Navigation and Seamanship

Aim

To study the general description of nautical charts.

Learning Outcome:   At the end of this practical the student will be able to understand the general description of the chart.

 Materials Required:

1) Parallel ruler: Parallel ruler is sliding double-edged scale which is used to transfer the courses and bearings across the chart . A line at any place in the chart can be transferred to any part of the chart simply by moving the ruler gently across the surface. It is essential for chart reading and they maintain the same angle on careful movement on the chart. Course and bearing can be transferred from different parts of chart to compass rose for taking the readings and vice-versa.

2) Divider and compass: Divider and compass are as essential as the parallel ruler for the chart reading purpose. They are used for measuring distances and to cut an arc of measured

3) Pencil and Eraser: Pencils are used to write and draw, on the chart. Good quality smooth erasers are used for cha rt reading as they are soft on rubbing with paper charts.

4) Nautical Chart: Given nautical chart.

5)Procedure Look for the following details and note down the descriptions for the given chart.

1. Title of chart : The title of each chart is printed in convenient, conspicuous place on a chart, where it does not hinder the navigational use of it. The title is printed clearly with bold letters.

2. Chart datum: This is a level of the water usually considered to be the depth at the lowest astronomical tide (Chart datum being an imaginary datum, beyond which the sea level rarely falls). All depths shown on a chart are below this level and these depths are called “soundings”. The unit of the soundings is as per the legend under the title.

3. Tide table : The height of tides in the tide tables are all above the chart datum and as the level of water is always more than that of the chart datum the error is always on the safer side. Tidal information is given usually in the form of tables.

4. Heights : All heights, unless otherwise stated are given in meter or feet above mean high water springs or in places where there is no tide, above mean sea level.

5. Drying height : Underlined figures on rocks and banks which uncover express the heights above the chart datum. These are called “drying heights”.

6. Bearing : Bearings are always from seaward and are true. 

7. Natural scale : Natural scale is the relationship between the actual length of something on the earth and the length by which that thing is shown on the chart. It does not matter what size the unit is or in what system it is measured for example, a scale of 1/80,000 or 1 : 80,000 means that one unit (inch, foot, meter, etc.) on a chart represent 80,000 such unit on the earth.

8. Projection: Type of projection followed in the chart.(Mercator projection, Gnomonic or any other projection)

9. Caution: Cautions are given in respect of the use of chart.

10. Notes: The information is printed on charts in the form of notes related to regulatory restrictions, cautions and warnings, unusual magnetic conditions.

11. Authority : A statement of the authorities on which a chart is based, is given in the title of each chart. If a chart is derived from a number of surveys ; the dates and areas of the surveys may be difficult to define concisely in the title : on the latest charts compilation diagrams showing the dates and coverage may be included in such cases.

12. Dates of survey : Dates of surveys on which the chart is based.

13. Number of the chart : Every nautical chart has a serial number for easy identification. This is printed at the bottom right hand and the top left hand corners of the chart.

14. Date of publication: The date of publication along with the name of the Hydrographer to the Government is printed at the bottom, in the middle just outside the margin. Recent publication would mean a more reliable chart, incorporating all corrections, large and small, up to that date.

15. Date of printing: This is shown as the number of the day in the year, printed at the top right hand corner, outside the margin. E.g. 335.88. This means that the chart was printed on the 335th day of 1988.

16. Plate dimension: The figures in brackets shown outside the lower right hand border of the chart thus (425.0 x 860.0 mm) or (34.46 x 25.49) express the dimension in millimeters or inches of the plates from which charts are printed.

17. Compass rose: The compass roses are printed on the chart, at two or three places, wherever it is possible, so that it does not interfere with any useful information given on the chart. Compass roses for laying off bearing and courses. The variation of the place is shown on the compass rose on the (90 o 270 o line) (east west line).

18. Depth contours: These are lines joining places of equal depth and most charts have depth contours to indicate the shallower areas on the chart.

19. Soundings: Soundings mean the depths of water below the chart datum and are thus one of the most important features of the navigational chart. The units used for soundings are clearly shown below the “Title” of the chart. Soundings are indicated by figures showings in meters and decimeters. The depths are indicated by a normal-sized sounding figure with a smaller figure below and to the right of it. For example, Fathoms: 5₂ (5 fathoms 2 feet = 32 feet) [1 fathom = 6 feet] Metres: 5₂ (5.2 metres = 17 feet approx.)

20. Nature of bottom: This is described on the chart in abbreviated form such as ‘S’ for sand, ‘Co’ for coral, ‘M’ for mud etc. The information is very useful when anchoring a ship and fishing.

21. Colour Shading: 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.

22. Small corrections : The charts are regularly updated for any changes that may have occurred after the publishing of the chart. When some small corrections are of immediate importance to safe navigation, they are announced various Hydrographic Departments by the issue of Notice to mariners. (Weekly notice to mariners by the Admiralty, U.K. and fortnightly Indian notice to mariners.

23. Meridians: These are imaginary lines, which pass from pole to pole. In the (mercator) charts these lines are drawn as vertical lines and parallel to each other. The prime meridian which passes from pole to pole through the site of the Royal observatory at Greenwich, England.

24. Parallels : These are also imaginary lines, which pass from east to west direction and parallel to the equator (The midway between the poles and divide the earth into two equal halves that is southern and northern hemisphere). In the (mercator) charts  there lines are drawn as horizontal lines and parallel to each other. The location of any position on the earth is specified by stating its location relative to the equator and the prime meridian.

25. Latitude (abbreviated as Lat.; symbol L) : The latitude of a position is defined as the angular distance measured from the equator north ward or south ward through 90^o . The direction of measurement is indicated by placing a suffix N (north) or S (south) after the angular measure. The equator is the 0^o latitude and the poles 90^o .

26. Longitude (abbreviated as Long.; symbol Lo) The longitude of a position is the angular distance measured from the prime meridian east ward (or) west ward trough 180^o . The direction of measurement is indicated by placing a suffix E (east) or W (west) after the angular measure. The longitude of any point on earth may be defined as the angular distance between the meridian of Greenwich and the meridian passing through the point. It is measured in degrees of arc, from 0^o to 180^o east (east longitude) or west (west longitude) from the prime meridian (Greenwich). Latitude and longitude are measured in degrees, minutes and seconds or in degrees, minutes and decimal fraction of a minute.

27. Sea mile or Nautical mile : This is the length of one minute of latitude at a place and is the unit of distance. (Sea mile is the length of one minute of latitude ; and is the principle mean of expressing distance on charts) The International Nautical mile (a sea mile) is equal to a length of 1.852 km or 1852 meters or 6080 feet.