Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
LESSON 02: Terrestrial 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 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. 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. 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. Terrestrial Coordinate System • Equator • Meridian Terrestrial Coordinate System • Small Circle- any circle not passing through the center of the earth. Terrestrial Coordinate System • Meridians of longitude are great circles • Parallels of latitude are small circles (with the exception of the equator) 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. 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 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. 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. Longitude Latitude 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. Measurement of Distance 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 Mercator Projection • 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. Mercator Projection 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. 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. • Three basic types, depending on point of tangency: – equatorial gnomonic (tangent at equator) – polar gnomonic (tangent at either pole) – oblique gnomonic (tangent somewhere else) Gnomonic Projection 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) Gnomonic Projection 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. Gnomonic vs. Mercator 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 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 1:9,000,001 and smaller –3 1:2,000,001 to 1:9,000,001 –4 misc and special non-nav charts –5 1:2,000,000 and larger 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). Ocean Basins of the World 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 Coastal Regions of the World 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 Chart Scale • An important point to remember: SMALL SCALE = LARGE AREA LARGE SCALE = SMALL AREA 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) 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. Chart and Publication Correction System • Semiannually, DMA publishes a five volume summary of all corrections for the previous six months. 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)