For a map to be useful in helping to navigate and plan a voyage (us sailors call them charts) it must be properly understood and be accurate for its purpose. Charts are physical (or electronic) representations of the physical characteristics of the earth & the water on it. They are a projection of a curved surface (the earth) onto a flat one (such as a sheet of paper or computer screen).
It's necessary to make some adjustments to how this information is presented so that this projection can be done whilst still making it a useful and navigationally accurate representation of the surface of this spherical planet.
On charts, magnetic north and its variance to True North is represented on a ‘compass rose’, printed on the chart. The difference between True and Magnetic North is called variation. This changes every year. The rate of change is also shown on the compass rose.
Lines of Latitude
Lines of latitude run parallel with the Equator in an east-west direction and are marked from 0 degrees at the equator to 90 degrees north (at the north pole) and 90 degrees south at the south pole).
Each degree of latitude represents sixty nautical miles in distance. There are 60 minutes in a degree, meaning each minute of latitude represents one nautical mile in distance. Therefore, the distance from pole to pole is around 10,800 nautical miles (180 degrees x 60).
Lines of Longitude
Lines of longitude, otherwise known as meridians, run from pole to pole in a north - south direction. Longitude starts at 0 degrees and runs east to 180 degrees east and west to 180 degrees west.
The earth rotates around an axis running from north pole to south pole meaning that about half of the earth is always in darkness and half in daylight. As the earth rotates day becomes night and night becomes day.
Greenwich, in London, was determined to be located at 0 degrees longitude. This is hardly surprising given that at the time all of this was decided the British Navy, based at Greenwich, was the largest navy in the World.
The virtual line of longitude running from pole to pole and running through Greenwich has its datum set at 0 degrees longitude. All parts of the planet that lie under this line of meridian are said to be on the Greenwich Meridian. They also share the same celestial time (Greenwich Mean Time).
Given that a full day comprises of 24 hours and rotation of a sphere is 360 degrees, we can see that for each hour that the earth spins on its axis, 15 degrees of longitude must have passed under the sun (360 degrees / 24hrs). It also, therefore, stands to reason that every 15 degrees west of London must be 1 hour behind GMT and every 15 degrees to the East of Greenwich must me 1 hour before GMT.
On the opposite side of the planet another meridian, known as the International Date line, divides the day before, from the day after! Confused yet?
Over the years there have been several different methods of projection used to create charts for navigation, all derived in different ways. However, for most forms of navigation, the most popular is still the Mercator projection. Many street mapping apps like Google and Bing use a slightly different methodology these days known as web mercator.
The Mercator chart is named after the Flemish geographer, Gerardus Mercator. In 1569, Mercator presented this mapping style, based on the work of Portuguese mathematician, Pedro Nunes, subsequently refined by English mathematician, Thomas Harriot.
The Mercator projection is based on the fact that a cylindrical representation of the earth’s surface is projected onto the chart. This manipulation has two practical effects, of which one should be aware, namely;
Because of this distortion, Mercator charts are of limited use when at a latitude greater than 70 degrees (i.e. closer to one of the poles).
Admiralty charts are periodically corrected and re-published. However, between publications, the competent mariner is expected to keep her paper charts up-to-date with reference to the Notices to Mariners. There is a specific but relatively simple methodology for correcting one’s charts.
Navigational charts will include information on everything from the depth of water (before allowing for tide and current), tidal flow rates, type of sea bed, navigational hazards, bouyage, lighthouses, major land features that can be seen from sea, national borders, specific shipping lanes and the location of ports, marinas and fishing areas. For Admiralty charts, the key to all this data is included in the booklet 5011 Symbols & Abbreviations.
Other charts might detail specific information useful when passage planning. These might represent the likely wind direction and speed and the flow of ocean currents at different types of the year.
Using GPS with paper charts
When plotting GPS-derived fixes onto paper charts it is important that the correct datum is used. In many cases in the UK this is WGS84. Check your charts and GPS.The coordinate origin of WGS 84 is meant to be located at the Earth's centre of mass. The error is believed to be less than 2 cm! The latest major revision of WGS 84 is referred to as "Earth Gravitational Model 1996" (EGM96), first published in 1996, with revisions as recent as 2004.
The Rhumb Line
The straight line driven between two points on a mercator chart is known as the rhumb line. In other words, it’s a line between two points that, when drawn on a mercator chart will cross each meridian at a constant bearing to north.
The Great Circle Route
When sailing short distances, say a few hundred miles, the difference between the great circle route and the rhumb line might be of little consequence in terms of extra distance sailed. However, over a few hundred or even a few thousands of miles, as when crossing an ocean for example, sailing the rhumb line as opposed to the great circle route, will add a considerable distance to your passage.
The Great Circle Route is the shortest distance between two points. When travelling across the surface of a sphere, such as the Earth, the shortest distance will always be that which, when plotted on a chart, has its 180 degree plane passing through the centre of the Earth. This distance is referred to as The Great Circle Route.
When plotted on a chart, the line representing the Great Circle Route will vary in bearing when compared to North on a Mercator chart because of the way it is projected onto a flat surface.
Gnomonic projection is said to be the oldest form of map projection, developed by Thales in the 6th century BC. Unlike a Mercator projection, Gnomonic projection has converging meridian lines of longitude (from the north or south pole to the equator) and curved parallels of latitude. Gnomonic charts are, therefore, most likely to be used when planning a longer passage where a navigator might want to determine waypoints along the Great Circle Route, plotted onto a Gnomonic Chart.
Modern electronic charts are usually displayed in such a way that whilst using mercator projection for local distances, the great circle route can be represented when drawing lines between two points on that chart. This helps when making a passage plan.
When using electronic charts it is important to consider what you would do if your vessel was to lose power or your electronics were to ‘crash’. Carrying paper charts as a back-up at the very least, is good seamanship.
Raster vs Vector Charts
There are two specific types of electronic chart/chart representation and it is critical that you know which one you are using. In recent years there have been several high-profile shipwrecks where professional sailors have found themselves wrecked on well-charted reefs because they did not follow simple codes of practice.
Raster Charts are, effectively, an electronic picture of a paper chart obtained through a detailed scanning process. What you see is exactly what is represented on a paper chart. The downside to raster charts is that more electronic storage is required to store the files. Also, they can become cluttered as all information is displayed at once, unlike a vector chart where information is layered.
Vector Charts present chart data selectively, dependent on what level of zoom the user has employed. This type of chart has the benefit of more clearly presenting relevant information to the user and keeping clutter to a minimum. The downside to vector charts is that if the user does not zoom into a larger scale he might miss important information, such as a reef in the middle of a large ocean for example! Vector charts can incorporate more detailed information at higher zoom, such as marina facilities, etc.
Whichever type of chart you use, make sure you know how to use it and the potential pitfalls of mis-using a vector chart!
And finally, what is the knot? Well, for the purposes of navigation 1 knot is defined as being the the speed required to travel one nautical mile in one hour. In other words, one knot is one nautical mile per hour. One knot is equivalent to 1.15 statute miles per hour.
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All content created by Mark Burkes, an RYA Yachtmaster™ Ocean Instructor. 200,000 miles logged over 30 years. Learn more...
All charts used in our videos and on this website are used under copyright from HMCO licence no. 35358. Not for navigational purposes.