General Navigation - Directions & Coordinates Flashcards
Directions & Coordinates
What are two terms for the shape of the earth? What do they mean?
Oblate spheroid
Ellipsoid
The earth is slightly fatter at the equator
What is the Geoid?
The shape that the ocean surface would take under the influence of gravitation and the Earth’s rotation alone, if other influences such as wind and tides were absent
What is the history of the Geoid?
It has been an important concept for 200 years, but has been defined to high precision only since advances in satellite geodesy
What’s a reference ellipsoid?
A mathematically defined surface that approximates the geoid
What are Reference Ellipsoids used for?
As a preferred surface on which geodetic network computations are performed
What’s the most common geodetic reference system?
World Geodetic System of 1984
WGS 84
How are the poles defined?
The extremities of of the axis of the Earth’s rotation
What is the direction of Earth’s rotation?
Anti-clockwise from above
What is true north?
The direction from any position on earth to the north pole
How might ‘East’ be defined? And West?
The direction is which Earth rotates
The opposite
What are Rhumb lines?
Lines of constant track
The Rhumb Line Track between two points of the same latitude would follow…
What would be the track?
A parallel of latitude
90degT
270degT
What would be the shortest route between two points of the same latitude?
a) The rhumb line
b) A Great Circle Track
B
What is another name for parallels of latitude other than the equator?
Small circles
What would your track be if tracking along a meridian?
000T or 180T
What are lines of longitude called?
Meridians
What’s a meridian’s opposite on the other side of the Earth called
Anti-meridians
How many parallels of latitude are there?
Infinite
Parallels of latitude and meridians of longitude are a…system
Position reference
What determines the accuracy of a position reference system which uses latitude and longitude?
The size of the units used
What are the units used for defining position using lat and long?
Degrees (°) and minutes (‘)
How are parallels of latitude described?
By their angle, measured at the centre of the Earth, above or below the equator
What are the maximum possible latitudes?
90°N or 90°S
How do we calculate changes in latitude?
If in the same hemisphere, we subtract the smaller from the larger
If in different hemispheres, we add one to the other
How might changes in latitude calculations be performed on a scientific calculator?
N 65° 30’ - = 65 DMS 30 DMS - 32 DMS 14 DMS
N 32° 14’
How are meridians of longitude described?
By their angle away from the Greenwich/Prime meridian
Find another way to phrase how meridians of longitude are described
Longitude is the angle measured at the polar axis, between the plane of the prime meridian and the local meridian
What’s the maximum longitude of a meridian of longitude? What else can this be called?
180°
The Greenwich anti-meridian
All meridians point towards…
How is this shown?
The north pole
With a north arrow at the end of each meridian, identifying north
How is made clear that a number is referring to a meridian of longitude?
By using three digits and E/W
What’s the change of longitude between 178°E and 178°W?
Why?
4°
Change of longitude is always expressed in the shortest way around Earth
What’s the rule for calculating change in longitude?
Add the two given and if the answer is more than 180°, subtract it from 360°
If in the same hemisphere, subtract the smaller from the larger
How accurately do modern navigation systems tend to display positions to?
One tenth of a minute
How might we find the exact opposite position of a spot on Earth?
Change the symbol for latitude e.g. N becomes S
Subtract longitude from 180
What’s the approximate circumference of Earth in KM?
40,000KM
What’s the approximate circumference of the Earth in NM?
21,600NM
How do we calculate radius from circumference?
By reverse engineering 2 x pi x r
What’s the other name for the equatorial radius?
The semi-major axis
What is the polar radius in NM? And the equatorial radius? What’s the difference?
3429NM
3440NM
11NM
What’s the definition of the ratio of ellipticity?
A ratio to describe the difference in radius between the polar and equatorial radius
How might the ratio of ellipticity be mathematically described?
(a-b)/a
a= Equatorial radius
b= Polar radius
What’s the ratio of ellipticity for earth, expressed as a ratio and as a decimal?
1/298
0.0033
What calculation might we use to calculate a semi-major axis from a semi-minor one?
x1.0033
Divide for opposite calculation
What’s the most commonly used reference model for GPS systems? What ratio of ellipticity does it use?
World Geodetic System of 1984
WGS 84
1/298.25
How is Geodetic latitude calculated as opposed to geocentric? How might this be known?
Draw a line at right angles to the Earth’s surface and extend it to the plane of the equator
The plumb-line on the ellipsoid
Where will geocentric and geodetic latitudes be most different?
45°N and 45°S
What system for latitudes and longitudes is used for the WGS 84 ellipsoid?
Geodetic
How is geodetic longitude defined?
The angular distance between the prime meridian and another meridian passing through a point on the Earth’s surface
Why is the value the same for geodetic and geocentric types of longitude?
Because the vertex of the angle is the centre of the Earth - the same as for geocentric longitude
