Lesson Plan 

for 

Chapter One 

Nautical Charts

 

Introduction - In this lesson we are going to learn all about Nautical Charts.  This lesson is, primarily, the foundation for future lessons.  Since this is a coastal navigation course, we are not going to spend much time on the various projections.  It is, however, important to understand the difference between Mercator Projection (used on all nautical charts except on the great lakes) and Polyconic Projection (used on Great Lakes Charts).  You can view an example of each here (hit your "back" button to return): 
Great Lakes Chart  Gulf of Mexico   

 

By the way, instructors comments will be in blue.  If, at any time, you have a question about the material here, Go to To the Questions Forum  

 

Lesson Objectives:

 

            To become familiar with the data found on a Nautical Chart.

 

            To understand chart orientation.

 

            To be introduced to Latitude and Longitude.

 

            To become familiar with Small Craft Charts.

 

            To be introduced to a Compass Rose and  to understand Variation.

 

            To become familiar with  the General Information Block.

 

            To understand how heights are measured on a chart.

 

            To understand how depths are shown on a chart.

 

            To understand the scale of a chart.

 

            To be introduced to chart symbols and abbreviations.

 

            To be introduced to horizontal and vertical Chart Datum.

 

            To become familiar with Aids to Navigation symbols.

 

            To become familiar with bottom characteristics.

 

            To briefly discuss Metrification of Charts.

 

1.     Why we need to know Nautical Charts
.

·       The nautical chart is the road map for the sea.  Without it, you are driving blind.

 

·       All it takes is one small rock or shoal to spoil a beautiful day and perhaps, put a large dent in your wallet.

 

·       There are many dangers at sea and along the coast most of which are shown on a nautical chart.

 

·       The nautical chart contains a wealth of information that can help you keep yourself,  your family, and your boat safe.

 

2.     Charts differ from a road map.

           

            Knowing how to read a chart is an important part of boating safety

 

so that you can:

 

·       Identify any hazards and dangers along your course line.

 

·       Effective plot course on your chart.

 

·       Get a clearer understanding of how electronic navigation equipment relates to a nautical chart.

 

3.     The Mercator Projection.

        

 

··       Earth’s surface is projected on a cylinder.

 

·       Tangent is at the Equator.

 

·       Distances between Meridians of Longitude are distorted as you move away from the equator.

 

·       True North is at the top of the projection.  True North is the point on which the earth rotates.

 

 

4.     The Earth as a Sphere

           

 

·       True Geographical North - the only place where you find it is on a Nautical Chart.  True North is "Geographic North" or the calculated location of the "top" of the earth.  There is a magnetic field surrounding the earth that flows northward.  The flow, however, is not consistent in either strength or direction and varies from point to point on the earth's surface.  This is the reason why charts are based on "true north" and the difference between true and magnetic north (called variation) is shown on each individual chart.

 

·       The circumference of the Equator is 21,614 nautical miles.

 

                        Divide by 360 degrees and you get 60.0389 nm per degree.

 

·       Earth’s diameter is 6,888 nautical miles. 

 

·       Your compass points to Magnetic North. 

Great Circle

A great circle is defined as a circle on the earth's surface the plane of which passes through the center of the earth.

For navigation purposes :

• The great circle track is the shortest distance between two places on the earth's surface.
  
• The great circle track appears as a straight line on Gnomonic (great circle) charts but will appear as a curved line (hence the name) on mercator charts.  
  
• The vertices of a great circle are the two points nearest to the poles which have a course on the great circle track due EAST / WEST.  We are not going to spend any time on great circle routes because they are only a concern for long voyages.  

 

                        “LAT is FLAT”.

 

·       The intersection of Latitude and Longitude defines position on the earth’s surface.

 

 

5.     Lambert Conformal Projection.

.

 

·       Projection of sphere on to a cone.

 

·       Used for charts of the Great Lakes.

 

·       Used for aircraft - (Aeronautical Charts).

 

 

 

6.     New Terminology

 

·       Longitude  [Meridians].

 

                        Vertical lines on a chart.

 

Numbered in degrees eastward and westward from the Prime Meridian.

 

Scale is found at the top and bottom of the chart.

 

Prime Meridian starts in Greenwich, England.

 

Distortion occurs as the Meridians approach the poles.

 

 

Great Circles.

 

Never used to measure distance.

 

A minute of Longitude does not equal a nautical mile.

 

Always labeled East or West depending on your location on the earth’s surface.

 

 

·       Latitude  [Parallels].

 

                        Horizontal lines on a nautical chart.

 

                        LAT is FLAT.

 

Numbered in degrees northward and southward from the Equator.

 

Scales are usually found on the right and left hand sides of a nautical chart.

 

Great Circle only at the Equator.

 

Small Circles toward the poles.

 

Equally Distant - Parallel.

 

Used to measure distance.

 

            One minute of latitude equals a nautical mile.

 

Always labeled as North or South depending on your location on the earth’s surface.       

 

The easiest way I can think of to visualize latitude and longitude is New York City.  New York labels its streets numerically running from south to north.  This is equivalent to latitude.  It uses avenues to label the streets that run from east to west (longitude).   When some one says to you, "meet me at 42nd St. and Fifth Avenue," you know exactly where to go.  The lines of longitude and latitude work exactly the same way - except instead of streets we have north and south lines of latitude and instead of avenues we have east and west lines of longitude.  Therefore, if you tell the coast guard, "I'm at 40 degrees north and 75 degrees west, they know where to go to find you.   

 

7.     Chart Orientation

 

·       True North is at the top of the chart. (except small craft charts)  See an example here - look at the compass rose and note how it points towards the east.

 

·       EAST is at the right-hand side of the chart.

 

·       WEST is at the left-hand side of the chart.

 

·       SOUTH is at the bottom of the chart.

 

·       One degree of Latitude is equal to sixty nautical miles. 

 

·       One minute of Latitude is equal to one nautical mile.

 

                        Nautical mile is 6,000 ft.

 

                        Statute mile is 5,280 ft.

 

                        Earth’s circumference is 360 degrees X 60 min = 21,600 nm.

 

 

8.     The Small Craft Chart.

 

·       The Small Craft chart is the exception to the rule.

 

·       True North orientation is not always at the top of this chart.

 

·       Orient True North with the Compass Rose.

 

9.     The Compass Rose

 

·       Every chart has a least one Compass Rose.

 

·       The star on the Compass Rose points to True North.

 

·       The Compass Rose has two circles:

The outer circle is oriented to True North.

 

  The inner circle is oriented to  Magnetic North.

 

·       True North and Magnetic North are different directions.

 

·       The angle between True North and Magnetic North is called Variation Error.

 

·       Variation Error is different for each position on the earth’s surface..

 

 

·       Variation is predictable on every position on the earth’s surface.

 

            Variation is printed in the center of each Compass Rose.

 

·       Variation is constantly changing.

 

The annual decrease or increase of Variation Error is also printed in the center of the Compass Rose.

 

·       The annual change is often so small that it is difficult to correct for using a compass on a small boat.

 

10. Chart Number

 

·       Located at the bottom left-hand corner of a nautical chart.

 

·       Chart Number is the official document number of the chart.

 

·       Edition Number.

 

·       Date of Last Update.

 

The last time any new date was added was at 10 AM on the date shown on the chart.

 

Information for correcting charts is found in the LNM - Local Notice to Mariners.

 

This is the reason that you should keep the latest edition charts aboard your boat.

 

 

10.    The General Information Block on a Nautical Chart.

 

·       Name of the large body of water covered by the chart.

                        i.e.  Block Island Sound and Approaches

 

·       Definition of how the soundings are measured.

                       

  i.e.  Soundings in feet at Mean Low Water [MLW].

 

Soundings can be shown in:

 

  Feetn  Meters Fathoms (3 feet)

 

Must reference the General Information Block to determine this fact.

 

Operator needs to calibrate depth sounding devices to reflect this Datum.

 

MLW - Mean Low Water is the mean [average] of the low tides depths for this area.

 

MLLW - Mean Lower Low Water is the mean of the lowest tide depth for each day.  It is a more conservative measurement,

 

Cautionary Note:

           

The term “mean” denotes an average.  An average tide depth infers that tides will be below this mean depth and will be above this mean depth.

 

Tide depths are also affected by weather, most specifically wind.  An onshore wind can increase the water depth and an offshore wind can decrease the water depth or delay its predicted time of tide.  Always use caution when using tide depth figures.

 

Tide depth projections from nautical catalogs are best professional guesses.  Always use two methods for determining water depth.

 

A charted depth and a depth sounder are acceptable as long as the depth sounder is calibrated to the chart datum.

 

Make it a practice to check the depth of water under your boat at your mooring or slip with your depth sounder before you go out.  Use a pole or lead line to check the reading.  Remember to add the extra depth to your readings for the distance that the transducer is mounted to your boat ‘s hull under the waterline.

 

 

·       Scale of the chart.  [A ratio of inches on a chart to actual inches on the earth]

 

1:40,000  means 1 inch on the chart represents 40,000 inches on the earth’s surface.  A note here:  This is confusing but important to know.  SMALL scale charts cover LARGE areas and LARGE scale charts cover SMALL areas.   A 1:5,000 chart would be considered a large scale chart while a 1:460,000 scale would considered a small scale chart. (the scale is smaller - not the area the chart covers)

 

·       Heights of objects are measured in Datum above Mean High Water [MHW].

 

Tidal Information Block provides the heights referred to in the sounding MLW.

 

MHW is designated as 9.4 ft. at Castle Island.

 

Use the designation closest to your position.

 

 

·       Clearances under Bridges are measured upward from MHW - Mean High Water.

 

·       The height of a light is measured from MHW to the focal plane of the light bulb.

 

This explains why some light towers look taller than their stated height.

 

           

·       Special Abbreviations.

 

Shows special abbreviations and notes peculiar to this charted area. 

 

·       Cautionary Notes.

 

            Special notes of uncharted dangers in the area. 

 

·       Special tidal and current information.

 

            Any unique tide of current phenomena in the area is noted. 

 

·       Magnetic interference in the area.

 

                        Any anomalies that affect your compass will be noted. 

 

·       LORAN interference in the area. 

 

·       Anchorage areas are noted.

 

11. Soundings and Water Depth.   Those of you who have chart #1, take a minute and look through it.  We will be discussing chart 1 in more detail in the next lesson.

 

·       Depth designations are shown on a Nautical Chart with:

 

Numbers which reflect the sounding datum shown in the General Information Block on the chart.

 

Colors

 

Shallow water is tinted a darker blue.

 

Deeper water is a lighter blue or white.

 

 

Contour Lines

 

A system of standardized symbols and abbreviations.

 

Dots and dashes.

 

 

·       Depths are expressed in:

 

                                    Feet

 

                                    Meters

 

                                    Fathoms [6 feet]

 

·       Datum for interpreting depths on a chart is found on the General Information Block on the chart.

 

12. Aids to Navigation [ATONs] (see the figures in your book)

 

·       Symbols for ATONs found on a nautical chart:

 

                        Buoy Symbol - Diamond with a small circle.

 

                        Cautionary Note:

 

                                    The Coast Guard moves ATONs periodically make your

nautical chart outdated.  The Local Notice to Mariners [LNM] tracks these changes.

 

·       Abbreviations for ATONs found on a nautical chart.

 

                        N - designates a nun or cone-shaped buoy.

 

                                    Will be red, even numbered, cone-shaped and, if lighted,

will have a red light.

 

                        C - designates a can or barrel-shaped buoy.

 

                                    Will be green, odd numbered, can-shaped, and, if lighted,

                                    will have a green light.

 

 

·       Abbreviations that indicate color of ATONs.

 

                        R - Red.

 

                                    Diamond symbol will be printed red.

 

                        G - Green

 

                                    Diamond symbol will be printed green.

 

                        RW - Red and White [Safe Water mark].

 

 

                         GR - Green buoy with Red band.

 

                                    Designates a junction buoy.  Top band is green

                                   

                                    Unlighted shape is a can.

 

                                    When lighted, the light will be green.

 

                        RG -             Red buoy with green band.

 

                                    Designates a junction buoy.  Top band is red,

 

                                    Unlighted shape is a nun.

 

                                    When lighted, the light will be red.

 

                        When an ATON is painted white or yellow, the diamond chart

symbol is left white or blank.            

 

·       Symbols for lighted buoys.

 

                        R - Light is red.

 

                        G - Light is green.

 

                        Y - Light is yellow.

 

When no color symbol is shown with a buoy, the light color is assumed to be white.

 

 

·       Other Features on buoys.

 

                        Sound Signals.

 

                                    Horns, whistles, and bells.     

 

                        Radar Reflectors.

 

                                    No symbol is shown on a nautical chart.

 

                                    Reference the Light List for this information.

 

 

                        Numbers.

 

                                    Nuns are even numbered.

 

                                    Cans are odd numbered.

 

                        Letters.

 

                        Light flash characteristics.

 

                        The numbers or letters, and flash characteristics that are

visible on an ATON are printed on the nautical chart beside the chart symbol.

 

·       Symbols for Lighthouses and other fixed ATONs.

           

 

                        Major Lights - Lighthouses.

 

                                    Black dot with a flare.  Looks like an exclamation point.

 

                                    Name of Light appears on the chart.

                                                i.e.  Boston Light.

 

                                    Will show height and range of light.

 

                        Minor Lights - Fixed lights.

 

                                    Black dot with a flare.

 

                                    Not named on the chart.

 

·       Ranges

               

 

                        Consists of two beacons or daymarks.

 

                        Shows dashed line in the direction of the range.

 

                        If unlighted, shows daymark symbols as square or triangle.

 

                                    Interpretation of panel colors found in the Light List.

 

                        If lighted, light symbols appear on the chart.

 

 

                        Cautionary Note:

 

                                    Ranges are usually fixed on land, rocks, or shoals. 

 

                                    Turn away to new channel so you don’t run aground.

 

                                    Always use your chart when transiting ranges.

 

 

·       Isolated Danger Marks

 

                        ATON erected on or moored over a danger that has navigable

water around it.

 

Horizontal bands of red and black.

 

White light.

 

Top mark of two black spheres.

 

Chart symbols has two black dots over the diamond symbol.

 

 

·       Daymarks - Day Beacons.

 

                        Permanently fixed structures.

 

                        Use square and triangle panels to designate use.

 

                        TR  and/or small triangle.

 

                                    Red - even numbered - red light if lighted.

 

                                    Treats as if a nun buoy.

 

                        SQ and/or small square.

 

                                    Green - odd numbered - green light if lighted.

 

 

 

 

                        Cautionary Note:

 

                                    Do note pass close aboard [near] Daymarks due to the

danger of striking the foundation, bottom structure, or protection riprap (rocks), or the obstruction marked by the ATON.

 

 

·       Nautical Purple print.

 

Nautical purple ink is used to color the symbol of lighted ATONs.

 

Stands out under a red light.

 

Red light is used on bridges in order to preserve night sight.

 

 

13.          Other Charted Features.

                   

 

·       Prominent Landmarks.

 

Water towers, smoke stacks, church spires, flag poles, public buildings, etc.

 

Symbol of circle with a center dot indicates a surveyed position.  Great for taking accurate bearings.

 

Symbol of circle without a center dot indicates an approximate position.

 

Landmarks on private property are not plotted.

 

  

·       Wrecks, Rocks, and Reefs.

 

May be shown on a chart with:

 

                                    Symbol

 

Abbreviation plus a number which indicates the depth of the water over the obstruction at low water datum.

 

Dashed or dotted lines around a symbol which calls attention to the hazardous nature of the obstruction.

 

·       Bottom Characteristics

 

Symbol or abbreviation used alone or in combination.

 

·       Structures.

 

Jetties, docks, drawbridges, and ramps.

 

Some docks do not appear on a chart depending on the scale of the chart.

 

·       Coastlines.

 

                        Identified at both high and low water.

 

Any prominent landmarks that will help you get a fix on your position may be shown.

 

Details located a short distance from the shoreline may not be shown.

 

·       Lettering on a Nautical Chart.

 

                        Vertical lettering indicates fixed objects.

 

                        Slanted lettering indicates floating objects.

 

 

14. Metrification of Charts

 

·       NOAA policies for change to the Metric System.

 

                        All charts will be converted to the Metric System.

 

                        Safety of navigation will be their primary goal.

 

Charts will be converted in logical groupings to minimize the need to shift between tow different measurement systems.

 

                        Conversion will be a 10 to 15 year effort.

 

·       Things that will change:

 

                        Depths

 

                        Depths over submerged objects.

 

                        Heights of objects.

 

                        Bridge clearances both horizontal and vertical.

 

                        Overhead cables and pipeline clearances.

 

                        Drying heights of objects.

                                    (How far they stick out of the water at low tide.)

 

                        Elevations.

           

                        Land contour intervals and labels.

 

                        Tide values [Tidal Information Block].

 

Charted notes associated with depth, distances, heights, and elevations.

 

 

15. Accuracy of a Nautical Chart.

 

·       Accuracy is based on the survey that was made to generate the chart.

 

·       Natural events are constantly changing things on a chart.

 

                        Hurricanes

 

                        Earthquakes

 

                        Erosion

 

                        Natural wave action

 

                        Floods.

 

·       Criteria used to determine the accuracy of a chart.

 

1.     Source and date of the survey.

 

2.     Amount of detail of the soundings in an area.

 

3.     Blank spaces should be regarded with suspicion.

 

4.     Term “Dumping Ground” indicates that material was dumped in the area and that the soundings in the area have not been verified.

 

·       North American Datum of 1983.

 

                        Vertical datum on a chart determines depth and heights.

 

Horizontal datum on a chart references the placement of line of Latitude and Longitude.

 

The 1983 Datum made changes to charts:

 

Relative distances did not change.

 

There was a shift in the Lat/Lon grid lines.

 

Be careful when using charts with different datums.

 

·       Effect of 1983 Datum shifts in Lat/Lon:

 

                        Great Lakes area                           15 yards

 

                        Atlantic Coast                                  35 yards

 

                        Gulf of Mexico                              35 yards

 

                        Pacific Coast                                  100 yards

 

                        Hawaii                                               500 yards

 

                        Alaska                                              500 yards

 

                        Puerto Rico                                                240 yards

 

·       The LNM - Local Notice to Mariners reports both datum positions when charted objects appear on charts with different horizontal datums.

 

·       Always use the latest edition of a nautical chart.