Lesson 2 Chart Projections and Numbering

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Lesson 2 -Chart Projections and Numbering

• Learning Objectives
• State and discuss the properties of the most
  commonly used chart projections.
• Comprehend the chart / publication correction
  system.
• Comprehend the chart numbering system.
• Applicable reading Hobbs, pp 19-50.

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Chart Projections and Numbering

• Desirable qualities of a chart projection
• Correct angular relationships
• Representation of areas in their correct
  proportions relative to one another
• True scale
• Rhumb lines (lines on the surface of the earth
  that cross all meridians at the same angle)
  represented as straight lines
• Great circles represented as straight lines
• The mercator projection is the most widely used
  projection in marine navigation.
• Its advantages are 1) Position, distance, and
  direction can all be easily determined, and 2)
  True shape of features is maintained for small
  areas (it is conformal) .
• Its disadvantages are 1) Distortion of true size
  of surface features increases with distance from
  the equator, and 2) Great circles appear as
  curved lines.

3
Rhumb Line
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Chart Projections and Numbering

• The mercator is a cylindrical 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 from within the earth.
• The amount of lateral distortion steadily
  increases with distance from the equator .
  Consequently, the latitude scale must be
  expanded to maintain conformality (true shape).
  The latitude scale is expanded mathematically on
  all mercator charts.

(Overhead 2-1)
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Chart Projections and Numbering

• The gnomonic projection is a geometrical
  projection in which surface features and the
  reference lines of the sphere are projected
  outward from the center of the earth onto a
  tangent plane. There are three general types of
  gnomonic charts based on the location of the
  point of tangency
• equatorial gnomonic (tangent at the equator)
• polar gnomonic (tangent at either pole)
• oblique gnomonic (tangent at any other latitude).

(Overhead 2-2)
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Chart Projections and Numbering

• Gnomic projection
• Its advantages are 1) Great circles appear as
  straight lines. Due to this attribute, the
  gnomonic chart is mainly used to plot the optimum
  great circle route (shortest path) between two
  points. These points are then picked off the
  gnomonic chart and plotted on a mercator chart,
  and 2) Distortion is tolerable within 1,000
  miles of the point of tangency.
• Its disadvantages are 1) Rhumb lines appear as
  curved lines, 2) Distance and direction cannot be
  measured directly, and 3) It is not conformal
  (true shapes are not presented).

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Chart Projections and Numbering

• Gnomic projection vs. Mercator projection

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Chart Projections and Numbering

• Two government activities are mainly responsible
  for producing nautical charts - the Defense
  Mapping Agency and the National Ocean Service.
• The Defense Mapping Agency (DMA) is concerned
  mainly with the production and upkeep of charts
  and related navigational publications covering
  all ocean areas of the world outside U.S.
  territorial waters.
• The National Ocean Service is concerned with
  charts covering inland and coastal waters of the
  United States and its possessions.

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Chart Projections and Numbering

• All charts produced by DMAHTC and NOS are
  assigned a number from one to five digits,
  according to the scale and area they depict. The
  relationship between the number of digits and the
  chart scale is shown below
• No. of digits Scale
• 1 No scale
  involved
• 2 1 9,000,001
  and smaller scale
• 3 1 2,000,001
  to 1 9,000,000
• 4 Miscellaneous
  and special
• 
  nonnavigational charts
• 5 1 2,000,000
  and larger scale
• Charts having a single digit I.D. number are
  not really charts, but supporting publications
  that do not have a scale.
• Large ScaleSmall Area
• Small Scale Large Area

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Chart Projections and Numbering

• Charts with two or three digit I.D. numbers are
  of suitable scale to depict large ocean basins
  and subdivisions . All ocean basins are
  included in one of nine basin areas. The first
  digit of a two or three digit chart indicates
  the basin in which the area is represented.
  These charts are of relatively small scale and
  depict large areas.
• Charts with a five digit I.D. number are of
  suitable scale to depict coastal regions with the
  great detail necessary for piloting. These
  charts are of relatively large scale and depict
  small areas. All coastal areas of the world are
  divided into nine coastal regions (their
  numbering is different from the ocean basins).
  The nine coastal regions are further divided into
  52 subregions. The first two digits of the
  chart identify the coastal region and subregion
  in which the charted area is located. The last
  three digits place the chart in geographic
  sequence in the subregion.
• The chart numbering system indicates the scale
  and area the chart depicts and also enables the
  navigator to organize the charts into portfolios.

(Overhead 2-4)
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Chart Projections and Numbering
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Chart Projections and Numbering

• CHART CORRECTION SYSTEM Navigation information
  periodically changes, but it is not practical
  to constantly reprint sets of charts. The means
  by which periodic corrections to DMAHTC and NOS
  charts and publications are disseminated are the
  Notice to Mariners and Local Notice to
  Mariners. These bulletins are in pamphlet form
  and distributed weekly by mail. They contain all
  corrections, additions, and deletions to DMAHTC
  and NOS charts. Notice to Mariners are issued by
  DMAHTC and contain all changes relating to
  oceanic and coastal areas worldwide, while Local
  Notice to Mariners are by the U.S. Coast Guard
  districts and contain changes pertaining to U.S
  inland waters .
• Notice to Mariners consist of three sections
  which list corrections by chart, publication and
  item number.
• Navigators are not required to enter every
  change on every chart. Instead, corrections to
  charts are entered on correction cards (a card is
  kept on file for each chart) when they are
  received.
• Changes are entered on a chart when a ship is
  scheduled to operate in the area the chart
  covers.

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Chart Projections and Numbering

• - The DMAHTC semiannually issues a set of five
  summary of correction volumes to ensure ships
  receive and record all chart corrections. The
  volumes are
• cumulative and contain corrections from
  previous volumes as well as all pertinent
  information from the last six months Notice to
  Mariners.

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Chart Projections and Numbering

• Radio Broadcast Warnings are used to quickly
  promulgate important changes affecting safety of
  navigation that cannot wait for then printing of
  the Notice to Mariners (the changes will be
  reflected in the next Notice to Mariners).
• Broadcast Notice to Mariners are transmitted as
  required by various Coast Guard, Navy and
  commercial radio stations.
• Worldwide Navigational Warning System is run by
  participating nations and applies to mid-ocean
  areas. For purposes of the system, the ocean is
  divided into 16 NAVAREAS with each nation
  assuming responsibility for certain areas.
• HYDROLANTS and HYDROPACS provide coverage that
  is somewhat redundant to the NAVAREA system.
• The DMA Automated Notice to Mariners System
  (ANMS) provides navigational data via satellite
  communication links and allows users to obtain
  and provide a variety of worldwide navigational
  information on a daily basis.

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Homework

• Reading
• Hobbs pp 75-105
• Workbook
• Chapter 2 2,3, 4
• Chapter 3 1, 2, 4, 5, 7,8,9,12
• Chapter 4 Section 1 - 2, 8 Section 2 -
  2,3,4,5,6
• Section 3 - 7, 8