orthodromic and gnomonic chart

Orthodromic and Gnomonic Charts: Essential Tools for Ocean Navigation

Orthodromic and gnomonic charts are specialized cartographic tools essential for ocean-going navigation and long-distance route planning. Offered by the Nautic Way Maritime Bookshop, these charts are designed for professional navigators, deep-sea captains, merchant navy officers, navigation schools, and maritime cartography enthusiasts.

Designed to accurately represent the Earth’s geometry, gnomonic charts make it possible to plot orthodromic routes, meaning the shortest paths between two points on the Earth’s surface. They are strategic tools for optimizing distance, sailing time, and fuel consumption.

What Is an Orthodromic and Gnomonic Chart?

A gnomonic chart is based on a specific projection in which the Earth’s surface is projected from the center of the globe onto a tangent plane. Its key feature is that every orthodromic route (great circle) appears as a straight line.

An orthodrome is the shortest path between two points on a sphere. It contrasts with a loxodrome, which crosses all meridians at a constant angle and appears straight on a Mercator chart but is longer over great distances.

Orthodromic charts are therefore intended primarily for route planning, not for real-time navigation at sea.

Uses of Orthodromic Charts in Maritime Navigation

Orthodromic and gnomonic charts are mainly used to:

• Plan transoceanic crossings
• Determine the shortest route between two ports
• Optimize voyage duration and fuel consumption
• Study routes in high latitudes
• Teach spherical and celestial navigation

In practice, the navigator plots the orthodromic route on a gnomonic chart and then transfers it in successive segments onto a Mercator chart, which is more suitable for everyday navigation.

Areas of Use for Orthodromic and Gnomonic Charts

Ocean and Deep-Sea Navigation

Orthodromic charts are indispensable for major crossings:

• Atlantic Ocean
• Pacific Ocean
• Indian Ocean
• Intercontinental shipping routes
• Subpolar and polar navigation

Over long distances, the difference between an orthodrome and a loxodrome can amount to hundreds of nautical miles.

Merchant Marine and Professional Navigation

Captains and ​​​​​​​officers use these charts to:

• Optimize commercial shipping routes
• Reduce operating costs
• Anticipate meteorological constraints
• Plan major maritime traffic lanes

Maritime Training and Education

Gnomonic charts are widely used in:

• Navigation schools
• Maritime academies
• Captain and officer training programs

They provide a clear understanding of the differences between chart projections and navigation routes.

Information Shown on a Gnomonic Chart

Orthodromic charts provide a deliberately simplified yet strategic representation of the globe:

• Continents and major landmasses
• Curved networks of meridians and parallels
• Direct visualization of great circles
• Major geographic reference points
• Global oceanic areas
• Sometimes climatic or seasonal information

They do not include detailed coastal features, aids to navigation, or soundings, as their purpose is global route planning.

Advantages of Orthodromic and Gnomonic Charts

The Shortest Route

Their main advantage is the ability to immediately plot the shortest route between two points, which is not possible on a standard Mercator chart.

Optimized Navigation

Over long distances, route reduction allows for:

• Time savings
• Fuel economy
• Reduced crew fatigue

Essential at High Latitudes

At high latitudes, loxodromic routes become highly inefficient. Orthodromic charts are therefore indispensable for northern and polar routes.

A Reference Educational Tool

These charts are ideal for understanding spherical navigation and the fundamental principles of marine cartography.

Limitations and Drawbacks of Gnomonic Charts

Significant Distortions

Gnomonic projection produces strong distortions of shapes, surfaces, and distances, especially far from the point of tangency.

Limited to Route Plotting

Orthodromic charts are not suitable for:

• Coastal navigation
• Pilotage
• Precise route monitoring

They must be used in conjunction with other nautical charts.

Technical Charts

Using these charts requires solid navigation training. They are less accessible to beginner sailors.

Complementarity with Mercator Charts

Traditional ocean navigation relies on the combined use of two chart types:

1. Gnomonic chart: for plotting the orthodromic route
2. Mercator chart: for sailing on constant headings

This combination ensures both theoretical efficiency and practical safety.

Who Are Orthodromic and Gnomonic Charts For?

These charts are mainly intended for:

• Professional navigators
• Deep-sea captains
• Offshore racing skippers
• Merchant navy officers
• Maritime schools and training centers
• Navigation and cartography enthusiasts

They become essential whenever navigation goes beyond coastal or regional waters.

Why Buy Your Orthodromic Charts from Nautic Way?

The Nautic Way Maritime Bookshop offers a carefully selected range of orthodromic and gnomonic charts, tailored to the demands of modern ocean navigation.

By choosing Nautic Way, you benefit from:

• Reliable, specialized nautical charts
• A range dedicated to deep-sea navigation
• Expert advice in maritime cartography
• A service trusted by nautical professionals

Conclusion

Orthodromic and gnomonic charts are essential tools for planning and optimizing long-distance maritime voyages. By allowing navigators to plot the shortest route between two points, they offer a major strategic advantage in ocean navigation.

Although technical and limited to planning purposes, their complementarity with Mercator charts makes them an indispensable reference for experienced navigators.

Available from Nautic Way, orthodromic and gnomonic charts support your voyage planning with precision, expertise, and true maritime professionalism.

​​​​​​​Before choosing your nautical chart, it is interesting to know the particularities and differences between nautical charts.

All nautical charts, paper or digital, are published to order and benefit from the latest updates. In accordance with the provisions of Article L221-28 of the Consumer Code, the right of withdrawal cannot be exercised for the supply of goods likely to be obsolete quickly. These charts are neither taken back nor exchanged. Consequently, these orders are concluded definitively as soon as the order is placed by the Customer according to the methods specified in these GCS.