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Direction, Latitude and Longitude Chapter *

On the Greenwich Meridian, the Eastern Longitudes are East of you. The Western Longitudes

are West of you. But now travel round the globe to the 180 E /W longitude (the Greenwich anti-

meridian). The direction of East has not changed. It is still 090 T, the direction of the Earth’s

spin. Nor has the direction of West. It is still 270 T, the opposite direction to the Earth’s spin.

But in Figure 1.13, the Eastern Longitudes are out to your left (West of you) and the Western

Longitudes are out to the right of you (East of you). So you get a situation where the Eastern

hemisphere to your West and the Western Hemisphere is to your East. It can cause confusion in

solving some navigation problems.

DIFFERENCE IN PRINCIPLE BETWEEN LATITUDE AND LONGITUDE

These 2 angular co-ordinate systems are quite diferent in principle. Lines of latitude are all

parallel to each other. Indeed, it is normal to talk of ‘parallels of latitude’. Lines of longitude,

however, emanate from a point (the North Pole), reach a maximum separation at the Equator,

and then converge back to a point again, at the South Pole.

You may fnd it a helpful illustration to think of latitude as like slicing a pineapple (parallels of

latitude) whilst longitude is like segmenting an orange (meridians of longitude).

POSITIONS IN LATITUDE AND LONGITUDE

When giving a position in latitude and longitude, the latitude is always quoted frst and the

longitude second : thus New York’s position is 41 N 074 W. Alternative forms are: 41 00’N

074 00’W, or 4100N 07400W which are used to allow more precision by the inclusion of minutes

of arc. For example, the Aerodrome Reference Point (ARP) for Oxford Airport is approximately

51 50’ N, 001 19’W or 5150N 00119W.

Even greater precision can be achieved by quoting position in seconds of arc although this is

not normally required in these notes. For example, the precise position for the Oxford ARP is

51 50’12”N, 001 19’11”W or 515012N 0011911W.

(As a point of interest, the ARP for Oxford is at the midpoint of R/W 02/20)

CONVERSION OF LATITUDE AND LONGITUDE TO DISTANCE ON THE EARTH

The defnition of a nautical mile has been chosen to make it relate to angular arc on a Great

Circle. It so happens that the mean radius of the Earth is approximately 20.9 million feet in

length. This means that on a Great Circle, 1 minute of arc corresponds to a distance on the

Earth’s surface of 6080 feet. This distance of 6080 feet is defned as 1 nautical mile.

Therefore you can relate angle along a Great Circle arc to distance on the Earth. For instance, all

meridians (lines of longitude) are Great Circles, so the distance up or down a meridian (which

is change of latitude) has this relationship. The diference in latitude between positions 5000N

00100W and 5005N 00100W is exactly 5 minutes, so they are exactly 5 nautical miles apart. This

is true of diferences in longitude only at the Equator, because the Equator is the only parallel of

latitude which is also a Great Circle.

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