Position Flashcards
Graticule
a network of lines representing meridians and parallels, on which a map or plan can be represented.
‘the graticule of latitude and longitude’
Latitude
Go around the earth
The distance from the equator, expressed as an angle, measured in degrees at the centre of the earth.
Longitude
The angle between the prime meridian and the meridian of the place.
What comes first, latitude or longitude?
latitude then longitude.
Horizontal Datums
Historical graticules of longitude and latitude that don’t always marry up.
Means that the lat/long of a fixed object may vary
What charts were used until recently in British waters?
Ordnance Survey Great Britain 1936 (OSGB 36)
What charts were used until recently in European waters?
European Datum 50 (ED 50)
Earth is slightly bigger and slightly more squashed than OSGB 36
Most widely used system for marine charts
World Geodetic System 1984 (WGS 84)
some use WGS 72
What are many new European waters charts based on?
European Terrestrial Reference System 1989 (ETRS 89)
Why are ETRS 89 and WGS 84 different?
There are technical differences between the two charting systems because Europe and America are drifting apart at the rate of several centimetres/year
When are chart datums important?
When using an electronic navigator/ chart plotter, it is important to make sure it is using the same datum as your positioning system.
When are chart datums not important?
When using traditional methods.
Where is the datum stated on British charts?
In the info section underneath the title.
What are the datum options on an electronic positioning system?
Automatic
Arithmetic
Manual
Datum automatic conversion example
If you were sailing between the south coast and channel islands your charts for each area might be in different datums
SC could be in OSGB 36
CI in ED 50
Your satnav is set to WGS 84, so first you have to change to OSGB 36
Then, halfway through the channel, you would need to switch from OSGB 36 to ED 50. Then the waypoints that you stored manually will look different on screen because they too will be converted AUTOMATICALLY from OSGB 36 to ED 50.
