Gnomonic Chart

A gnomonic chart is a specialist navigation chart used to plan long-distance maritime routes and ocean crossings. It is particularly useful for plotting a great-circle route, also known as an orthodromic route. Its main characteristic is that great circles are represented as straight lines, making it possible to visualise directly the shortest geographical route between two points on the Earth’s surface.

The gnomonic projection is therefore very different from the Mercator projection, which is widely used for conventional navigation charts. On a Mercator chart, a straight line represents a constant-course route, or rhumb line. On a gnomonic chart, by contrast, a straight line represents a great-circle route. This makes the gnomonic chart particularly useful for planning long crossings of the Atlantic, Pacific and Indian Oceans.

Gnomonic charts are primarily planning charts. They are used to determine the general geometry of a long-distance route before transferring intermediate positions to charts suitable for practical navigation.

What is a gnomonic chart?

A gnomonic chart uses an azimuthal projection obtained by projecting the Earth's surface onto a plane tangent to the Earth. This geometric construction gives the projection one essential property for navigation: every great circle is represented as a straight line.

A great circle is a circle whose plane passes through the centre of the Earth. The Equator is a great circle, as are pairs of opposite meridians. Between two distant points, the great-circle arc represents the shortest geographical route when the Earth is considered as a sphere.

A gnomonic chart can therefore be used to:

• plot a great-circle route as a straight line;

• determine an orthodromic route graphically;

• visualise intermediate positions during a long passage;

• compare a great-circle route with a constant-course route;

• prepare a series of courses to be transferred to a Mercator chart;

• work efficiently with very long ocean distances.

Gnomonic navigation is therefore directly associated with great-circle navigation, although the two concepts are not exactly the same: orthodromy describes a route, whereas gnomonic is a cartographic projection.

Why use a gnomonic chart?

On a Mercator chart, the meridians are parallel and a constant-course route appears as a straight line. This property makes the Mercator projection extremely useful for practical navigation, but it does not represent a great-circle route as a straight line.

On a gnomonic chart, the situation is reversed: a great-circle route appears as a straight line.

The navigator can therefore plot the departure point and destination, then draw a straight line between them. This line represents the great-circle route.

The Admiralty uses this property for its gnomonic charts designed for passage planning. Its range includes coverage of major ocean areas such as the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean. The current Admiralty catalogue includes dedicated gnomonic charts for these major ocean regions.

Gnomonic chart and great-circle route

The relationship between a gnomonic chart and great-circle navigation is fundamental.

A great-circle route follows a great circle and represents the shortest geographical route between two distant points. On a gnomonic chart, this route appears as a straight line.

It is therefore possible to plot the great circle between two ports and identify several intermediate positions along the route.

These positions can then be transferred to a Mercator chart. The navigator can determine the successive courses required to approximate the great-circle route throughout the passage.

The gnomonic chart is therefore not normally used to track the vessel throughout the entire voyage. It is primarily a route-planning tool.

Gnomonic projection and Mercator projection

The difference between the two projections is fundamental.

The Mercator projection is particularly well suited to practical navigation. A straight line represents a rhumb line, or constant-course route, making it possible to plot and measure courses conveniently. Most conventional nautical charts use this projection.

The gnomonic projection serves a different purpose. A straight line represents a great circle and therefore an orthodromic route.

The navigator can consequently use the two projections successively:

1. the gnomonic chart to plot the great circle;

2. the selection of several intermediate positions;

3. the transfer of these positions to a Mercator chart;

4. the calculation of successive courses;

5. the preparation of the practical navigation route.

This complementary use of gnomonic and Mercator projections is a traditional method for planning long ocean passages.

Example: crossing the North Atlantic

The North Atlantic provides a particularly good example.

A passage between Brest, Cherbourg, Southampton, Lisbon or La Coruña and Halifax, Boston, New York or Québec may cover several thousand nautical miles.

On a Mercator chart, the great-circle route appears as a curve. On a gnomonic chart covering the relevant area, the same route appears as a straight line.

The navigator can therefore plot the great-circle route and select several intermediate positions. These positions can then be transferred to a Mercator chart to construct a series of constant-course navigation legs.

This method makes it possible to approach the theoretical minimum geographical distance while retaining a practical navigation method.

Gnomonic charts for the Atlantic

The North Atlantic is one of the classic areas where gnomonic charts can be used to prepare passages between Western Europe and North America.

Routes may involve areas around Brest, Cherbourg, Southampton, La Coruña, Lisbon, Halifax, Boston or New York.

The South Atlantic may also require this type of planning for routes between Africa, South America and the Atlantic islands.

Ports such as Dakar, Cape Town, Recife, Salvador da Bahia, Rio de Janeiro and Buenos Aires may serve as departure or destination points for long ocean routes.

In every case, the geometrical route must subsequently be considered alongside meteorological and oceanographic conditions.

Gnomonic charts for the Pacific

The enormous distances of the Pacific make great-circle planning particularly relevant.

In the North Pacific, planning may involve routes between the west coast of North America and Japan, including areas around Vancouver, Seattle, San Francisco, Anchorage or Tokyo.

In the South Pacific, routes may involve Hawaii, Tahiti, French Polynesia, New Zealand or Australia.

A great-circle route nevertheless provides only a geometrical starting point. The actual route must also take prevailing winds, depressions, tropical systems, ocean currents and diversion possibilities into account.

Gnomonic charts for the Indian Ocean

The Indian Ocean is another important area for planning long-distance routes.

Passages may connect southern Africa, Madagascar, the Mascarene Islands, India, Sri Lanka, Southeast Asia or Australia.

Ports such as Durban, Port Louis, Mumbai, Colombo, Singapore and Fremantle may be involved.

In this region, seasonal conditions are particularly important. The shortest geometrical route may be modified to account for monsoon winds, tropical cyclones and ocean currents.

Distortion of the gnomonic projection

The main limitation of the gnomonic projection is its strong distortion as distance increases from the point of tangency.

The chart therefore cannot represent a very large geographical area with uniform accuracy. The projection is primarily designed to make great-circle routes easy to plot.

This explains why a gnomonic chart is generally used as a planning document rather than as a detailed coastal navigation chart.

It should be used together with appropriate charts for the different stages of a passage, particularly when approaching ports or navigating close to shore.

Gnomonic charts and weather routing

A great-circle route represents the shortest geographical path, but it is not necessarily the best maritime route.

The navigator must also consider:

• prevailing winds;

• depressions and storms;

• ocean currents;

• swell;

• tropical cyclones;

• sea ice;

• shipping traffic;

• restricted areas;

• vessel characteristics;

• opportunities for landfall and diversion.

Admiralty Routeing Charts provide complementary information on shipping routes, distances between major ports, winds, storms, temperatures, currents and ice limits according to the month of the year. They cover the major ocean basins.

The final route may therefore differ considerably from the theoretical great-circle route when meteorological or oceanographic conditions require a different track.

Gnomonic charts and electronic navigation

Modern navigation software, ECDIS and some route-planning systems can calculate great-circle routes automatically.

This automation does not make the gnomonic chart irrelevant. Understanding the projection allows the navigator to interpret the calculated route, check waypoints and distinguish a great-circle route from a constant-course route.

The traditional method therefore remains useful both operationally and educationally for understanding the principles of ocean navigation.

It also provides a better understanding of the differences between the various cartographic representations of the same route.

Hydrographic offices and chart publishers

Gnomonic charts for planning major ocean passages are mainly produced by hydrographic offices and specialist nautical publishers.

Admiralty, published by the UK Hydrographic Office, provides gnomonic charts for the major oceans as well as complementary planning documents. The Admiralty catalogue includes gnomonic charts specifically intended for great-circle route planning and for preparing routes made up of successive rhumb-line segments.

Other national hydrographic offices, including the French Hydrographic and Oceanographic Service (SHOM), together with specialist publishers such as Imray, provide complementary documents for offshore and ocean navigation according to the areas concerned.

How to choose a gnomonic chart

The choice of a gnomonic chart depends primarily on the navigation area and the intended route.

The following points should be checked:

• the ocean covered;

• the relevant latitudes and longitudes;

• the departure point;

• the destination;

• the chart scale;

• the projection used;

• the possibility of transferring positions to a Mercator chart;

• coverage of the entire passage;

• seasonal conditions;

• possible ice areas;

• vessel-specific constraints.

For a transatlantic, transpacific or Indian Ocean crossing, the gnomonic chart is therefore one planning document among several required for a complete passage plan.

Why use a gnomonic chart?

A gnomonic chart is primarily used to visualise and construct a great-circle route.

It can be used to:

• plot the shortest geographical route between two points;

• plan a great-circle passage;

• identify intermediate positions;

• compare great-circle and rhumb-line routes;

• prepare successive courses;

• transfer a route to a Mercator chart;

• analyse a long ocean passage;

• understand the geometry of a maritime route.

It is therefore particularly suited to navigators preparing passages covering several thousand nautical miles.

Conclusion

A gnomonic chart is a specialist planning tool for long-distance maritime routes. Its fundamental property is that great circles are represented as straight lines. It therefore provides a simple way of plotting a great-circle route between two distant points.

It is particularly useful for crossings of the North and South Atlantic, North and South Pacific and Indian Ocean. Routes may involve ports such as Brest, Southampton, Halifax, New York, Dakar, Cape Town, Rio de Janeiro, Tahiti, Auckland, Durban or Colombo.

In practice, the gnomonic chart is generally used together with a Mercator chart. The great circle is first plotted on the gnomonic projection, after which intermediate positions are transferred to a chart suitable for practical navigation.

The gnomonic chart is therefore an important tool for understanding and planning major ocean passages, combining the geometry of great-circle navigation with the practical requirements of maritime navigation.

FAQ – Gnomonic Chart

What is a gnomonic chart?

A gnomonic chart is a chart using a projection in which great circles appear as straight lines. It is mainly used to plan great-circle routes during long ocean passages.

What is a gnomonic chart used for in navigation?

It is primarily used to plot a great-circle route between distant points and determine intermediate positions that can subsequently be transferred to practical navigation charts.

What is the difference between a gnomonic chart and an orthodromic chart?

Gnomonic refers to a cartographic projection. Orthodromy refers to a route following a great circle. A gnomonic chart is therefore particularly suitable for plotting an orthodromic route.

Why does a great circle appear as a straight line on a gnomonic chart?

This is a geometric property of the gnomonic projection: points on the Earth's surface are projected onto a tangent plane from the Earth's centre, causing great circles to appear as straight lines.

What is the difference between a gnomonic chart and a Mercator chart?

On a gnomonic chart, great circles are represented by straight lines. On a Mercator chart, straight lines represent constant-course routes, or rhumb lines.

Can a gnomonic chart be used directly for navigation?

It is primarily intended for long-distance route planning. For practical navigation and port approaches, charts appropriate to the scale and area concerned are required.

Why is a gnomonic route transferred to a Mercator chart?

Because the Mercator projection is better suited to practical course plotting and navigation. Intermediate positions along the great circle can be used to construct a series of constant-course legs that approximate the great-circle route.

Which oceans are covered by Admiralty gnomonic charts?

Admiralty's gnomonic chart portfolio includes major ocean areas such as the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian Ocean.

Is a great-circle route always the route actually sailed?

No. The great-circle route is a geometrical solution. Wind, currents, storms, tropical cyclones, ice, shipping traffic and vessel characteristics may require a different route.

Why are gnomonic charts particularly useful for long ocean passages?

As distance increases, the difference between a great-circle route and a constant-course route can become significant. A gnomonic chart makes the great-circle route immediately visible.

Which publishers provide documents useful for planning major ocean passages?

Admiralty provides a dedicated series of gnomonic charts together with ocean routeing charts. SHOM, Imray and other hydrographic offices or specialist publishers also provide complementary documents depending on the navigation area.

Is a gnomonic chart useful with GPS or ECDIS?

Yes. Electronic systems can calculate great-circle routes automatically, but understanding the gnomonic projection allows the navigator to understand, check and interpret the calculated routes.