Marine Charts & Navigation
“Believe me, my young friend, there is nothing –
absolutely nothing – half so much worth doing as
simply messing around in boats.”
Water Rat in The Wind in the Willows
Background
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Charting US coastal waters began in 1807
• Office of Coast Survey (OCS)
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Oldest Scientific Organization in the US
Part of the National Ocean Service
Navigation – the art and science of
locating the position and plotting the
course of a vessel
• Where are You?
• What direction do you need to head to get to
where you want to be?
• How far away is it and how long will it take to
get there?
• How do we maneuver to safely reach a desired
destination?
Modern Nautical Charts
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Graphical portrayal of the marine environment
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General configuration of the sea bottom
Water depths
Currents
Locations of dangers to navigation (wreck, rocks,
sandbars)
• Information about aids to navigation (buoys, beacons,
and conspicuous objects)
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Traditional Paper Charts and Electronic Formats
• http://chartmaker.ncd.noaa.gov/staff/charts.htm
• United States Coast Pilot
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Supplemental information
• nowCOAST – map-based internet gateway to real-time
observations and forecasts of marine weather and
oceanographic conditions
The Coordinates
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Meridians of Longitude
• Set of circles that pass through the north and south
poles
• Prime Meridian - Reference point is the meridian passing
through Greenwich, England. Represented by 0
degrees.
• Measured by how many degrees east or west of PM
(maximum is 180 degrees)
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Parallels of Latitude
• Set of circles that would lie on plane surfaces cutting
through the Earth perpendicular to the polar axis
(perpendicular to meridians of longitude)
• Equator is the reference point.
• Measured by how many degrees north or south of the
Equator
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Minutes and Seconds – fractions of degrees (60 minutes in
one degree and 60 seconds in one minute)
1° latitude = 60 nautical miles (1' =1 NM)
1 Nautical mile = 1.15 Statute Mile (1 degree = 60 NM =
69 SM)
Knot (speed used at sea) = 1 NM per hour
• Useful conversion – D=RT
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D=Distance (in nautical miles)
R=Rate (in knots)
T=Time (in hours)
Relationship between Longitude
and Time
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360 degrees in a circle
24 hrs in the circle that a person standing on the
earth travels between noon one day and noon the
next
360/24 hours = 15 degrees/hr
0 degrees is the starting point for time zones
Local apparent noon is when the sun is directly
overhead
• Noon GMT = 0600 on DI (88/15=6hrs)
• So if you know when the sun is directly overhead and
the time in Greenwich, you can calculate longitude (the
difference in time is distance)
Evaluate Your Chart
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Locale
Unit of Soundings
(may be in Fathoms)
Datum Plane - A
permanently established
horizontal plane, surface,
or level to which
soundings, ground
elevations, water surface
elevations, and tidal data
are referred
Date of the Survey
Title Block
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Isobaths – connect
similar depths
Channels and Markers
– Odd/Green,
Even/Red
Nature of the Bottom
Obstructions
Landmarks
Compass Rose
Shows clearly the equivalent magnetic and true north directions
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Magnetic North – to which the north-seeking pole
of a magnetic compass points
True North – geographic north
Cardinal points - N(0º), E(90º), S(180º), W(270º)
Intercardinal points – northeast, southeast,
southwest, northwest
Vocabulary
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Course – intended direction of travel (expressed
in degrees). Wind, ocean currents and pilot error
may change the ship’s course.
Heading – actual direction of travel, regardless of
course.
Drift – deviation from course due to wind or
current
Bearing – direction from one point to another
expressed as an angle from north
Range – distance from one point to another
Waypoint – point between legs of a route where
direction is changed
How to “fix” a vessel’s
position on a chart
Find the desired latitude on
the scale on the sides of
the chart and connect
them.
Note that the scales on the
sides of the charts will be
in minutes and seconds –
not degrees!
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Do the same for longitude
(top and bottom)
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Where the two lines cross
will be the designated
position
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How to determine
distance
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Use a drafting
compass or pair of
dividers
Spread the points so
that they touch the
ends of the length to
be converted
Transfer the points
directly to the bar
scale to obtain
distance (or 1 minute
= 1 NM)
Use distance and bearing lines of
position to fix position
Within site of land:
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Technique 1:
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Take bearings with a compass on at least 2 landmarks and plot the
lines on a chart
The vessel is located where the two lines cross
Technique 2:
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Use a range finder to get the distance to two land objects
Draw circles with those radii
The vessel is at the intersection of the arcs
Beyond site of land:
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RDF – Radio direction finding
Dead Reckoning – uses D=RT (difficult to account for drift)
Loran – Long Range navigation
GPS – Global Positioning System
GPS
Receiver computes the distance to a satellite based on
amount of time required for a radio signal from the satellite
to reach the receiver
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Based on Trilateration
24 Satellites – launched, operated and maintained
by US Air Force
Global Network of ground monitoring stations –
precisely determined
CORS (continuously operating GPS reference
stations) – stationary and permanently operating
receivers that receive GPS signals and transmit
position data
Deployment
Constellation
•21 Satellites (Plus
3 Operational
Spares
•55º Inclination
•Repeating Ground
Track
•4 Satellites
always in view
Geocaching
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New generation of outdoor game –
“treasure hunt”
Use GPS receiver to navigate to the
specific latitude and longitude of a
hidden container
www.geocaching.com
Sample Questions
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Deploy a buoy 500 yards N of a
platform at coordinates X in 54 ft of
water. What are the coordinates of
the buoy? (1nm=2027 yards)
How long does it take to get there
from DISL at 9 knots?
How long does it take to get from
30º30’N to 30º40’N at 5 kts?