Welcome to Coastal Safety - here's our guide to latitude/longitude and GPS coordinates
the topics we'll discuss are, what is latitude
what is longitude the fact that one degree can be split into smaller
sixty minutes and the minutes again can be divided into decimals
and finally we'll talk about how to give a position by radio
first we'll look at an example of latitude longitude then we'll explain how the grid system works
discover where the starting point zero degrees zero degrees is
demonstrate through some work examples Then we'll talk about how to measure distances
and we'll finish off with some top tips
So here's a buoy, this is the south Cardinal boy and the question is what is the latitude?
you can see from the top-left corner the whole degrees is 24 degrees
here we can see the minutes. The minutes is between 28 and 29
so we're going to call it 28 minutes
and over on the right hand side we're going to put some red numbers on there to show the decimals
and it's point 8 minutes
so the latitude is 24 degrees 28 minutes point 8 north
now we're going to look what is the longitude?
so we' have drawn in the meridian of longitude and you can see from the top left it's 54 degrees
here's 28 minutes 29 minutes and 30 minutes
so we're 29 minutes
and we will put some decimals on there on the top right
there you can see we're going to be decimal 3 or point 3
so that means our latitude and longitude or the latitude longitude of that buoy is
24 degrees, 28 point 8 minutes North and 54 degrees 29 minutes point 3 East
notice that we give the latitude first followed by the longitude
so here's the world and let's try and put some lattice on to the world
you can see here's the North Pole and here's the South Pole
will cover the earth in half and put the two halves together. The join if you like, would be the Equator separating the northern and the southern halves.
Now we can put the Greenwich Meridian which is zero degrees longitude
so we'll start off with a blank earth just to make things easy for us
here's the equator splitting the top and the bottom
this is zero degrees latitude
Between the North and the South Pole running through Greenwich in London UK we'll put the Greenwich Meridian
this is called the Greenwich meridian you know probably know this from the Greenwich Mean Time
this is where all the longitude starts
from this is zero longitude
so now let's look at latitude in greater detail
we can draw a "parallel" to the equator and that one would be 20° North
this one would be 45° North
this one will be 70° and this is 80° North
and right at the very top that's the North Pole which is 90° 90° North
now let's look at longitude again the Prime Meridian going through Greenwich
that angle there, will be 20 degrees east this would be something like 45 degrees east
this would be something like 80 degrees east
and of course we can go right round the corner, all the way to the other side of the earth, and that'll be 180 degrees
here's 20 degrees west 45 degrees west, 80 degrees west
all the way around the back will be a hundred and eighty west
now let's look at the latitude longitude on the actual globe
there so here is zero degrees latitude here is 20 degrees north
here is 40 degrees north here is 60 degrees north
getting up by the pole we're going to have 80 degrees north
and right at the very top, that would be the North Pole
we can also go south
here's 20 degrees south and here's 40 degrees south
here's 60 degrees south and all the way down to the South Pole - which would be 90 degrees south
now let's look at the longitude
this direction would be west and this direction would be east measured from the Greenwich Meridian
this is the Greenwich Meridian passing through London the Greenwich Observatory in London
so here is 20 degrees east
40 degrees east 60 degrees east
here's 80 degrees east and of course we can go all the way around to the Dateline
which is a 180°
here's the 20° 40° and 60° heading in the West direction
so let's look at latitude in a little bit greater detail
North Pole - South Pole - the equator
so we wanted to know "X" that's London
what is the latitude of London
so we draw a parallel to the equator
and we measure the angle at the centre of the earth and if that angle is 51 degrees we found London,
so London is 51 degrees north which is a parallel to the equator
now let's have a look at the longitude again
we've got the North Pole - South Pole - draw a line between the two passing through Greenwich
and that's the zero longitude, the Greenwich Meridian - some people will call it prime meridian
and let's have a look at the position for Dubai
if we want to measure the longitude for Dubai we measure the angle from the center of the earth
between Greenwich and Dubai, we find it's 55 degrees east
so let's have a three-dimensional look at that
so as we mentioned before we've measured the angle at the center of the earth
so if you want to know the latitude of X you can see that angle there would be 20 degrees
20 degrees North north from the equator towards the North Pole
and if we wanted to find the longitude this longitude here is 30 degrees so
so from the Greenwich Meridian heading east along the equator that would be 30 degrees
an angle of 30 degrees - 30 degrees east
one degree is a bit of a big unit so we split it into smaller units
just like a clock one hour can be split in to 60 minutes one degree can also be split into 60 minutes
so one degree and 60 minutes it's the same thing as far as angles are concerned
here's some bonus information for you one minute of latitude is also one nautical mile
so between this minute and this minute this is one minute of latitude this is one nautical mile
it's as simple as that
we can write it as one minute we can say it's one mile or we can write it down as one ' or one nm or one big M
we'll look at that a little bit later so let's put it into practical use
what's the distance between these two points of land
we put the dividers on there - bring the dividers round to the latitude scale
and you can see that's looking at 0.6,
0.6 of a minute which is also point six of a nautical mile
really nice and easy
here's another example here's Weligama in Sri Lanka
what's the distance between the Yalla Rock here and Marissa point here
put the dividers on there take them around to the latitude scale
and you can see that's 55 minutes - this is the full degree so this will be ... lets count them,
55 minutes, 56 minutes, 57 58 59 and up to 60 minutes
so our distance lies between 55 minutes and 57 minutes
simple mathematics - 57 less 55 is 2 so that distance will be 2 nautical miles
so here we've got a bit of an empty chartlett with some latitude/longitude on the scale
and let's place a little golden disk on there
and lets say for example so what is the latitude of this
well here's the full degrees which is 24 degrees here's the full minutes which is 29 minutes
so this is latitude 24 degrees in 29
now let's look at the longitude
here's 54 degrees
and here's 28 minutes
so that position here will be at those coordinates as simple as that
now we've moved it up a little bit
so you can see now that the we are onto a decimal
so 24 degrees 29 point 5 minutes north
the longitude didn't change because we moved directly north
now let's just bring the golden this down a little bit
we've got a new latitude
and this is point three
so the new latitude is 24 degrees 29 minutes point three north
notice that the longitude didn't change
now let's move the golden disc along the towards east a little bit
so we'll now move our meridian of longitude across as well
and you can see
that this is going to be 54 degrees 29 minutes point 4 east
so now let's have add some points of land our points of interest there
for example what is this navigation boy here?
see if you can work this one out yourself
here is the longitude sorry the latitude line that we've put on
so what will be the latitude of that buoy?
24 degrees 29 minutes point 0 north
now what's the longitude?
longitude is zero five four degrees twenty nine point one minutes east
there's some points to remember when you're working on a chart plotter
the coordinates that you are looking for -
up here you can see it says the vessel latitude and longitude
but down here is the cursor latitude and longitude
here we've got a boat moving across
but notice the latitude and longitude of the cursor is different to the latitude and longitude of the vessel
so make sure that you give the right coordinates
too so many people - by mistake give the cursor coordinates rather than the vessel coordinates
something else we need to be a bit careful of
when you've got a chart within a chart or a Chartlet as we call it
you can come over here but you must use the right scale
because there's two latitude scales side-by-side
one is a Charlet scale
and one is the main chart scale
so we'll zoom into this a little bit
so we can see in greater detail
now we've measured on the actual chocolate (the smaller chart)
so we need to use this scale here
so you can see that's roughly what one mile something like that
but if we went over to the main chart all of a sudden it becomes 5.9 miles
so here are some top tips
always check the latitude scale
and make sure you know how many of the individual segments equals one minute or which equals one mile
remember that east longitude increases as you move towards the right of the chart
remember that north latitude increases as you move upwards on a chart
check that you use the correct scale
always count the minutes between the written numbers on the side of the chart
ie 25 minutes 26 minutes 27 minutes 28 minutes 29 minutes 30 minutes
one day it will save you from making a mistake
always give your latitude first and the longitude second
really that's what everybody does in the world - might as well do it the same way
when measuring distances always use the latitude scale
directly to the left or directly to the right of
the position or area that you're measuring
be safe
enjoy navigating