navigation nau 102 lesson 2. the earth news flash! it isn’t flat. but, it isn’t a perfect sphere...
TRANSCRIPT
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" 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?