LESSON 02: Terrestrial Coordinate System Chart Projections and Numbering

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LESSON 02Terrestrial Coordinate System/ Chart
Projections and Numbering

• Learning Objectives
• Comprehend the terrestrial coordinate system
• Comprehend the location of positions on the earth
  using latitude and longitude
• Comprehend the basic properties of the most
  commonly used chart projections

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Terrestrial Coordinate System

• The earth is an oblate spheroid, but for
  navigational purposes it is considered a perfect
  sphere with a circumference of 21,600 NM.
• On a perfect sphere at rest, all points on the
  surface are similar reference points must be
  designated in order to make any type of
  measurements.

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Terrestrial Coordinate System

• When rotation is introduced, the spin axis
  introduces two reference points, the north and
  south poles.
• The spin axis of the earth, together with its
  poles, constitutes the basic reference points on
  which the terrestrial coordinate system is based.

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Terrestrial Coordinate System

• Great Circle the intersection of a plane
  passing through two points on the surface of the
  earth and the center of the earth.
• Some key points
• A great circle is the largest circle that can be
  drawn on the face of the earth.
• A great circle represents the shortest distance
  between two points on the surface of the earth.

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Terrestrial Coordinate System

• Equator
• Meridian

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Terrestrial Coordinate System

• Small Circle- any circle not passing through the
  center of the earth.

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Terrestrial Coordinate System

• Meridians of longitude are great circles
• Parallels of latitude are small circles (with the
  exception of the equator)

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Latitude and Longitude

• The equator divides the earth into the northern
  and southern hemispheres and is the reference for
  parallels of latitude.
• The prime meridian passes through the original
  position of the Royal Greenwich Observatory. It
  serves as the reference for meridians of
  longitude.

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Latitude and Longitude

• Meridians are divided in half
• upper branch
• lower branch
• Prime Meridian
• upper branch is known as the Greenwich meridian
• lower branch is the 180th meridian

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Longitude Defined

• The angular distance between the Greenwich
  meridian and the meridian passing through a
  particular point on the earths surface.
  Longitude is measured in degrees of arc from 0 to
  180 degrees, either east or west.

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Latitude Defined

• The angular distance between the Equator and the
  parallel passing through a particular point on
  the earths surface. Latitude is measured in
  degrees of arc from 0 to 90 degrees, either north
  or south.

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Longitude
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Latitude
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Measurement of Distance

• Since latitude lines are parallel, the length of
  one degree of latitude is the same everywhere on
  earth (60 NM).
• As the distance from the equator increases, the
  length in miles of one degree of longitude
  decreases, so NEVER use the longitude scale to
  determine distances on a chart.

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Measurement of Distance
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Chart Projections

• Desirable qualities of a chart projection
• Correct angular relationships
• Representation of areas in their correct
  proportions relative to one another
• True scale
• Rhumb lines represented as straight lines.
• Note Rhumb lines are lines on the surface of
  the earth that cross all meridians at the same
  angle. Ships on a constant course follow rhumb
  lines.
• Great circles represented as straight lines

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Mercator Projection

• Imagine a cylinder rolled around the earth,
  tangent at the equator, and parallel to the
  earths axis.
• Meridians appear as straight vertical lines when
  projected outward onto the cylinder.

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Mercator Projection
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Mercator Projection

• Advantages
• Position, Distance, and direction easily
  determined.
• True shape of features is maintained for small
  areas (conformality)
• Rhumb lines plot as straight lines.

• Disadvantages
• Distortion of true size of surface features
  increases with distance from the equator.
• Great circles appear as curved lines.

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Gnomonic Projection

• Surface features and reference lines on the
  earths surface are projected outward from the
  center of the earth onto a tangent plane.
• Three basic types, depending on point of
  tangency
• equatorial gnomonic (tangent at equator)
• polar gnomonic (tangent at either pole)
• oblique gnomonic (tangent somewhere else)

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Gnomonic Projection
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Gnomonic Projection

• Advantages
• Great circles appear as straight lines (shortest
  distance between two points)
• Tolerable distortion within 1000 miles of the
  point of tangency

• Disadvantages
• Rhumb lines appear as curved lines
• Distance and direction cannot be measured
  directly
• Not conformal (true shapes are not presented)

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Gnomonic Projection
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Gnomonic vs. Mercator

• Gnomonic
• Since great circles appear as straight lines,
  used to determine the shortest route between two
  points. This information is then transferred to
  a Mercator chart.

• Mercator
• Used for everyday navigation, due to the ease of
  measurement of position, distance, and direction.

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Gnomonic vs. Mercator
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Nautical Charts

• Two government activities are responsible for
  producing charts
• Defense Mapping Agency (DMA)
• ocean areas of the world outside U.S. territorial
  waters
• National Ocean Service (NOS)
• inland and coastal waters of the U.S. and its
  possessions

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Chart Numbering System

• All charts are assigned a 1 to 5 digit number,
  depending on the scale of the chart
• No. of digits Scale
• 1 no scale (supporting publications)
• 2 19,000,001 and smaller
• 3 12,000,001 to 19,000,001
• 4 misc and special non-nav charts
• 5 12,000,000 and larger

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Chart Numbering System

• 1 Digit supporting publications
• 2 or 3 Digits depict large ocean basins and
  subdivisions. First digit is the ocean basin
  (there are nine worldwide).

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Ocean Basins of the World
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Chart Numbering System

• 5 Digits are of suitable scale to depict
  coastal regions with the great detail necessary
  for piloting.
• There are nine coastal regions in the world
• Each is divided into several subregions
• The first number is the region
• The second number is the subregion
• The last three numbers represent the geographic
  sequence of the chart

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Coastal Regions of the World
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Chart Numbering System

• Thus, the chart numbering system
• indicates the scale of the chart (by the number
  of digits in the chart number)
• indicates the area of the world depicted (ocean
  basin, subdivision, coastal region, and
  subregion)
• enables the navigator to organize the charts into
  portfolios

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Chart Scale

• An important point to remember
• SMALL SCALE LARGE AREA
• LARGE SCALE SMALL AREA

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Chart and Publication Correction System

• Navigation is constantly changing!
• It is not practical to constantly reprint
  navigational charts and publications.
• DMA and NOS disseminate corrections using two
  publications
• Notice to Mariners (DMA, worldwide coverage)
• Local Notice to Mariners (USCG, changes
  pertaining to U.S. inland waters)

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Chart and Publication Correction System

• Corrections must be made by hand to the affected
  chart or publication.
• Fortunately, not all changes are made. The
  corrections are kept on file, using a correction
  card for each chart.
• Changes are then entered on a chart when a ship
  is scheduled to operate in the area the chart
  covers.

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Chart and Publication Correction System

• Semiannually, DMA publishes a five volume summary
  of all corrections for the previous six months.

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Other Correction Resources

• Broadcast Notice to Mariners
• Worldwide Navigation Warning System (NAVAREAS)
• HYDROLANTS and HYDROPACS
• DMA Daily Memoranda
• Special Warnings (USN or USCG)
• Automated Notice to Mariners System (ANMS)