Navigation NAU 102 Lesson 2

The Earth News Flash! It isn’t flat. But, it isn’t a perfect sphere either.

The Earth The Earth is an oblate spheroid. Equatorial diameter: 6,888 nm Polar diameter: 6,865nm Considered a perfect sphere for navigation purposes. A sphere flattened at the poles. (1 nm = 6,076.1 ft = 1.15 statute mile)

Where on Earth? The rotation of the Earth can be used to distinguish reference points and lines. Coordinate systems require reference (starting) points. Locations can be identified using coordinate systems.

Where on Earth? The Earth rotates on an axis. North & South Poles – the extreme ends of the axis at the surface of the earth. That gives us 2 reference points!

Reference Lines Great Circle A line on the surface formed by the intersection of a plane passing through the center of the earth.

Reference Lines Great Circle The largest circle that can be drawn on the surface of a sphere. The shortest path between any 2 points on a sphere.

Reference Lines Small Circle A line on the surface formed by the intersection of a plane which does not pass through the center of the earth.

Reference Lines Meridian A great circle passing through the poles.

Reference Lines Meridians Upper Branch – that half of a meridian extending from the north pole to the south pole, passing through a particular point on the surface. Lower Branch – the other half of the meridian, on the opposite side of the earth.

Reference Lines Prime (or Greenwich) Meridian – the upper branch of the meridian which passes through the Royal Observatory in Greenwich, England.

Reference Lines Equator A great circle, perpendicular to the axis and equidistant from the poles.

Reference Lines Parallels Small circles, parallel to the equator. Also called parallels of latitude.

Reference Lines Latitude (Lat) - The angular distance from the equator to a parallel. 0° at the equator 90° at the poles Labeled north (N) or south (S) Measured along a meridian

Reference Lines Longitude (Lon) - The angular distance from the prime meridian to the meridian of a point. Measured along a parallel 180° at the lower branch of Greenwich 0° at prime meridian Measured east (E) or west (W)

Geographic Coordinates Stated in Degrees (°) 360° in a circle 60 seconds (") in a minute 60 minutes (') in a degree

Geographic Coordinates Position Conventions Latitude then longitude Degrees (2 digits), minutes (2) and seconds (2) e.g. 08° 15' 30" N Or, Degrees (2 digits), minutes (2) and tenths (1) e.g. 08° 15.5' N Latitude format:

Geographic Coordinates Degrees (3 digits), minutes (2) and seconds (2) e.g. 096° 24' 42" W Or, Degrees (3 digits), minutes (2) and tenths (1) e.g. 096° 24.7' W Longitude format:

Distances How long is a degree of latitude? (6865 nm * π) / 360° = nm / ° Lat = nm / ' Lat 60 '/° For navigation purposes 1' Lat = 1 nm

Distances How long is a degree of longitude? It Varies! At the equator: (6888 nm * π) / 360° At the poles 1° = 0 nm = 60.1 nm / ° Lon

Great Circle vs. Rhumb Line Great Circle the shortest line between 2 points direction constantly changes cannot steer a great circle route Rhumb Line crosses all meridians at same angle greater distance than GC

Great Circle vs. Rhumb Line

Rhumb Line If a vessel steers one course continually, it will spiral to the pole. A loxodrome.

Directions Measured in degrees of arc from a reference. Ship’s heading = relative direction (R) References: Geographic North = true direction (T) Direction – angular orientation between two points. Magnetic North = magnetic direction (M)

Directions Definitions Course made good (CMG) – resultant direction traveled between 2 points. Course over ground (COG) – actual path of travel Track (Tr) – intended path of travel Heading (Hdg.) – direction the vessel is pointed. Course (C)– intended direction of vessel.

Direction – Which way?

Directions Definitions (cont.) Bearing - Direction of one object to another

Geodesy The science of measuring the Earth and positioning of points. Measured mathematically based on geometric models The actual Earth is irregular (mountains, valleys, etc.) An ellipsoid is used to simplify the math.

Datums North American Datum, 1927 (NAD 27) Coordinates computed from a point in Kansas. Latitude and Longitude of locations are computed by triangulation from known points. European DatumTokyo Datum Indian Datum, etc.

Datums

Different starting points Different ellipsoids We have a Problem! + = Different computed latitude and longitude

World Geodetic System Based on multiple points One worldwide ellipsoid One system for the entire world. Satellite technology for determining position Result – a standardized method of computing position WGS - 84

Datum Shift No problem when piloting Different datum = different computed position Big problem when using GPS! GPS uses WGS - 84 GPS positions must be adjusted prior to plotting on older charts.

Datum Shift

Introduction to Navigation Questions?