Sunday, 25 August 2013

Give your position away

If you can determine your position, it's important to ensure that you know how to describe it. This is best illustrated with latitude and longitude, the most internationally recognised coordinate system used on all sea and air charts as well as most GPSes and mobile phones. Many instruments display position in different format and if you say it wrong, it's possible to send the rescuers to the other side of the planet, which isn't going to help. If you follow this rule of the numbers and symbols, it doesn't matter what format the device uses, you'll always be able to convey your correct position so that the search and rescue crew are tasked to the right location. 

Let us suppose that we were sailing from Miami to Bermuda, skipping along the edge of the Bermuda Triangle and it all goes wrong. The ropes knot, the sails rip, the engine floods, but there is enough power to get a reading from the GPS and broadcast a Mayday. Unfortunately, you don't have one of those fancy DCS digital type radios which transmit the position with the emergency broadcast, so you have to describe your position. The GPS reads:
27° 10.452'  -078° 02.532'  
The first component is latitude and the second this longitude. Latitude describes how North or South we are and Longitude tells us how West or East we are. Latitude always comes first, though there is not generally an indication as to which is which other than maybe the number of digits, but now always, see below. In this case, if we switched them around, we'd be telling people we were in the antarctic directly south of Africa. An easy way to remember the order is that they are in alphabetical order.

Everything about the way the coordinates are displayed is important and all of it needs to be conveyed. This is how we would correctly and unambiguously describe our position as displayed above:
"latitude two seven degrees, one zero decimal four five two minutes, longitude negative zero seven eight degrees, zero two decimal five three two minutes"
Let's break it down. First we are saying which component of the position is to follow. By saying 'latitude' and 'longitude' explicitly, we are giving confidence to the recipient that we are giving the position correctly. Next we are breaking down the numbers into individual digits. It would be unfortunate to mix up fifteen and fifty, say, as that would put our position some considerable distance away, over 2000 nautical miles and nearly 4000 kilometers, in fact.

Now, we are separating the degrees from the minutes. First we say the number of degrees, then the words 'degrees' prompted by the symbol, then we say the minutes, in this case unambiguously placing the decimal point and finishing with the word 'minutes' to show we are using that convention. This is prompted by the apostrophe.

There are three ways in which the same number of degrees can be described. Different GPS units or chart plotters may display in different ways, so it's important to know what to say. The first is a pure decimal number of degrees. In the case of this latitude, we would be at 27.17°. This is not 27 degrees and 17 minutes and that is 11 nautical miles (20km) away. This may not see much, but it is at sea, in the dark or fog, I assure you. If your display showed this, you would say "Latitude two seven decimal one seven degrees". It's very simply read left to right with a word for each number or symbol. So long as you say the word 'degrees' where you see the symbol, you're doing the right thing. This system is most commonly used by Google.

As you might expect, there are sixty minutes in each degree. Historically and still used on some devices, each degree is split into seconds. If this were the case, the above position would be written 27° 10' 27", -078° 02' 32" which would be said "two seven degrees, one zero minutes (and) two seven second ...", etc.

The third version is the one most commonly available today which employs a whole number of degrees and a decimal number of minutes. It is important not to forget the information after the decimal point as half a degree represents almost one kilometer. By stating the words 'degrees', 'decimal' and 'minutes' in the correct positions, you will allow the listener to correctly identify this format.

With this particular GPS, positive numbers are interpreted as north and east whilst south and west coordinates are represented by negative numbers. The alternative is to state specifically the north/south and east/west distinction. The above position would be displayed as 27° 10.452' N 078° 02.532' W and spoken "latitude two seven degrees, one zero decimal four five two minutes north, longitude zero seven eight degrees, zero two decimal five three two minutes west". If the position is displayed this way it is critical to state the compass directions or the coordinates will be interpreted as north and east as they will always be positive. In this case, missing those off would place you at the Taj Mahal.

A final point is the importance of stating the leading zeros. As there are only 90 degrees north and south of the equator, there will at most two figures in the whole part of the number of degrees latitude. Longitude has a range of 180 degrees in both directions (meeting at the international date line) and therefore has up to three figures. Minutes and seconds range up to 60 and have two before any decimal place. If you state all of the leading zeros, so 12 becomes "zero one two" and 4 becomes "zero zero four", the receiving party can be assured that no digits have been lost in transmission. Any less and it's possible and you position is not definite without further interpretation. If it were the case that your device, especially a mobile device, were to leave these zeros off, then do add them in. Remember, everything has two figures apart from longitudinal degrees which has three. There would be nothing too bad about adding extras to others, just ensure that you don't leave any out.

So yes, there are a number of confusing formats, but one simple set of rules.
  1. say everything in the order you see it
  2. say 'latitude' and 'longitude' before each coordinate
  3. say 'negative' if there is a '-'
  4. say each number as individual digits
  5. say the leading zeros even if they are not there
  6. say 'decimal' at the decimal point
  7. say 'degrees' where you see °
  8. say 'minutes' where you see '
  9. say 'seconds' where you see "
  10. say 'north', 'south', 'east' or 'west' where you see 'N','S','E' or 'W' respectively if displayed
If you follow these rules the format used by the device will not matter and you will be clear about your actual position without need for additional clarification. Of course, speaking slowly and clearly will give you the best possible chance for being understood first time.

Look out of further articles on navigation and rescue and a special coming in Self Reliance Illustrated.

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