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LESSON 02: Terrestrial Coordinate System/ Chart Projections and Numbering Learning Objectives: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.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.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.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.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.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: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 EquatorEquator MeridianMeridian

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Terrestrial Coordinate System Small Circle- any circle not passing through the center of the earth.Small Circle- any circle not passing through the center of the earth.

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Terrestrial Coordinate System Meridians of longitude are great circlesMeridians of longitude are great circles Parallels of latitude are small circles (with the exception of the equator)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 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.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:Meridians are divided in half: –upper branch –lower branch Prime MeridianPrime 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 earth’s surface. Longitude is measured in degrees of arc from 0 to 180 degrees, either east or west.The angular distance between the Greenwich meridian and the meridian passing through a particular point on the earth’s 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 earth’s surface. Latitude is measured in degrees of arc from 0 to 90 degrees, either north or south.The angular distance between the Equator and the parallel passing through a particular point on the earth’s 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).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.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: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 earth’s axis.Imagine a cylinder rolled around the earth, tangent at the equator, and parallel to the earth’s axis. Meridians appear as straight vertical lines when projected outward onto the cylinder.Meridians appear as straight vertical lines when projected outward onto the cylinder.

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Mercator Projection

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Advantages Position, Distance, and direction easily determined.Position, Distance, and direction easily determined. True shape of features is maintained for small areas (conformality)True shape of features is maintained for small areas (conformality) Rhumb lines plot as straight lines.Rhumb lines plot as straight lines.Disadvantages Distortion of true size of surface features increases with distance from the equator.Distortion of true size of surface features increases with distance from the equator. Great circles appear as curved lines.Great circles appear as curved lines.

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Gnomonic Projection Surface features and reference lines on the earth’s surface are projected outward from the center of the earth onto a tangent plane.Surface features and reference lines on the earth’s surface are projected outward from the center of the earth onto a tangent plane. Three basic types, depending on point of tangency: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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Advantages Great circles appear as straight lines (shortest distance between two points)Great circles appear as straight lines (shortest distance between two points) Tolerable distortion within 1000 miles of the point of tangencyTolerable distortion within 1000 miles of the point of tangencyDisadvantages Rhumb lines appear as curved linesRhumb lines appear as curved lines Distance and direction cannot be measured directlyDistance and direction cannot be measured directly Not conformal (true shapes are not presented)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.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.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:Two government activities are responsible for producing charts: Defense Mapping Agency (DMA)Defense Mapping Agency (DMA) –ocean areas of the world outside U.S. territorial waters National Ocean Service (NOS)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 chartAll charts are assigned a 1 to 5 digit number, depending on the scale of the chart No. of digitsScaleNo. of digitsScale –1no scale (supporting publications) –21:9,000,001 and smaller –31:2,000,001 to 1:9,000,001 –4misc and special non-nav charts –51:2,000,000 and larger

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Chart Numbering System 1 Digit: supporting publications1 Digit: supporting publications 2 or 3 Digits: depict large ocean basins and subdivisions. First digit is the ocean basin (there are nine worldwide).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.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 regionThe first number is the region The second number is the subregionThe second number is the subregion The last three numbers represent the geographic sequence of the chartThe 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 systemThus, 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: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!Navigation is constantly changing! It is not practical to constantly reprint navigational charts and publications.It is not practical to constantly reprint navigational charts and publications. DMA and NOS disseminate corrections using two 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.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.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.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.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 MarinersBroadcast Notice to Mariners Worldwide Navigation Warning System (NAVAREAS)Worldwide Navigation Warning System (NAVAREAS) HYDROLANTS and HYDROPACSHYDROLANTS and HYDROPACS DMA Daily MemorandaDMA Daily Memoranda Special Warnings (USN or USCG)Special Warnings (USN or USCG) Automated Notice to Mariners System (ANMS)Automated Notice to Mariners System (ANMS)

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