Knowing how to read nautical charts is a core skill for any boater or angler who goes beyond a small, familiar pond. A chart is a map of the water, showing depth, the shape of the bottom, hazards, channels, buoys and landmarks, and it can also reveal the structure where fish gather. Whether you use a paper chart, a digital chart on a chartplotter or a phone app, the same principles apply. This guide explains the main elements of a chart, how to read depth numbers and contour lines, how to understand symbols and colours, how to find hazards and safe routes, how to use charts for fishing and how to avoid common mistakes. Always combine charts with a lookout, a compass and local knowledge.
How to read nautical charts: start with the basics
What a chart shows
A chart shows land, water, depth, navigation aids and hazards. Unlike a road map, it focuses on what lies beneath the surface.
Paper and digital charts
Paper charts show everything at once, while digital charts let you zoom and overlay data. Both follow similar conventions. See our chartplotter buying guide for digital options.
Scale and orientation
Check the scale, which tells you how much detail you can expect, and note which way is north. Most charts have north at the top, but check.
Depth numbers and datum
Soundings
Small numbers scattered on the water are depths, called soundings. They show how deep the water is at that point.
Units and datum
Charts use feet, metres or fathoms and measure from a chart datum, often a low-water reference. Check the legend. Actual depth can be greater or less depending on tide or water level.
Tide and water level
In tidal water, add or subtract tide height to get actual depth. In lakes and reservoirs, water levels change with rainfall and releases. See our tides and currents guide.
Contour lines
Lines of equal depth
Depth contours connect points of the same depth. Closely spaced lines indicate steep slopes, and widely spaced lines indicate gentle slopes.
Colour shading
Many charts shade shallow water in a different colour to highlight hazards. Learn what each shade means on your chart.
Fishing value
Contours show ledges, humps, points and channels, which are prime places to fish. See our fishing drop offs guide and contour trolling guide.
Symbols and colours
Buoys and markers
Buoys and markers show channels and hazards, and their shapes, colours and numbers carry meaning. Learn the system used where you boat.
Hazards
Rocks, wrecks, obstructions and shallow areas have their own symbols. Treat them with respect and keep well clear.
Landmarks and lights
Lighthouses, towers and prominent buildings help you confirm your position.
The legend
Every chart has a legend, which explains symbols. Study it before you rely on a chart.
Plan a route
Choose safe water
Plan a route through deeper water, away from hazards, and note alternate routes.
Use waypoints
Mark turning points and hazards as waypoints. See our marine GPS waypoints guide.
Check clearances
Check bridges, power lines and shallow spots along your route.
Plan for tide and weather
Check conditions and plan departure and return times. See our boat navigation safety basics.
Use charts with GPS and sonar
Compare chart and reality
Check what the chart shows against what you see around you and on sonar. If they disagree, trust your eyes and sonar and slow down.
Use sonar to verify depth
Sonar shows actual depth at your position. See our fish finder with GPS guide.
Keep charts updated
Channels shift and aids move. Update charts regularly.
Charts for fishing
Find structure before you launch
Study the chart for points, humps, creek channels and ledges, and mark candidates.
Combine with sonar
Use the chart to find areas and sonar to confirm what is there. See our how to find fish with a fish finder guide.
Record what works
Add waypoints and notes. See our fishing journal guide.
Limits of charts
Charts can be out of date
Survey dates vary. Some areas have not been surveyed recently.
Detail varies
Charts for small lakes may be less detailed than coastal charts.
Charts are not a substitute for a lookout
Always watch the water, and carry backups such as a compass. See our marine compass backup guide.
Common mistakes
- Ignoring the chart datum and tide.
- Following a chart blindly.
- Using outdated charts.
- Missing the legend.
- Cutting corners near hazards.
A practice exercise with a chart
Open a chart of a lake or coastal area you know. Find your launch ramp and mark it. Trace a route to a favourite fishing spot, noting the soundings along the way, any hazard symbols and the nearest channel markers. Identify two places where contours bunch together, and think about why fish might hold there. Then compare the chart with what you remember seeing on the water. This exercise teaches you to connect symbols with real features, and it takes only a few minutes. Repeat it with a new area before your next trip, and notice how quickly the symbols become familiar.
Check the chart against your sonar
On your next outing, run a short line across a contour and watch the depth on your fish finder. Does it match the chart? Differences can come from water level, tide, chart age or survey detail. Understanding these differences builds trust in the chart and in your own judgement.
Teach someone else
Explaining how to read a chart to a friend or family member is one of the best ways to learn it. It also improves safety on board, because more people understand where you are and where you are going.
Digital chart tips
On a chartplotter, adjust the chart detail level so you can see soundings and symbols clearly without clutter. Use the cursor to select an object, such as a buoy or a hazard, and read its information. Turn on shallow-water shading if available, and set depth alarms. Learn how to zoom in and out quickly, and how to switch between north-up and heading-up orientation. A little practice with the controls means you can use the chart quickly and confidently when it matters.
Frequently asked questions
What do the numbers on a chart mean?
They are depth soundings, in the units shown on the chart.
How do I read contour lines?
They connect equal depths. Close lines mean steep slopes.
Do I need paper charts?
They are useful as a backup, and some boaters like to carry them.
How often should I update charts?
Check each season, and whenever your supplier offers updates.
Read the water before you go
Learning how to read nautical charts helps you navigate safely, plan trips and find fish. Study the legend, understand depth and contours and always compare the chart with reality. Browse GPS units and chartplotters in our shop and contact us if you need help choosing equipment.
Related reading from Fishtrail Outfitters
- Marine GPS Chartplotter Buying Guide: Features, Sizes and What to Skip
- Boat Navigation Safety Basics: Using GPS, Charts and Common Sense
- CHIRP Fish Finder Explained: How CHIRP Sonar Works and Why It Matters
- Portable Fish Finder Guide: Wireless, Handheld and Kit Options Explained
Further reading: U.S. Coast Guard Navigation Center · Take Me Fishing: how to get started and where to fish
Questions before you buy? Contact Fishtrail Outfitters or browse the shop.
