Globe

Giving a position in Latitude/Longitude may seem straightforward, not having to worry about all those different grid systems with their individual position formats, but is it really that straightforward? Let’s take a look. Certainly, the Latitude/Longitude system covers the world in the same format, and degrees, minutes and seconds are universal worldwide. Aren’t they?

In Times of Old

It is often the case that as things get more advanced they get much more complicated; this is, without doubt, the case here. The original Latitude/Longitude system was, I think, devised by the Greeks in around the third century BC based on celestial positions; the true Latitude/Longitude is still based on this.

What Went Wrong?

Well, nothing really, it just became more complicated. With the onset of electronic navigation equipment and computer based mapping systems we now have three formats, these are:

1). DDD  MM SS.S General use.

2). DDD  MM.MMM Generally the norm when using electronic navigation equipment.

3). DDD.DDDDD  Generally the norm when using computer mapping systems.

We now have to use a ‘reference frame’ to keep everything accurate and consistent; this being WGS 84 (World Geodetic System 1984). The navigational satellites use the WGS 84 frame and all new navigational charts etc. should now be in WGS 84. This is not the only frame by any means but is now the accepted standard. WGS 84 became the result of development from the earlier GRS 80 (Geodetic Reference System 1980). The same numerical Latitude/Longitude position will often produce different positions on maps and charts based on different reference frames; an obviously undesirable state of affairs. Offsets can be applied to change from one reference datum to another, but as always, the best way is to use maps and charts based on WGS 84. When everything is going well it’s not too much of a problem; it’s when you have to navigate under extreme duress that datum information and offsets can often be overlooked.

Is it Accurate and Consistent?

The honest answer is no but we are talking small amounts. The reason for this is due to movement in the Earth’s crust, plate tectonics to be more specific. As WGS 84 is a worldwide frame it cannot accommodate independent landmass movements, so, while the WGS 84 frame is accurate and steadfast, the landmasses underneath are moving about all over the place. It obviously isn’t as bad as I’m making it sound but sometimes it makes things a little easier to understand if they are exaggerated somewhat.

Is Latitude/Longitude a Grid?

Many term it a grid but really a ‘grid’ should be perfectly uniform and predictable; unfortunately, the Latitude/Longitude overlay is not. The ‘grid’ squares are not squares at all and are various shapes and sizes; this can be seen easily when working with small scale charts. This does not make the system useless, in fact far from it; very accurate navigation indeed can and is achieved all the time. Its main advantage is when travelling through vast featureless environments such as oceans and deserts.

Why Then Do We Need Grids?

In normal land navigation we are generally working with relatively small areas and the grid overlay is completely independent of the rest of the world so plate tectonics can be largely disregarded as the grid will move with the landmass. Remember, the grid references only exist within the grid system and are isolated from the rest of the world. We can alter the scale easily by making the squares larger without changing the numerical part of the grid; we can also calculate accurately grid convergence if we need to but this is more of an academic exercise than a practical one. Another reason is linked to the spheroid on which the map is based, different spheroids fit the curvature of the Earth’s surface better at that particular location; this increases the accuracy of the map and reduces distortion.

Can We Work Either?

Yes, without doubt. A good navigator will work with whatever system he needs to and practice with both is always a good idea. There are a few basic things to look out for when giving a position in either format.

• When giving a grid reference we always give the Eastings first, followed by the Northings. When giving a position in Latitude/Longitude we always give the Latitude (Northings) first and the Longitude (Eastings) second.
• When working with grids all Northings and Eastings will be positive. When working with Latitude and Longitude some give positions using a minus sign (-) to express positions South of the Equator or West of the prime meridian; this, in my opinion, is extremely poor practice as it can leave the system open to huge errors. The general method is to express Latitude as either N (North) or S (South) of the equator and Longitude as either E (East) or W (West) of the prime meridian.
• When giving a position in Latitude/Longitude, always be clear about the units (as above).
• It is always good practice when giving a position in Latitude/Longitude to include the zeros. A Longitude of 12 degrees 4.47 minutes East would be expressed as Longitude 012 degrees 04.470 minutes East. This ensures that the total number of digits are used at all times; it helps eliminate the error of reading it as Longitude 120 degrees 44.7 minutes East or variations thereof. Of course, Latitude cannot be more than 90 degrees N or S of the equator but the system holds true and should be expressed as 090 degrees all the same. It may seem pedantic but often this information is given and received under duress and a simple mistake could result in a needless loss of life; if we use the system all the time it becomes second nature.
• It is generally the case that if you are on land and a grid system is available, a position would normally be given as a grid reference; however, if you are off the coast you would normally give your position in the Latitude/Longitude format.

The last Word? Perhaps Not!

The subject of Latitude and Longitude is extremely complicated and covers many, many things that you would think were entirely unconnected. The development of incredibly accurate electronic navigational aids has made it necessary to develop extremely technical models relating to the shape of our world; these developments will continue in the quest for better and more accurate systems. I have tried to give a simplified overview to provide a basic understanding; in doing so, some statements are technically incorrect and tell nothing like the full story. That said, the basic navigator needs only a limited understanding of the overall mechanism to accomplish
their goals. The subject of navigation is enthralling to those who are interested but incredibly boring to those who are not. I hope I have not crossed the line into the latter.