Orthodromic Chart

An orthodromic chart is a specialist navigation chart used to plan long-distance maritime routes, particularly ocean crossings. It is based on the concept of the great-circle route, also known as orthodromy, which represents the shortest possible route between two points on the Earth’s surface.

Orthodromic navigation is particularly relevant when planning passages across the North Atlantic, South Atlantic, North Pacific, South Pacific or Indian Ocean. For long passages, the great-circle route can significantly reduce the distance sailed compared with a constant-course route. However, because the course continuously changes along a great-circle route, the route must generally be divided into a series of practical legs for actual navigation.

The Nautic Way Maritime Bookshop offers specialist nautical charts and navigation documents for sailors, professional mariners, ocean sailors, offshore navigators and anyone preparing a long-distance passage.

What is an orthodromic route?

An orthodromic route is the shortest route between two points on the surface of a sphere. On the Earth, it follows an arc of a great circle.

A great circle is a circle whose plane passes through the centre of the Earth. The equator is a great circle, as are all meridians considered in pairs. The shortest route between two distant points therefore follows a great-circle arc.

This has several practical consequences:

• the orthodromic route is the shortest geometrical route between two points;

• the route follows an arc of a great circle;

• the course changes continuously along the route;

• the difference from a constant-course route becomes increasingly significant over long distances;

• the route is particularly useful when planning ocean passages;

• for practical navigation, the great-circle route is normally converted into a series of successive courses.

The term “orthodromic chart” is sometimes used generally to describe a chart used for planning an orthodromic route. More precisely, the chart most commonly used for plotting a great-circle route is a gnomonic chart.

Orthodromy and the gnomonic chart

The gnomonic projection is particularly useful for planning great-circle routes because a great circle appears as a straight line on a gnomonic chart.

This makes it possible to plot the departure point and destination, draw a straight line between them and obtain the corresponding great-circle route.

The Admiralty portfolio includes gnomonic charts specifically designed for passage planning. These charts cover major ocean areas including the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean.

The gnomonic chart is therefore mainly a route-planning tool. It is not normally the chart used for detailed coastal navigation or for plotting the vessel’s position throughout an entire voyage.

Once the great-circle route has been established, intermediate positions can be transferred to a Mercator chart. The navigator can then determine the successive courses required to follow the planned route.

Orthodromic route versus rhumb-line route

The distinction between orthodromy and loxodromy is fundamental to navigation.

A rhumb line, or loxodrome, crosses every meridian at the same angle. The vessel therefore maintains a constant compass course, subject to the effects of wind, current and steering corrections. On a Mercator chart, a rhumb line appears as a straight line.

An orthodromic route, by contrast, follows a great circle. It is the shortest geometrical route, but the course changes continuously.

For short coastal passages, the difference between the two routes may be negligible. For long ocean passages, however, the difference can become considerable.

This is why long-distance passage planning often combines both approaches: the great-circle route is used to determine the shortest theoretical track, while Mercator charts are used to establish practical navigation legs and courses.

How is an orthodromic route planned?

Planning an orthodromic route generally involves several stages.

First, the navigator identifies the exact departure and destination points. These may be ports, waypoints, offshore positions or other geographical reference points.

A suitable gnomonic chart is then selected for the ocean area concerned. The departure and destination positions are plotted on the chart.

The great-circle route between the two points appears as a straight line on the gnomonic projection. Intermediate points are then selected at suitable intervals along this line.

These positions are subsequently transferred to a Mercator chart or another practical navigation chart. The navigator can then determine the individual courses and distances between the successive waypoints.

This method combines the geometrical advantage of the great-circle route with the practical advantages of the Mercator projection.

Example: an Atlantic crossing

Consider a passage between Europe and North America.

A vessel leaving Brest, Cherbourg, Southampton, Lisbon or La Coruña and heading towards Halifax, Boston, New York or Québec may cover several thousand nautical miles.

If the route is plotted as a constant-course line on a Mercator chart, it may appear relatively simple. However, this does not necessarily represent the shortest geographical route.

A great-circle route may initially take the vessel further north before gradually turning towards its destination. On a gnomonic chart, the same great-circle route appears as a straight line.

The navigator can therefore determine a series of intermediate waypoints and transfer them to a Mercator chart. The resulting route consists of successive practical courses rather than a single constant heading.

This technique is particularly useful for long Atlantic crossings where even a relatively small percentage reduction in distance can represent a substantial number of nautical miles.

Gnomonic projection and Mercator projection

The gnomonic and Mercator projections have different purposes in maritime navigation.

The gnomonic projection is particularly useful for planning great-circle routes. Its main advantage is that great circles appear as straight lines. Its main limitation is increasing distortion away from the point of tangency.

The Mercator projection is widely used for practical navigation because rhumb lines appear as straight lines and because bearings and courses can be conveniently plotted.

On a Mercator chart, however, a great-circle route appears as a curved line. The navigator must therefore divide the route into a series of courses if the intention is to approximate the great-circle track.

This explains why an orthodromic route can appear different depending on the projection used. The route itself has not changed; its graphical representation has.

Ocean areas where orthodromic planning is useful

Orthodromic navigation is particularly relevant to long passages across the major oceans.

North Atlantic

Typical routes include Europe to eastern Canada or the United States, for example:

• Brest to Halifax;

• Cherbourg to New York;

• Southampton to Boston;

• Lisbon to New York;

• La Coruña to Halifax.

Weather systems, seasonal conditions, sea state and ice must also be considered when determining the practical route.

South Atlantic

Great-circle planning can be relevant to passages between Europe, West Africa and South America, as well as routes linking southern Africa with Brazil and the southern Atlantic.

Typical ports and areas include Dakar, Cape Town, Recife, Salvador, Rio de Janeiro and Buenos Aires.

North Pacific

Long-distance routes between North America and Asia can benefit significantly from great-circle planning because of the very large distances involved.

Routes may involve areas around Alaska, the Aleutian Islands, Japan, Hawaii and the western Pacific.

South Pacific

Orthodromic planning can also be useful for passages involving French Polynesia, New Zealand, Australia and the Pacific islands.

For offshore sailors, the geometrical route is only one part of the planning process. Prevailing winds, tropical systems, ocean currents and the availability of suitable ports or anchorages can significantly influence the final route.

Indian Ocean

The Indian Ocean presents other major long-distance routes, including passages between southern Africa, Mauritius, India, Sri Lanka, Southeast Asia and Australia.

Ports and areas such as Durban, Port Louis, Mumbai, Colombo, Singapore and Fremantle may form part of long ocean passages where great-circle planning can be useful.

Orthodromy and weather routing

The shortest geographical route is not necessarily the most appropriate route to sail.

An orthodromic calculation only considers the geometry of the Earth. It does not automatically account for wind, waves, ocean currents, tropical cyclones, depressions, sea ice, restricted areas or maritime traffic.

A navigator therefore combines the great-circle calculation with meteorological and oceanographic information.

Admiralty Routeing Charts, for example, provide information on seasonal weather and ocean conditions, including prevailing winds, storms, sea and air temperatures, pressure, ice limits and ocean currents. They are available for the major ocean regions and for individual months of the year.

The final route may consequently be longer than the theoretical great-circle route if this provides safer, faster or more practical passage conditions.

Orthodromic navigation and modern electronic systems

Modern navigation software, ECDIS and many onboard routing systems can calculate great-circle routes automatically.

The navigator can enter departure and destination coordinates and obtain a calculated route, often together with intermediate waypoints and distances.

Nevertheless, understanding orthodromy remains important. A navigator who understands the geometry of the route can better interpret the proposed track, identify why the course changes and evaluate whether the calculated route is appropriate for the vessel and the prevailing conditions.

For professional navigation, electronic route calculation therefore complements rather than replaces fundamental knowledge of nautical chart projections and route planning.

Orthodromic charts and nautical publishers

Great-circle and route-planning charts are published by specialist hydrographic organisations and nautical publishers.

Admiralty is particularly well known for its gnomonic charts and ocean plotting charts used for long-distance passage planning. Its current reference and plotting chart portfolio includes gnomonic coverage of the major ocean basins.

Other hydrographic offices and specialist publishers may provide complementary navigation charts, routeing information and nautical publications.

For an ocean passage, an orthodromic or gnomonic planning chart should therefore be considered as part of a wider set of navigation documents, alongside practical navigation charts, routeing information, sailing directions, tide information and weather data.

How to choose an orthodromic chart

Several criteria should be considered when selecting a chart for great-circle route planning:

• the ocean basin covered;

• the departure and destination areas;

• the scale of the chart;

• the projection used;

• the geographical limits of the chart;

• the distance between the departure and destination points;

• the possibility of transferring intermediate positions to practical Mercator charts;

• seasonal weather conditions;

• ocean currents;

• ice limits where relevant;

• the availability of detailed charts for the subsequent navigation legs.

For a transatlantic, transpacific or Indian Ocean passage, the planning chart should therefore be considered as one element of a complete navigation system rather than as a replacement for detailed navigation charts.

Why use an orthodromic chart?

The main advantage of an orthodromic or gnomonic planning chart is that it makes the geometry of a great-circle route immediately understandable.

It allows the navigator to:

• determine the shortest theoretical route;

• visualise the great-circle track;

• identify intermediate positions;

• compare the great-circle route with a rhumb-line route;

• estimate the potential distance saved;

• prepare successive courses;

• transfer the route to Mercator charts;

• integrate geographical planning with weather and oceanographic information.

For long ocean passages, this method provides a clear link between mathematical navigation and practical route planning.

Conclusion

An orthodromic chart is an important planning tool for long-distance maritime navigation. Orthodromy follows a great-circle route, representing the shortest geometrical distance between two points on the Earth's surface.

Because a great circle appears as a straight line on a gnomonic chart, this projection is particularly useful for plotting an ocean route. The resulting positions can then be transferred to Mercator charts, where the route is divided into practical navigation legs and successive courses.

The method is especially relevant to passages across the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean. In real navigation, however, the theoretical shortest route must always be considered alongside weather, currents, sea state, ice, traffic and navigational constraints.

Understanding the relationship between orthodromy, gnomonic projection and Mercator projection therefore remains an essential part of advanced passage planning, whether the route is prepared traditionally on paper charts or calculated using modern electronic navigation systems.

Frequently Asked Questions

What is an orthodromic route?

An orthodromic route is the shortest geographical route between two points on the Earth's surface. It follows an arc of a great circle.

What is a great circle?

A great circle is a circle whose plane passes through the centre of the Earth. The shortest route between two distant points follows a great-circle arc.

What is a gnomonic chart?

A gnomonic chart uses a projection in which great circles appear as straight lines. It is therefore particularly useful for planning great-circle routes.

What is the difference between an orthodromic route and a rhumb line?

An orthodromic route follows a great circle and represents the shortest geographical distance. A rhumb line maintains a constant angle with the meridians and appears as a straight line on a Mercator chart.

Why does an orthodromic route appear curved on a Mercator chart?

The difference results from the map projection. A great circle is generally curved when represented on a Mercator chart, even though it is the shortest route on the Earth's surface.

Does an orthodromic route require a changing course?

Yes. Because the great-circle track continuously changes its angle relative to the meridians, the navigator normally divides it into successive courses.

When is orthodromic navigation particularly useful?

It is most useful for long-distance ocean passages where the difference between a great-circle route and a constant-course route can become significant.

Which oceans are covered by gnomonic planning charts?

Specialist gnomonic charts are available for major ocean areas including the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean.

Can an orthodromic route be used directly for detailed coastal navigation?

Generally, no. Great-circle planning is primarily used to establish the long-distance route. Detailed coastal navigation is normally carried out using appropriate large- and medium-scale navigation charts.

Does the shortest route always mean the fastest route?

No. Wind, waves, currents, storms, tropical cyclones, ice and traffic can make a longer route more appropriate for a particular vessel and passage.

Are orthodromic routes still useful with modern electronic navigation?

Yes. Modern systems can calculate great-circle routes automatically, but understanding orthodromy and chart projections remains valuable for checking and interpreting the proposed route.