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Chapter 3 of Bowditch
The Nautical Chart
A representation of a portion of the navigable waters of the Earth and adjacent coastal areas on a specified map projection, and designed specifically to meet requirements of marine navigation. Included on most nautical charts are:
depths of water
characteristics of the bottom
elevations of selected topographic features
general configuration and characteristics of the coast
the shoreline (usually the mean high water line)
dangers
obstructions
aids to navigation (buoys, day markers, etc…)
limited tidal data
information about magnetic variation in the charted area
Basic definition of a Nautical Chart – a miniature representation of a portion of the Earth’s navigable waters on a plane surface and in accordance with some definite system of projection and development.
Chart Scale
The scale of a chart is the ratio of a given distance on the chart to the actual distance that it represents on the earth. It may be expressed in various ways.
Natural or Fractional Scale – a simple ratio of fraction. 1:80,000 or 1/80,000 means that 1 unit on the chart (usually an inch) represents 80,000 units on the earth.
Numerical Scale – A statement of that distance on the earth shown in one unit (usually an inch) on the chart or vice versa. For example, “30 miles to the inch” means that 1 inch on the chart represents 30 nm on the earth’s surface.
You can convert a Natural Scale to a Numerical Scale & vice versa:
1 nautical mile = 6076 feet
6076 feet X 12 = 72912 inches
If the natural scale is 1:80,000 then 1inch on the chart is equal to 80,000 inches on the earth. To get the inches per nautical mile, divide the scale by the number of inches per mile. 80,000 inches/ 72,912 inches/nm = 1.097 nm. That means that on a chart with a scale of 1:80,000 one inch is equal to 1.097nm (approx. 1.1 nm). If you divide the other way you will get the number of inches per nautical mile (72912 in/nm / 80,000 in = 0.911 inches = 1 nm)
Numerical to Natural – 60 nm to an inch 60 nm X 72912 in/nm = 4,374,720 so the ratio is 1:4,374,720.
Bowditch has a table that allows you to figure this out and avoid the math!
Graphic Scale – a line or bar may be drawn at a convenient place on the chart and subdivided into nautical miles, yards, etc. All charts vary somewhat in scale from point to point, and in some projections the scale is not the same in all directions. Since 1 minute of latitude is very nearly equal to 1 nautical mile, the latitude scale serves as an approximate graphic scale. The scale is normally printed on a chart whose scale does not appreciably change over the length of the chart.
A chart covering a large area is known as a Small Scale Chart.
A chart covering a small area is known as a Large Scale Chart.
The larger the number on a natural scale (1:80,000) the smaller the scale.
Most Navigation Charts range in scale from 1:2,500 to 1:14,000,000.
Small-scale charts are used for planning and offshore navigation where detailed information is not as important. Charts of Larger scale should be used as the vessel approaches pilot waters. The Largest Scale Charts should be used for an area to ensure all necessary information is included.
Types of Scales -
Name of Scale & Uses
Sailing Chart - For planning, fixing position at sea, plotting DR’s when proceeding on a long voyage. Ocean Navigation.
General Chart - Coastwise navigation outside of outlying reefs and shoals.
Coast Chart - Inshore coastwise navigation where the course may lie inside outlying reefs and shoals, for entering/leaving bays & harbors of considerable width, and for navigating large inland waterways.
Harbor Charts - Navigation and anchorage in harbors and small waterways
The DMA, who used to make US charts, defined General Charts as those smaller that 1:150,000 (Sailing & General Charts) and referred to the Coast and Harbor Charts as Approach Charts. |
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How accurate are charts?
Suitability of the scale for the design and intended navigational use – The same detail cannot be shown on a small-scale as on one of a larger scale. On small-scale charts detailed information, including minor aids to navigation, is omitted or generalized in the areas covered by larger scale charts. The Largest Scale Charts should be used for an area to ensure all necessary information is included.
They are only as accurate & up-to-date as the surveys that are included on the chart. If information is based on very old surveys, special care should be taken when transiting the area. Charts are constantly being updated by mariners who report their findings to the Chart Makers (NIMA – NOAA) and by USCG/NOAA vessels that do surveys of US waters. The updates can be found in the Local Notice to Mariners and Notice to Mariners. These publications are put out by NIMA/USCG on a weekly basis and include information to allow the 2nd Mate to correct the charts.
Notice to Mariners cover all the US published charts and publications. The include:
changes in aids to navigation (lights, buoys, daymarks, ranges)
dangers to navigation (rocks, shoals, reefs, wrecks)
Selected items from the Local Notice to Mariners
Important new soundings
Changes in channels
Harbor construction
Radio navigation information
New and revised charts & publications
Special warnings and notices
HYDROLANT, HYDROPAC, NAVAREA IV & XII messages
In general, any information that affects mariners on charts
The “Local Notice to Mariners” is put out by the USCG Districts for their area. It is published as often as needed, usually weekly. It includes:
Changes and deficiencies in aids to navigation under the authority of the USCG District
Channel depths
New charts
Naval operations
Regattas
Temporary information, known or expected to be of short duration
Who makes charts?
In the United States the Defense Mapping Agency (DMA) used to print the charts. Now, the National Oceanographic & Atmospheric Administration (NOAA) is responsible for the charts. They work with NIMA (National Imagery and Mapping Agency.) They are now printed by NOS, the National Ocean Service. You may still hear the “old timers” refer to US Charts as “DMA Charts.”
Did NIMA replace the Defense Mapping Agency (DMA)? NIMA was formed through the consolidation of the following: the Defense Mapping Agency (DMA), the Central Imagery Office (CIO), the Defense Dissemination Program Office (DDPO) and the National Photographic Interpretation Center (NPIC) as well as the imagery exploitation and dissemination elements of the Defense Intelligence Agency (DIA), the National Reconnaissance Office (NRO), the Defense Airborne Reconnaissance Office (DARO) and the Central Intelligence Agency.
The other commonly used charts are the British Admiralty or “BA” charts. Most foreign governments produce nautical charts for their waters. The US & BA charts use those foreign charts to make their own charts for the areas. The foreign charts are often in the native language of the county that is issuing the chart so the US & BA charts have to translate them. They also update the charts to conform to US/BA coding. Using only one manufacturer’s charts is preferred so that you do not have to use different publications to correct all your charts.
Dates on Charts – The date of a chart is always printed in the lower left hand margin of the chart. BA must, by law, be sold correct and up-to-date the day you buy it. It will include next to the date, the NM numbers for all corrections included on the chart.
First edition – original date of issue
New Edition – When the corrections are too numerous or too extensive to print in the Notice to Mariners. This makes all previous printings obsolete
Revised Print – May contain corrections that had been listed in the Notice to Mariners but does not supersede the current edition. You may obtain “Print on Demand” charts now. These charts are revised prints that are printed the day you order them. They are the latest editions with all Notice to Mariner Corrections already included. This is a great thing for navigators to buy, especially when they are going to a new area – no need to make years of corrections!
Reprint – this is initiated by a low stock situation and is a reprint of the chart with limited correction, and updated magnetic variation.
Chart Sounding Datum (a.k.a. Chart Datum) – The tidal datum to which soundings and drying heights on a chart are referred. It is usually taken to correspond to a low water stage of a tide.
Datum is any numerical or geometrical quantity or set of such quantities, which may serve as a reference or base for other quantities.
Sounding is a measured or charted depth of water or the measurement of such depth.
The following datum’s have been averages over a 19-year period
Mean Low Water – MLW – the average of all the low tides. This was used for years on all Atlantic & Gulf Coast Charts as well as most DMA charts. This is being discontinued as the DMA charts are discontinued (replaced by NOS charts.)
Mean Lower Low Water – MLLW – The average of only the lower of the two daily tides. This was always used on the Pacific Coast Charts & is now used by all NOS Charts.
Mean Low Water Spring – MLWS – The low water at spring tide
Mean Lower Low Water Spring – MLLWS – the average of the lower of the two daily low tides at springs. Most BA charts use one of the spring datum’s.
Know the chart numbering system:
1 digit charts - Products in the chart system that are not actually charts. Example – Chart 1, Nautical Chart Symbols and Abbreviations.
2 digit charts - Ocean Basins. The 1st number identifies the ocean. Not all Ocean basins are big enough for that scale chart so the unused numbers (30-49 & 70-79) are assigned to special world charts (42 Magnetic Variation, 76 Standard Time Zone of the World.)
3 digit charts - Ocean Basins or a Large area. Despite the scale, 3 digits are used for a series of position plotting sheets, which are larger scale that this range but they are used for ocean navigation.
4 digit charts - Non-navigational and special purpose Example – 5090 Maneuvering Boards, 5101 Gnomonic Plotting Chart North Atlantic, 7707 Omega Plotting Chart
5 digit charts - Cover portions of the coastline rather than significant portions of ocean basins. These charts are based on the regions of the nautical chart index.
1st digit of 2 & 3 digit charts represent the Ocean Region of the Chart.
1st digit of 5 digit charts represent the Land Region of the Chart.
2nd digit of 5 digit charts represent the Land Sub-Region of the Chart.
3rd digit of 5 digit charts represent the Area of the Chart.
Miscellaneous Information – This can all be found in Chart 1
Traffic Separation Schemes – show routes to increase safety of navigation, particularly in areas of high-density shipping. They are shown standard on charts of scale 1:600,000 and larger and are printed in magenta. The arrows on the chart indicate that proper direction of flow.
Buoys – (this will be gone over in depth in lecture 10) Nun & Can, Red & Green – Even Nun Blush! IALA buoyage system. International Association of Lighthouse Authorities – this is a system to standardize the world with buoys to promote safe navigation worldwide. There are 2 systems – Region A & B. The US is Region B, where the red (nun shaped) buoys are kept on the starboard side when entering port. Region A is used in England and Japan where the red can buoy is kept on the port side when entering port. The shape & numbering is the same in both regions, it is just the color that changes.
Lateral marks – used in conjunction with a conventional direction of buoyage, are generally used for well-defined channels. Where a channel divides, a modified lateral mark may be used to indicate a preferred route. These differ within region A & B by color only.
Cardinal marks – used in conjunction with the mariner’s compass, indicate where the mariner may find navigable water. These are black & yellow with triangle shaped top marks indicating the direction.
Isolated Danger marks – indicate isolated dangers of limited size that have navigable water all around them.
Safe water marks – indicate that there is navigable water abound their position (example – mid-channel markers.)
Special marks – not primarily intended to assist navigation, indicate an area or feature referred to in navigational documents.
Soundings - a measured or charted depth of water or the measurement of such depth.
Notes – need to be read with care, as they may be important information not graphically presented. Notes in connection with the chart title include such information as scale, sources of charted data, tidal information, the unit in which soundings are given, cautions, etc. Those in the margin give chart number, and (sometimes) publication, and edition notes, identification of adjoining charts, etc. Another class of note is those referring to local magnetic disturbance, controlling depths of channels (called tabulation), measured miles, dangers, dumping grounds, anchorages, etc. (Measured Mile – a measured nautical mile accurate within 6 feet, most were made before 1959 in the US.)
Vertical Clearances for bridges – given in mean high waters to indicate the minimum amount area between the top of the water and the lower part of the bridge. Lets ships know if they will fit under the bridge.
Shoals – offshore hazards to navigation on which there are less than 16 fathoms or 30 meters or less, composed of unconsolidated material. Shoaling “causing to be less deep.”
Wrecks – the ruined remains of a vessel, which has been rendered useless. In hydrography, the term is limited to a wrecked vessel, either submerged or visible, which is attached to or foul of the bottom or cast up on the shore.
Anchorage Areas – Indicate areas for vessels to anchor, magenta colored
Compass Rose – placed at convenient locations on Mercator charts to facilitate the plotting of bearings and courses.
True Direction – outer circle graduated in degrees with zero at True North
Magnetic Direction – inner circle graduated in degrees with zero at Magnetic North for the chart’s edition year
Variation – isogonic lines are lines of equal variation, agonic line is a line of 0 variation
Annual Change – the magnetic north pole is not stationary, it moves a little each year so variation changes each year. This tells you the correction to apply & the direction to apply it to for each year past the edition date.
Edition Number and Date – lower left margin
Title Block – see notes
Use of Color – Red is never used on a navigation chart because we use red lights on the bridge at night so we don’t ruin our night vision. Magenta is the purple writing. The color of anything on the chart is just as important to describing it as the shape/design is.
Character of the bottom
Courses on charts are given in true direction to the nearest minute of arc
Bearings are in true directions toward (not from) the object
Rules of the Road – Lines of Demarcation between the areas where International & Inland Rules apply are shown only when they cannot be adequately described in the notes on the chart.
Lettering on chart
Block or Vertical – used for features, which are dry at high water and not affected by movement of the water, except for heights above water.
Slant of Italic – used for water, underwater, and floating features, except soundings. Generally, objects that cover and uncover with the tide.
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