Ahead of its time when it was published, a boon for navigators when they eventually bolted down the problems of determining longitude, criticised for its distortions, it is still the most popular projection used today, and almost every map of the world showing the distribution of the land masses and oceans utilises this method.  Find out how this genius insight made everyone's lives a great deal easier.  >>>

Mercator Projection of the World

Whilst preparing the Mapwork courses for Rock and Sky, I have been trying to plot a route which takes us through the requirements of the syllabus, but also brings the subject to life.  Having done Survey 101 at university, and lived around maps and surveyors ever since, I have had an ongoing interest in cartography and geodesy.  In fact some of the great discoveries of the earth sciences were thanks to the great trigonometrical surveys of the 18^th and 19^th centuries. 

Ahead of its time, but a boon for navigators

A quick Google search on the history of map making throws up a bewildering pile of information on mapmakers from China, Arabia and Europe.  It is a fascinating read if you have the inclination and the time, but with a great deal of work ahead of me I didn’t have the luxury of tacking this way and that to explore these fascinating waters.  What I needed was information on the maps and projections that we have to teach in the school syllabus, and one of the most famous, perhaps the most famous, is the Mercator Projection.  Ahead of its time when it was published, it was a boon for navigators when they eventually bolted down the problems of determining longitude. Although it was criticised for its distortions, it is still the most popular projection used today, and almost every map of the world showing the distribution of the land masses and oceans is a Mercator Projection.

The distance between a line of longitude varies

The Mercator Projection shows lines of Latitude and Longitude at right angles to one another – if we look at a Mercator Projection map of the world, we will see that this is so.  But remember that Earth spins on its axis, with the North Pole at the top and the South Pole at the bottom.  Also remember that all the lines of Longitude converge on the poles.  So the distance between a line of longitude varies from 111 km (69 miles) at the equator, to 0 km at the poles.  

Gerardus Mercator

Mercator’s genius was that he was able to come up with a projection that ‘stretched’ the map at the top and the bottom, with the amount of stretch increasing proportionally as we move north and south, or to what is known as higher latitudes.  Increasingly higher in terms of the degrees – 0 degrees at the equator and 90 degrees at the poles.

The great challenge for any map maker is how to represent an essentially spherical Earth on a flat sheet of paper.  A number of map makers have taken up the challenge, but it was Mercator who came up with arguably the best solution, which is why it is used to this day, and which is why the projection bears his name.

On constant lookout for hidden reefs

By stretching things proportionally, the lines of longitude were always orientated north-south, the shape of the land masses remained correct, and it was now possible for navigators to plot a course on their chart, and sail that course using a compass until such time as their destination hove into view.  Well, that was the general idea, but life is never that simple – ocean currents drove ships off course, storms did likewise, and of course those earlier seafarers had to be on constant lookout for hidden reefs and low-lying islands or atolls.  Little wonder that there were so many shipwrecks back in those early days.  

But for navigators, Mercator’s maps were a real boon – they could plot courses and navigate their way across oceans with much greater confidence than before.  One of the problems they did have though was an inability to calculate longitude.  It was simple enough to get the latitude based on the height of the sun and stars above the horizon, but the longitude calculation required an accurate clock to determine the local time at noon whilst at sea (or on land for that matter) which was then compared to the time at a known longitude – which generally was that of Greenwich.

They became totally unreliable

Now is not the time to dive into the intricacies of navigation, but suffice to say, it was only in 1762 that a suitably accurate chronometer was built that kept accurate time at sea.  Up until then all the clocks that had been taken on voyagers lost or gained time and became totally unreliable.

One of the peculiarities of sailing a compass bearing on a map produced using Mercator’s Projection is that the route sailed is not actually the shortest route.  Crossing lines of longitude at the same angle, which is what happens when sailing a compass bearing, actually produces a ‘spiral’ route.  Check out this Wikipedia image showing how this works. For relatively short voyages this doesn’t make that much difference, but for modern, long haul flights from one side of the globe to the other, this can add a considerable distance to the journey and pilots now fly what are called ‘great circles.’

In spite of the issues around determining longitude, and the fact that the plotted routes of ocean voyages were not necessarily the shortest routes, Mercator’s projection caught on and became the ‘go-to’ map projection for cartographers ever since.  Another disadvantage of the Mercator Projection is that is stretches out the land masses in the north and south, so that Britain for instance is shown disproportionately large compared to Africa, which straddles the equator.

A light at the centre of the Earth

I have borrowed an image from Encyclopaedia Britannica to illustrate the mechanics of the projection. The mathematics we don’t want to get involved in, but the idea of a light at the centre of the Earth projecting the intersections of the lines of latitude and longitude, as well as the outlines of the land masses onto a flat sheet of paper wrapped around the globe is a fantastic way of illustrating how the projection works.

We owe Gerardus Mercator a debt of gratitude for his insights and hard work in creating his map that was gifted to the world in 1569.  Well done Gerardus Mercator. 

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