NS310

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NS100
Fundamentals of Naval Science
The Nautical Chart
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Objectives
1. Understand the Terrestrial coordinate system
as used to determine position on the surface of
the earth. 2. Understand the concepts of
latitude and longitude, and how distances and
direction on the surface of the earth are
computed.
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Objectives
3. Understand the nautical chart with respect
to chart projections, symbology, rhumb
lines, great circles, chart scales and chart
accuracy.
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Reference Points
Equator - passes through the center of the
earth and is perpendicular to the axis of
rotation. 0o Latitude at the
equator. Poles - the earth rotates on its axis
and the North and South poles are where
the axis touches the surface of the
earth. 90o Latitude at the poles.
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Reference Points
Prime Meridian - Greenwich Meridian passes
through England and is the 000o
longitude. International Date Line - passes
through the middle of the Pacific Ocean and is
180o Longitude.
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Coordinate System
Latitude - (parallels) measured from the equator
from 00o to 90o north or south depending on the
which hemisphere.
Longitude - (meridians) measured from the Prime
Meridian from 000o to 180o east or West depending
on which hemisphere.
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North Pole
Longitude line
Equator
Latitude line
South Pole
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Coordinate SystemCont.
Position - the concept of latitude and Longitude
can be used to describe any point on
earth. Example Degreeso Minutes Seconds
39o 58 55 N 075o
28 45 W
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Great Small Circles
Great Circle - a line on the surface of the
earth formed when a plane passes through the
center of the earth.
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Great Small CirclesCont.
Great Circle - Divides the earth into equal
halves and represents the shortest distance
between two points on earth. Example The
equator separates the Northern from the
Southern Hemispheres. Example The Prime
Meridian separates the Eastern from the
Western Hemispheres.
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Great Small CirclesCont.
Small Circle - any other circle that is not
a great circle.
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Rhumb Lines Great Circles
Rhumb line - Crosses every meridian at
the same angle. A rhumb
lines course remains
constant. Great Circle - Shortest
distance between two
points but plot as a
curved line on Mercator charts.
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Nautical Charts

• A two dimensional representation of the earths
  surface (water). A chart should include
  soundings, coastlines, harbors, plus navigation
  aids and hazards.

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Chart Projections
Desirable Characteristics for Charts - True
shape of physical features. - True scale and
size of areas. - Correct angular
relationships. - Great circles plot as straight
lines. - Rhumb lines plot as straight lines.
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Mercator Chart (Cylindrical Projection)
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Mercator Chart (Cylindrical Projection)
Advantages - Rhumb line plot as straight
lines. - Ease of measurement (distance,
direction position). - Latitude/longitude
drawn as straight lines. Disadvantages - Great
circles plot as curves. - Distortion of features
at extreme latitudes.
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Mercator Projection
Imagine wrapping a sheet of paper around the
earth, with a light bulb in the center to project
the image of the earths surface.
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Mercator Projection
Unwrap the paper, and you have a mercator
projection chart.
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Mercator Chart Cont.
Composite Track 2495 NM
San Diego
Great Circle Route
Rhumb Line 2730 NM
Oahu, HI
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Gnomonic Chart (Tangent Point Projection)
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Gnomonic Chart (Tangent Point Projection)
Advantages - Great Circles plot as straight
lines, which ideal for long
voyages. Disadvantages - Rhumb Lines plot as
curves and do not plot easily on the
chart. - Distortion of features for the point
of tangency.
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Gnomonic Projection
Imagine placing a flat sheet of paper against
the surface of the earth, with the light bulb
still projecting an image of the earths surface.
point of tangency
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Gnomonic Chart Cont.
San Diego
Great Circle Route
Oahu, HI
Point of Tangency
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Chart Agencies
- National Imagery Mapping Agency NIMA -
National Ocean Service NOS - National Oceanic
Atmospheric Administration NOAA - Army Corps of
Engineers
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Chart Classification

• Sailing charts - scale 1600,000
• General charts - scale 1150,000 to
  1600,000
• Coastal charts- scale150,000 to 1 150,000
• Harbor charts - scale larger than 150,000
• Small craft charts - scale 180,000 and larger

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Chart Symbols
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Review Questions
1. What are two reference points for
latitude? 2. Describe a great circle? 3. Do
navigators normally use Mercator or
Gnomonic Projection Charts and why? 4. What is a
rhumb line? 5. Where would you look up a chart
symbol?