How to Read Nautical Charts and Use Chart Plotters

Introduction

Nautical charts are the foundation of marine navigation. Whether you’re using traditional paper charts or modern digital chart plotters, understanding chart symbols, scales, and how to interpret information is essential for safe boating. This guide covers chart fundamentals and practical chart usage for Pacific Northwest waters.

Chart Fundamentals

What is a Nautical Chart?

A nautical chart is a detailed map of coastal waters, harbors, and navigable areas. Unlike road maps, charts emphasize:

• Water depth (critical for safety)
• Hazards (rocks, wrecks, reefs)
• Navigation aids (buoys, lights, ranges)
• Bottom type (affects anchoring)
• Current patterns
• Magnetic variation

Chart Projections and Accuracy

Most nautical charts use Mercator Projection, which:

• Represents Earth’s curved surface on flat paper
• Preserves direction (rhumb line courses are straight lines)
• Distorts distance in high latitudes (less important in PNW)
• Makes charts reliable for navigation

Chart Scales

Chart scale indicates how much real-world area is represented:

Scale Types

• 1:10,000 (large-scale): 1 inch on chart = 10,000 inches real distance

  • Maximum detail; limited area coverage
  • Used for harbors and tight channels
  • Smallest area per chart sheet

• 1:50,000 (medium-scale): 1 inch = 50,000 inches

  • Balance of detail and coverage
  • Typical cruising chart scale
  • Good for passages and route planning

• 1:100,000+ (small-scale): 1 inch = 100,000 inches

  • Minimum detail; wide area coverage
  • Used for coastal overview
  • Largest area per chart sheet

Scale Rule Most navigation charts include a graphic scale printed on the chart itself. Measure distance between two points using dividers, then compare to scale bar (no math required).

Reading Chart Symbols and Information

Depth Measurements

Depths are shown in numbers across the chart:

Depth Units

• United States charts: Depths in feet (for shallow water) or fathoms (1 fathom = 6 feet)
• Modern charts increasingly use metric (meters)
• Always verify units before navigation; critical for safety

Interpreting Depth Numbers

• 25 = 25 feet of water depth
• Numbers placed throughout chart, concentrated in known shallow areas
• Areas with no numbers typically 100+ feet

Color Coding (Most modern charts)

• Green/blue: Deep water (navigable)
• Light blue/white: Shallow water (use caution)
• Yellow/tan: Very shallow or uncharted areas (dangerous)
• Brown/beige: Land areas above water

Bottom Type Symbols

Bottom type is critical for anchoring. Standard abbreviations:

Good Anchoring

• M = Mud (excellent holding)
• C = Clay (excellent holding)
• S = Sand (good holding)
• MdSd = Mud and Sand (good holding)

Marginal Anchoring

• St = Stones
• G = Gravel
• Grs = Grass (eelgrass - poor holding)

Poor Anchoring

• R = Rock (very poor)
• Rk = Rocky
• Ky = Kelp (poor holding)
• Wd = Weed (poor holding)

Hazard Symbols

Charts indicate specific hazards:

Rocks and Wrecks

• Solid circle with X = Rock (always awash or above water)
• Solid dot with X = Rock (periodically covered by water)
• Outline circle with X = Rock (submerged at high tide)
• Crossed anchors or wreck symbol = Sunken vessel
• “Wreck Unk” or “Wreck PA” = Wreck position approximate or unknown

Obstructions

• Irregular line with X = Shallow water/shoal
• Dots with X = Foul ground (debris, wreckage)
• Small line segments = Wrecks/obstructions

Navigation Aids

Charts show buoys, lights, and other navigational markers:

Buoy Types

• Red triangle symbol = Nun buoy (right/starboard side of channel)
• Green square symbol = Can buoy (left/port side of channel)
• Red/green striped = Mid-channel buoy
• Yellow/black = Special purpose buoy (mooring, research, etc.)

Light Structures

• Lighthouse symbol = Major light structure
• Small circle/dot = Minor light or beacon
• Chart shows light characteristic (flashing pattern)
• Color and range provided in legend (green light, red light, etc.)

Ranges and Alignment Objects

• Two vertical symbols = Range markers (align to follow channel)
• Alignment is clearer navigation than relying on compass
• Particularly useful in Puget Sound where ranges common

Chart Symbols for Common Features

Anchorages and Harbors

Anchorage Symbols

• Circle with a mark inside = Recommended anchorage
• Dashed circle = Emergency anchorage
• Text “Anchorage” labels areas
• No symbol = Undesignated (but may be suitable)

Harbor Information

• Detailed harbor insets provided for major harbors
• Larger scale allows clarity on dock locations, hazards
• Basin names and approach descriptions provided

Restricted Areas

Important Restrictions

• Red/magenta shading = Military or restricted area (may have access restrictions)
• No Anchoring symbol = Area where anchoring prohibited
• Fish trap areas = Fishery operations (marked as such)
• Wildlife refuge = Protected area (specific rules apply)

Magnetic Variation

Every chart displays magnetic variation (difference between true north and magnetic north):

Understanding Variation

• Magnetic compass points to magnetic north (not true north)
• Variation changes by location and year
• Chart shows variation as rose symbols
• Variation in PNW: 15-17 degrees east (varies by location)

Applying Variation

• When plotting courses: Convert between true and magnetic
• Most modern GPS/chart plotters do this automatically
• Traditional compass users must account for it
• Small error from ignoring variation compounds over distance

Using Paper Charts

Chart Care and Storage

Preservation

• Store charts flat or rolled (never folded; degrades at creases)
• Keep dry; laminate or use waterproof covers for active charts
• Store in cool location (UV and heat damage charts over time)
• Organize by region for easy reference

Chart Updates

Charts are updated periodically:

Chart Corrections

• NOAA publishes “Notice to Mariners” weekly
• Critical changes (new hazards, light failures) posted immediately
• Chart owner responsible for keeping charts current
• Digital charts update automatically (advantage over paper)

Understanding Chart Dates

• Chart edition number and date printed at bottom
• Older charts may have outdated information
• Buoy positions sometimes change (marked on active charts)
• Hazards discovered after publication posted in Notices

Plotting Positions

Using Latitude and Longitude

• Every chart includes latitude/longitude grid
• Latitude lines run east-west (horizontal)
• Longitude lines run north-south (vertical)
• Use chart’s scales to measure distance

Dividers Method (Traditional)

1. Using dividers (compass), measure distance between two latitude lines
2. Step dividers along your course line
3. Note latitude/longitude at each step
4. Current GPS makes this less critical but useful backup

Course Plotting

Laying Down a Course

1. Use parallel rulers or course plotting tool
2. Align tool with desired course line on chart
3. Walk tool to compass rose on chart
4. Read bearing where tool intersects rose
5. Add variation correction (typically -15 degrees in PNW)
6. Magnetic bearing is what compass should read

Using Digital Chart Plotters

Chart Plotter Advantages

Modern GPS/chart plotters have largely replaced paper charts:

Benefits

• Automatic position display (no manual plotting required)
• Waypoint storage and course automation
• Depth alarm (warns if approaching shallow water)
• Automatic route planning
• Real-time tidal current visualization (advanced units)
• Weather overlay capabilities

Limitations

• Battery dependent (paper charts never need battery)
• Screen glare in bright sunlight (polarized screens help)
• Data entry errors can send you wrong direction
• Technical failure removes navigation capability

Essential Chart Plotter Functions

Waypoint Management

• Create waypoints at current position or entered coordinates
• Name waypoints for easy identification
• Navigate to waypoints (autopilot-compatible)
• Organize waypoints by region or trip

Route Planning

• Plot multi-waypoint routes before departure
• Autopilot can follow route automatically
• Modify routes underway if conditions change
• Distance and estimated time calculations automatic

Depth Alarm

• Set alarm to warn when water shallower than threshold
• Critical safety feature preventing grounding
• Typically set to 1.5x your vessel’s draft
• Example: 5-foot draft vessel sets alarm to 8 feet

Track Recording

• Most plotters record your path (track) automatically
• Useful for retracing route or analyzing passage
• Can view historical tracks to learn optimal routes

Chart Plotter Types

Standalone GPS/Chartplotter

• Dedicated device for navigation only
• Examples: Garmin, Raymarine, Furuno brands
• Range: $500-5,000+ depending on features
• Ideal for dedicated navigation need

Integrated Marine Electronics

• Combines GPS, radar, sonar, autopilot in single system
• Expensive ($3,000-15,000+) but comprehensive
• Professional-grade units offer advanced features
• Overkill for casual cruising

Tablet-Based Solutions

• iPad/Android apps with offline chart capability
• Lower cost ($100-500) but less rugged
• Screen size/brightness limitations
• Good backup to dedicated hardware

Smartphone Integration

• Many plotters integrate with smartphones via wifi
• Allows viewing chart data on secondary screen
• Reduces need for large plotter screen
• Adds cost and complexity

Backup Navigation Without Electronics

Dead Reckoning

If electronics fail, navigate by compass and distance:

1. Note starting position (latitude/longitude from chart plotter before failure)
2. Maintain compass course
3. Track speed through water
4. Calculate distance = Speed × Time
5. Plot position on chart every hour
6. Account for current (known from tide tables or observed)

Limitations

• Compass course can drift (steer discipline critical)
• Speed estimation inaccurate (log reading preferred)
• Current estimation adds error
• Position accuracy decreases over hours of passage

Traditional Navigation Methods

Visual Bearings

• Identify visible landmarks
• Measure compass bearing to them
• Plot bearing on chart (should intersect at current position)
• Crossing bearings from multiple objects provides fix

Depth Contours

• Follow known depth line along coast
• Match soundings to chart depth numbers
• Confirms position in fog or low visibility

Celestial Navigation

• Using stars or sun position to estimate location
• Requires astronomical tables and calculations
• Backup method if all else fails
• Training required; not practical for most recreational boaters

Chart Plotter Best Practices

Pre-Departure Setup

Before Each Trip

• Load offline charts (if subscription-based)
• Input planned waypoints
• Verify battery status
• Test depth alarm functionality
• Check compass calibration
• Confirm GPS is acquiring satellites (takes 1-2 minutes)

Backup Plan

• Have paper charts as backup (all areas you’ll navigate)
• Know how to navigate without electronics
• Understand chart symbols before relying solely on plotter
• Verify plotter accuracy using traditional navigation occasionally

Maintenance and Updates

Chart Updates

• Download chart updates before season
• Subscription services update automatically if connected
• Older charts may have outdated information
• Verify plotter chart version matches chart edition

Device Care

• Keep plotter dry (use cover in rain)
• Protect screen from physical damage
• Battery maintenance (charge after each use)
• Protect from extreme temperature (don’t leave in hot car)

Further Reading

• Navigation Basics — Core navigation concepts
• Tides and Currents — Using chart data for current planning
• Anchoring Techniques — Selecting anchorages from charts