Chuck Hawley demonstrates the use of traditional chart navigation tools including circular parallel rules, standard parallel rules, rolling plotters, dividers, sextants, and star charts for offshore navigation.
GPS chartplotters have made offshore navigation dramatically more accessible, but they have also created a generation of boaters who cannot navigate without them. When electronics fail — as they can offshore — the boater who understands traditional chart navigation tools has options. The one who doesn't may be left without a reliable backup means of navigation.
This guide covers the traditional tools used for paper chart navigation: what each one does, how to use it, and why understanding them makes you a safer and more capable mariner regardless of how much technology you carry aboard.
Why Traditional Navigation Skills Still Matter
GPS and chartplotters are remarkable tools, but electronic navigation systems share common vulnerabilities: they depend on power, functioning hardware, and, in the case of GPS, satellite signals. Any of those three can fail. A chartplotter that loses power 40 miles offshore can leave you without a reliable means for navigation — unless you have paper charts, a compass, and the skills to use traditional plotting tools.
Beyond the backup argument, traditional navigation builds a fundamental understanding of how position, course, speed, and time relate to each other. Navigators who learned on paper charts before GPS understand what the chartplotter is telling them and can recognize when it's wrong. That situational awareness is genuinely difficult to develop through electronic-only navigation.
Celestial navigation is generally not required for entry in mainstream offshore sailing races, although individual races may establish their own navigation requirements. Sailors still have the opportunity to show off their traditional navigation prowess in retro and vintage offshore races, such as the Golden Globe Race and Ocean Globe Race, which deliberately restrict modern navigation technology and emphasize traditional methods such as sextant navigation and paper charts.
Browse our full selection of nautical charts and chart navigation plotting tools to equip yourself for paper navigation.
Parallel Rules: Transferring Bearings on a Paper Chart
Parallel rules are the most fundamental chart plotting tool. Their purpose is to transfer a direction — a course line, a bearing, a range — from one point on a paper chart to the compass rose, or from the compass rose to a point on the chart, while maintaining the exact angle throughout.
How Parallel Rules Work
Standard parallel rules consist of two straight rules connected by pivot arms of equal length. The arms allow the rules to open and close while remaining exactly parallel — any angle established with one rule is preserved in the other regardless of separation. To use them:
- Place one rule along the course line you've drawn on the chart.
- Hold that rule firmly in place and "walk" the other rule away from it by alternating which rule you anchor.
- Walk the rules across the chart until one edge passes through the center of the nearest compass rose.
- Read the degree marking where the rule intersects the compass rose — that is your course or bearing.
The challenge with parallel rules is that the chart moves on a boat. Any slip during the walking process introduces angular error. Practice on a flat, stable surface first, and brace the chart against the chart table before working underway.
Circular Parallel Rules
Circular parallel rules have a rotating compass dial integrated into the instrument. Rather than walking to the compass rose, you set the dial to the desired bearing and use the straight edge to draw the line. This is faster and introduces fewer opportunities for slip error than walking standard parallel rules across a large chart.
Fixed vs. Adjustable Parallel Rules
Standard parallel rules have fixed-length connecting arms and are the most common type. Longer arm length allows more distance to be covered per step, reducing the number of transfers needed across a large chart. Adjustable rules offer variable spacing but require more care to avoid misalignment and are less common on working chart tables.
Rolling Plotters: Faster Course Plotting for the Chart Table
A rolling plotter combines a rotating compass rose with a roller mechanism that allows it to move smoothly across the chart while maintaining the orientation established by the navigator. Unlike parallel rules, a rolling plotter moves as a single unit — no walking, no step-by-step advancement.
How to Use a Rolling Plotter
- Align the plotter's grid lines with the chart's meridians (lines of longitude) to establish the north-south reference.
- Roll the plotter to the position where you want to draw or read a course line.
- Read the bearing directly from the compass dial — no walking to a compass rose required.
- Draw your course line along the straight edge.
Rolling plotters are the preferred tool for many offshore navigators who do regular route planning on paper. They are particularly well-suited to larger charts where walking parallel rules would require many steps and risk cumulative error.
Dividers: Measuring Distance on a Nautical Chart
Dividers are two arms joined at a pivot with a point at the end of each arm. Their function is to measure distances on a paper chart by spanning two points and transferring that span to the chart's latitude scale. On a nautical chart, one minute of latitude corresponds to one nautical mile, making the latitude scale the standard reference for measuring distance.
How to Use Dividers
- Place one point on your starting position, the other on your destination.
- Without changing the spread, move the dividers to the latitude scale on the chart's left or right margin.
- Count the minutes of latitude spanned — each minute equals one nautical mile.
- For long distances, "walk" the dividers along the route in steps of a known distance, counting steps.
Always use the latitude scale at the same approximate latitude as the portion of chart you're measuring. On Mercator projection charts, scale varies with latitude, so use the latitude scale at approximately the same latitude as the distance being measured.
Single-Handed Dividers
Single-handed dividers have a spring-loaded mechanism that lets you adjust the spread with thumb pressure on one hand. For navigators working on a pitching chart table, they are significantly easier to use accurately than traditional two-handed dividers.
The Sextant: Celestial Navigation for Offshore Passages
The sextant is the instrument of celestial navigation — determining position by measuring the angles of celestial bodies above the horizon. It is the most technically demanding traditional tool covered here and is particularly valuable for offshore passages where landmarks and navigational aids may be unavailable.
How a Sextant Works
A sextant measures the angle between a celestial body and the horizon through a system of two mirrors: a fixed horizon mirror and a movable index mirror on an arc arm. Adjusting the index arm brings the reflected image of the celestial body down to the horizon in the eyepiece; the angle is read from the arc. The instrument is named for its original arc span of one-sixth of a circle — 60 degrees.
Taking a Sun Sight
A common celestial observation for practical offshore navigation is the noon Sun sight — measuring the Sun's maximum altitude around local apparent noon to determine latitude.
- Know your approximate longitude so you can estimate when local apparent noon will occur.
- Beginning a few minutes before estimated noon, track the sun's altitude as it rises. Continue taking sights as the Sun rises. When its altitude reaches its maximum and begins to decrease, record the highest observed altitude — this is your meridian-altitude, or noon, sight.
- Use the corrected noon altitude and the Sun's declination from the Nautical Almanac to determine latitude. The calculation depends on the Sun's declination and the observer's hemisphere.
What You Need for Celestial Navigation
- A current Nautical Almanac — jointly published annually by the U.S. Naval Observatory and the UK Hydrographic Office, providing the astronomical data needed for celestial navigation
- A reliable UTC time source accurate to the second — four seconds of time correspond to one minute of longitude, with the resulting distance error depending on latitude.
- Sight reduction tables (HO 229 or HO 249) or a sight reduction calculator to convert observations into lines of position
- A clear horizon — haze, swell, or obstruction introduce errors
The United States Power Squadrons and U.S. Sailing both offer celestial navigation courses for recreational mariners.
Nautical Star Charts: Identifying Celestial Bodies for Navigation
A nautical star chart maps the positions of the 58 navigational stars listed in the Nautical Almanac — bright, well-distributed stars used for celestial fixes. It also shows principal planets, the moon's orbital path, and constellation patterns used as reference frames for locating navigation stars.
How to Use a Planisphere
- Set the date and time according to the planisphere's instructions. Make sure you are using a planisphere designed for your approximate latitude.
- The planisphere shows the portion of the sky that should be visible above the horizon at that date and time.
- Orient the planisphere with north at the top to approximate what you see overhead.
- Use familiar reference stars and constellations to help identify navigation stars, such as Polaris in Ursa Minor, which lies close to true north; Sirius near Orion; and Vega in Lyra, which is prominent in the summer sky in the Northern Hemisphere.
At twilight — when the horizon is still visible and stars are beginning to appear — an experienced navigator can take sights on several stars in a relatively short period to obtain a multi-body fix. This requires star identification to be nearly automatic. You have limited time at twilight, so star identification should be quick and practiced.
Dead Reckoning: Estimating Position Between Fixes
Dead reckoning (DR) estimates your current position from a known previous position, your course, speed, and elapsed time. It is a technique, not a physical tool, but it underpins all traditional navigation and makes the physical tools meaningful.
The formula is straightforward: distance = speed × time. Steering 240° magnetic at 6 knots for 3 hours puts you approximately 18 nautical miles from your last known position on a 240° bearing. Plot that with parallel rules and dividers and you have a DR position. DR accumulates error over time as current, leeway, and compass deviation cause actual track to differ from the intended track. A disciplined DR plot should therefore be updated whenever a reliable position fix is available.
About Chuck Hawley
Chuck Hawley served as West Marine's Vice President of Product Information, becoming one of the most widely recognized experts in recreational marine safety, navigation, and seamanship in the United States. He holds a USCG 100-ton Master's License and has contributed to safety standards development for U.S. Sailing and the International Sailing Federation. Chuck has made numerous offshore passages and considers traditional navigation skills essential for any mariner operating beyond sight of land. His video guides reflect decades of practical on-the-water experience.
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Chart Navigation Tools — Frequently Asked Questions
What are parallel rules used for in chart navigation?
Parallel rules transfer directional information between a plotted course line and the compass rose on a paper nautical chart. By walking one rule against the other across the chart while maintaining the original angle, you can read the magnetic or true bearing of a plotted line on the chart without moving the line itself. They are the fundamental tool for plotting courses and reading bearings on paper charts.
How does a rolling plotter differ from standard parallel rules?
Standard parallel rules must be walked step-by-step across the chart, alternating which rule holds while the other advances, until the edge reaches the compass rose. A rolling plotter moves as a single unit on a roller and has an integrated compass rose you align with the chart's meridians — you can then read bearings directly from the plotter's dial without walking to the chart's printed compass rose. Rolling plotters are faster and introduce fewer opportunities for slip error on large charts.
What are dividers used for in navigation?
Dividers measure distances on nautical charts. You span the distance between two points, then transfer that span to the chart's latitude scale — where one minute of latitude equals one nautical mile. For long distances, walk the dividers along the route in steps of a known distance. Single-handed spring-loaded dividers are easier to use accurately on a moving vessel.
What is a sextant and how is it used at sea?
A sextant is a precision optical instrument that measures the angle between a celestial body and the visible horizon. That angle, combined with accurate UTC time and data from the Nautical Almanac, allows a navigator to calculate a line of position. Multiple observations of different celestial bodies produce intersecting lines of position — a celestial fix — that determines the vessel's latitude and longitude independently of GPS or any electronic system.
Why do navigators still use paper charts and traditional tools?
Electronic navigation systems depend on power, satellite signal, and functioning hardware — all of which can fail offshore. Paper charts and traditional tools don't depend on electrical power or satellite signals, making them a valuable backup when electronic navigation fails. Beyond providing a reliable backup, traditional navigation builds situational awareness and chart-reading skills that make navigators more capable and safer even when electronics are working perfectly.
How accurate are traditional navigation tools?
Parallel rules and dividers used carefully can provide highly accurate results, although practical accuracy is limited by chart scale, plotting technique, and the precision of the chart itself. With a properly adjusted sextant, accurate time, good observations, and practiced sight reduction, celestial navigation can produce fixes accurate to within a few nautical miles and sometimes considerably better. Dead reckoning accumulates error over time, particularly when current, leeway, and other factors cause the vessel's actual track to differ from its intended course. Careful DR provides a useful estimate of position between reliable fixes.
Do I need to learn traditional navigation if I already use GPS?
Yes, particularly for offshore or extended coastal passages. GPS chartplotters fail — power outages, antenna failures, software crashes, and saltwater intrusion all happen at sea. A navigator with parallel rules, dividers, a compass, and paper charts can continue navigating after these types of electronic failures. The U.S. Power Squadrons and U.S. Sailing both offer navigation courses covering traditional methods alongside modern electronic navigation.
What is the difference between true north and magnetic north on a nautical chart?
True north points to the geographic North Pole. Magnetic north points to the magnetic North Pole, which is offset from geographic north by an amount called magnetic variation or declination that varies by location and changes slowly over time. Nautical charts print compass roses with both true (outer ring) and magnetic (inner ring) degree markings. When plotting courses to steer with a magnetic compass, use the magnetic rose. GPS-derived courses and bearings are commonly referenced to true north, although the reference used depends on the equipment and its settings. When converting a true course to a magnetic compass course, apply the chart's local magnetic variation.