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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" NOr,

•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° = 59.91 nm / ° Lat

59.91 = 0.9985 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 Datum Tokyo Datum

Indian Datum, etc.

Datums

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?