its distinguishing feature is that both the meridians and parallels are expanded at the same ratio with increased latitude.
The expansion is equal to the secant of the latitude, with a small correction for the ellipticity of the earth. Since the secant of 90˚ is infinity, the projection cannot include the poles.
Since the projection is conformal, expansion is the same in all directions and angles are correctly shown.
Rhumb lines appear as straight lines, the directions of which can be measured directly on the chart. Distances can also be measured directly if the spread of latitude is small. Great circles, except meridians and the equator, appear as curved lines concave to the equator.
Small areas appear in their correct shape but of increased size unless they are near the equator.
Earth as a Sphere
At the equator a degree of longitude is approximately equal in length to a degree of latitude.
As the distance from the equator increases, degrees of latitude remain approximately the same, while degrees of longitude become progressively shorter.
Earth spread out as a Flat Mercator Chart
Since degrees of longitude appear everywhere the same length in the Mercator projection, it is necessary to increase the length of the meridians if the expansion is to be equal in all directions.
Thus, to maintain the correct proportions between degrees of latitude and degrees of longitude, the degrees of latitude must be progressively longer as the distance from the equator increases.
The length of a meridian, increased between the equator and any given latitude, expressed in minutes of arc at the equator as a unit, constitutes the number of meridional parts (MerPart) corresponding to that latitude. Meridional parts, given in the Nautical Tables for every minute of latitude from the equator to the pole, make it possible to construct a Mercator chart and to solve problems in Mercator sailing.
If a plane is tangent to the earth, and points are projected geometrically from the center of the earth, the result is a gnomonic projection.
The distance scale, however, changes rapidly. The projection is neither conformal nor equal area. Distortion is so great that shapes, as well as distances and areas, are very poorly represented, except near the point of tangency.
The usefulness of this projection rests upon the fact that any great circle appears on the map as a straight line, giving charts made on this projection the common name great-circle charts.
Gnomonic charts are most often used for planning the great-circle track between points. Points along the determined track are then transferred to a Mercator projection.
The great circle is then followed by following the rhumb lines from one point to the next.
ECDIS - A graticule is the network of latitude and longitude lines laid out in accordance with the principles of any projection.
Types Of Scales
The scale of a chart is the ratio of a given distance on the chart to the actual distance, which it represents on the earth. It may be expressed in various ways. The most common are:
1. A simple ratio or fraction, known as the representative fraction. For example, 1:80,000 or 1/80,000 means that one unit (such as a meter) on the chart represents 80,000 of the same unit on the surface of the earth. This scale is sometimes called the natural or fractional scale.
A statement that a given distance on the earth equals a given measure on the chart, or vice versa. For example, “30 miles to the inch” means that 1 inch on the chart represents 30 miles of the earth’s surface. Similarly, “2 inches to a mile” indicates that 2 inches on the chart represent 1 mile on the earth. This is some-times called the numerical scale.
A line or bar called a graphic scale may be drawn at a convenient place on the chart and subdivided into nautical miles, meters, etc. All charts vary somewhat in scale from point to point, and in some projections the scale is not the same in all directions about a single point. A single subdivided line or bar for use over n entire chart is shown only when the chart is of such scale and projection that the scale varies a negligible amount over the chart, usually one of about 1:75,000 or larger. Since 1 minute of latitude is very nearly equal to 1 nautical mile, the latitude scale serves as an approximate graphic scale. On most nautical charts the east and west borders are subdivided to facilitate distance measurements.
On a Mercator chart the scale varies with the latitude. This is noticeable on a chart covering a relatively large distance in a north-south direction. On such a chart the border scale near the latitude in question should be used for measuring distances.
Of the various methods of indicating scale, the graphical method is normally available in some form on the chart. In addition, the scale is customarily stated on charts on which the scale does not change appreciably over the chart. The ways of expressing the scale of a chart are readily interchangeable. For instance, in a nautical mile there are about 72,913.39 inches. If the natural scale of a chart is 1:80,000, one inch of the chart represents 80,000 inches of the earth, or a little more than a mile. To find the exact amount, divide the scale by the number of inches in a mile, or 80,000/72,913.39 = 1.097. Thus, a scale of 1:80,000 is the same as a scale of 1.097 (or approximately 1.1) miles to an inch. Stated another way, there are: 72,913.39/80,000 = 0.911 (approximately 0.9) inch to a mile. Similarly, if the scale is 60 nautical miles to an inch, the representative fraction is 1:(60 x 72,913.39) = 1:4,374,803.
A chart covering a relatively large area is called a small-scale chart and one covering a relatively small area is called a large-scale chart. Since the terms are relative, there is no sharp division between the two. Thus, a chart of scale 1:100,000 is large scale when compared with a chart of 1:1,000,000 but small scale when compared with one of 1:25,000.
As scale decreases, the amount of detail which can be shown decreases also. Cartographers selectively decrease the detail in a process called generalization when producing small scale charts using large scale charts as sources. The amount of detail shown depends on several factors, among them the coverage of the area at larger scales and the intended use of the chart.
Chart Classification By Scale
Charts are constructed on many different scales, ranging from about 1:2,500 to 1:14,000,000. Small-scale charts covering large areas are used for route planning and for offshore navigation. Charts of larger scale, covering smaller areas, are used as the vessel approaches land. Several methods of classifying charts according to scale are used in various nations. The following classifications of nautical charts are used by the US National Ocean Service.
Sailing charts are the smallest scale charts used for planning, fixing position at sea, and for plotting the dead reckoning while proceeding on a long voyage. The scale is generally smaller than 1:600,000. The shoreline and topography are generalized and only offshore soundings, the principal navigational lights, outer buoys, and landmarks visible at considerable distances are shown.
General charts are intended for coastwise navigation outside of outlying reefs and shoals. The scales range from about 1:150,000 to 1:600,000.
Coastal charts are intended for inshore coastwise navigation, for entering or leaving bays and harbours of considerable width, and for navigating large inland waterways.
The scales range from about 1:50,000 to 1:150,000.
Harbour charts are intended for navigation and anchorage in harbours and small waterways. The scale is generally larger than 1:50,000.
In the classification system the sailing charts are incorporated in the general charts classification (smaller than about 1:150,000); those coast charts especially useful for approaching more confined waters (bays, harbours) are classified as approach charts. There is considerable overlap in these designations, and the classification of a chart is best determined by its use and by its relationship to other charts of the area. The use of insets complicates the placement of charts into rigid classifications.
Factors Relating To Accuracy
The accuracy of a chart depends upon the accuracy of the hydrographic surveys used to compile it and the suitability of its scale for its intended use.
Estimate the accuracy of a chart’s surveys from the source notes given in the title of the chart. If the chart is based upon very old surveys, use it with caution. Many early surveys were inaccurate because of the technological limitations of the surveyor.
The number of soundings and their spacing indicates the completeness of the survey. Only a small fraction of the soundings taken in a thorough survey are shown on the chart, but sparse or unevenly distributed soundings indicate that the survey was probably not made in detail. Large blank areas or absence of depth contours generally indicate lack of soundings in the area.
Operate in an area with sparse sounding data only if operationally required and then only with the most extreme caution. Run the echo sounder continuously and operate at a reduced speed. Sparse sounding information does not necessarily indicate an incomplete survey. Relatively few soundings are shown when there is a large number of depth contours, or where the bottom is flat, or gently and evenly sloping. Additional soundings are shown when they are helpful in indicating the uneven character of a rough bottom.
Even a detailed survey may fail to locate every rock or pinnacle. In waters where they might be located, the best method for finding them is a wire drag survey. Areas that have been dragged may be indicated on the chart by limiting lines and green or purple tint and a note added to show the effective depth at which the drag was operated. Changes in bottom contours are relatively rapid in areas such as entrances to harbors where there are strong currents or heavy surf. Similarly, there is sometimes a tendency for dredged channels to shoal, especially if they are surrounded by sand or mud, and cross currents exist. Charts often contain notes indicating the bottom contours are known to change rapidly.
The same detail cannot be shown on a small-scale chart as on a large-scale chart. On small-scale charts, detailed information is omitted or “generalized” in the areas covered by larger scale charts. The navigator should use the largest scale chart available for the area in which he is operating, especially when operating in the vicinity of hazards. Charting agencies continually evaluate both the detail and the presentation of data appearing on a chart. Development of a new navigational aid may render previous charts inadequate. The development of radar, for example, required upgrading charts, which lacked the detail required for reliable identification of radar targets.
After receiving a chart, the user is responsible for keeping it updated. Mariner’s reports of errors, changes, and suggestions are useful to charting agencies. Even with modern automated data collection techniques, there is no substitute for on-sight observation of hydrographic conditions by experienced mariners. This holds true especially in less frequently travelled areas of the world.
Upon receiving a new paper chart, verify its announcement in the Notice to Mariners and correct it with all applicable corrections. Read all the chart’s notes; there should be no question about the meanings of symbols or the units in which depths are given. Since the latitude and longitude scales differ considerably on various charts, carefully note those on the chart to be used. Prepare piloting charts and open ocean transit charts. Place additional information on the chart as required.
Arcs of circles might be drawn around navigational lights to indicate the limit of visibility at the height of eye of an observer on the bridge. Notes regarding other information from the light lists, tide tables, tidal current tables, and sailing directions might prove helpful.
The preparation of electronic charts for use is determined by the operator’s manual for the system. If the electronic chart system in use is not IMO-approved, the navigator is required to maintain a concurrent plot on paper charts.
Maintaining Paper Charts
A mariner navigating on an uncorrected chart is courting disaster. The chart’s print date reflects the latest Notice to Mariners used to update the chart; responsibility for maintaining it after this date lies with the user. The weekly Notice to Mariners contains information needed for maintaining charts. Radio broadcasts give advance notice of urgent corrections. Local Notice to Mariners should be consulted for inshore areas. The navigator must develop a system to keep track of chart corrections and to ensure that the chart he is using s updated with the latest correction. A convenient way of keeping this record is with a chart/Publication Correction Record Card system. Using this system, the navigator does not immediately update every chart in his portfolio when he receives the Notice to Mariners. Instead, he constructs a card for every chart in his portfolio and notes the correction on this card. When the time comes to use the chart, he pulls the chart and chart’s card, and he makes the indicated corrections on the chart. This system ensures that every chart is properly corrected prior to use.
A Summary of Corrections, containing a cumulative listing of previously published Notice to Mariners corrections, is published bi-annually.
Thus, to fully correct a chart whose edition date is several years old, the navigator needs only the Summary of Corrections for that region and the notices from that Summary forward; he does not need to obtain notices all the way back to the edition date. The mariner bears the responsibility for ensuring that his charts are the current edition. The very fact that a new edition has been prepared indicates that there have been changes that cannot adequately be shown by hand corrections.
Use And Stowage Of Charts
Use and stow charts carefully. This is especially true with digital charts contained on electronic media. Keep optical and magnetic media containing chart data out of the sun, inside dust covers, and away from magnetic influences.
Placing a disk in an inhospitable environment will destroy important data. Make permanent corrections to paper charts in ink so that they will not be inadvertently erased. Pencil in all other markings so that they can be easily erased without damaging the chart. Lay out and label tracks on charts of frequently-travelled ports in ink. Draw lines and labels no larger than necessary. Do not obscure sounding data or other information when labeling a chart. When a voyage is completed, carefully erase the charts unless there has been a grounding or collision. In this case, preserve the charts without change because they will play a critical role in the investigation.
When not in use, stow charts flat in their proper portfolio. Minimize their folding and properly index them for easy retrieval.
CHAPTER V: Safety of Navigation
All ships shall carry adequate and up-to-date charts,* sailing directions, lists of lights, notices to mariners, tide tables and all other nautical publications necessary for the intended voyage.
*Refer to the Recommendation on performance standards for electronic chart display and information systems (ECDIS) (resolution A.817 (19), as amended) and resolution MSC.86 (70).
The ECDIS is the only system recognised under International law, as an acceptable replacement for paper charts.
This is an important legal point. International law requires that a paper chart be used at all times (SOLAS chapter V, Reg.20), unless an IMO approved ECDIS is onboard and in use.
Should an accident occur, the authorities will determine, if one of these system was in use at the time.
Brief understanding of IMO Res.A817 (19)
As per IMO Resolution A817 (19)
Noting that the up to date charts required by SOLAS regulation V/20 can be provided and displayed electronically on board ships by electronic chart display and information systems (ECDIS), and that the other nautical publications required by regulation V/20 may also be so provided and displayed.
Recognizing the need to prepare performance standards for ECDIS in order to ensure the operational reliability of such equipment, and to ensure that the information provided and displayed electronically is at least equivalent to that of up to date charts and, when also provided and displayed, other nautical publications, and to avoid, as far as practicable, adverse interaction between ECDIS and other ship borne navigational and communication equipment
ECDIS should be capable of displaying all chart information necessary for safe and efficient navigation originated by, and distributed on the authority of, government authorised hydrographic offices.
Equivalency of electronic navigation charts to paper charts,
ECD1S, with adequate back up arrangements, may be accepted as complying with the up to date charts required by regulation V/20 of the 1974 SOLAS Convention.
Emphasising that the data is as reliable as of the date of survey.
As with paper charts the warnings and caution that are issued with the above holds true also for ENC.
Meaning that the government authorised hydrographic office would obviously take all the care possible to present on a chart (data base) up to date information of the area covered.
The IMO has devised a training schedule for usage and handling of ECDIS
Prior an officer is allowed to use and handle the ships ECDIS, the officer has to go an IMO approved ECDIS course. This is generally a 40-hour course, which includes theory as well as practical classes on a simulator. However since this is conducted on a generic model, specific ship ECDIS familiarisation is also a must. The officer is encouraged to use the ECDIS to become proficient in using the ECDIS – including updating of charts as well as the back up arrangement in case of an ECDIS failure.