1:1 ON YOUR DATE
Latitude and Longitude
To define a position on the earths surface we use Latitude and Longitude.
There are 360 degrees in a circle. Latitude is measured in degrees and minutes North or South of the Equator the North Pole is 90º North, the Equator is 0º.
Longitude is measured in degrees and minutes West or East of Greenwich which is zero degrees, the Prime Meridian.
Latitude denotes North / South from the
Equator: the sides of
the chart. eg Latitude 51º 55' .70 N
Longitude denotes East / West from Greenwich: top and bottom of the chart. Longitude degrees are given in three figures eg 004º 35'.35
NB Parallel of Latitude, Meridian of Longitude. Greenwich is the Prime Meridian ( 0 degrees )
On the chart below London is approximately 50º N, 000º W. Tasmania is 40ºS, 150º E.
Since a degree of latitude is defined as 60 Nautical Miles (M), we need more precision. Each degree is divided into 60 minutes of arc; one minute of latitude is 1 Nautical Mile. Each minute is then divided up to 2 places of decimals.
1 Nautical Mile (M) = 1 minute
of arc of latitude
= 1852 metres
= 2000 yards
On RYA Chart 3 look at the bottom right and top left of the chart. The latitude runs from 45º 37.00 North to 46º 30.00 North, ie 53 minutes (53 Miles) in total from bottom to top.
Longitude runs from 005º 35.00 West to 006º 25.00 West, ie 50 minutes of longitude. (Do not use this for measuring distances).
Gilly Point (Southern Peninsula) is 45º 46.20 N, 005º 50.00 W.
The centre of the Compass Rose off Cape Woodward is 46º 07.20 N, 006º 15.00 W.A good exercise is to check the positions of the tidal stream diamonds the Lat and Long is given above each diamond (A to R) in the table at the top of the chart. The most accurate way is to use dividers, but a ruler will do quite well.
Posted 5th May 2011