How to Read a Marine Chart
A practical guide to interpreting nautical charts — the essential skill for safe navigation on the water.
Quick Reference
Distance
1 minute of latitude = 1 nautical mile (1.852 km)
Depths
Metres below LAT (lowest astronomical tide)
System
IALA Region A (port = red, starboard = green)
Interactive Tools
Distance Calculator
Great-circle distance between two points
True → Magnetic Converter
Convert true bearing to magnetic compass bearing
Latitude & Longitude
- •Marine charts use a grid of latitude (horizontal lines) and longitude (vertical lines) to pinpoint any location on the water.
- •Latitude is measured in degrees north or south of the equator (e.g. 36°50′S).
- •Longitude is measured in degrees east or west of the Greenwich meridian (e.g. 174°46′E).
- •Each degree is divided into 60 minutes (′), and each minute into 60 seconds (″). One minute of latitude equals one nautical mile.
- •Read coordinates as latitude first, then longitude — e.g. 36°50′S 174°46′E.
Scale & Distance
- •The chart scale tells you how much the real world has been reduced. A 1:25,000 chart shows more detail than a 1:150,000 chart.
- •Use the latitude scale on the left and right margins to measure distance — one minute of latitude = one nautical mile (1.852 km).
- •Never use the longitude scale for distance — it compresses towards the poles.
- •Use dividers to step off distance along a route, then read the total against the latitude scale.
Depth Contours & Soundings
- •Numbers on the chart are depth soundings. On New Zealand charts these are in metres.
- •Contour lines connect points of equal depth, giving you a visual picture of the seabed shape.
- •Shallow areas (shoals, reefs) are shown with tighter contour lines and lighter shading.
- •Always check the chart datum — depths are typically measured below Lowest Astronomical Tide (LAT), meaning water should be at least that deep at low tide.
Hazards & Symbols
- •Rocks: an asterisk (✱) or cross (+) marks a rock. A dotted outline means it covers/uncovers with the tide.
- •Wrecks: shown as a half-hull symbol (∿) or the letters "WK".
- •Kelp, weed, and obstructions are marked with specific symbols — always consult the chart legend (Chart 5011 for NZ).
- •Restricted or caution areas (firing ranges, pipelines, cables) are shown with dashed boundaries and labels.
Buoys, Beacons & Marks
- •Lateral marks (port/red, starboard/green) guide you in and out of channels following the IALA Region A system used in NZ.
- •Cardinal marks (north, south, east, west) indicate the safe water side of a hazard — named after the quadrant of the compass they occupy.
- •Isolated danger marks are black and red with two black spheres on top.
- •Safe water marks (red and white vertical stripes) indicate navigable water all around.
- •Special marks (yellow) indicate features like recreation zones, mooring areas, or cable areas.
Compass Rose & Bearings
- •The compass rose shows true north (outer ring) and magnetic north (inner ring).
- •The variation figure printed inside the rose tells you the difference between true and magnetic north for that area (e.g. "Var 20°E 2020, decreasing 3′ annually").
- •To convert true to magnetic: add east variation, subtract west variation ("Error east, compass least; error west, compass best").
- •Plot bearings as true bearings on the chart, then apply variation to get the magnetic bearing for your compass.
Tidal & Current Information
- •Tidal streams are shown with arrows indicating direction, and figures showing the rate in knots at spring and neap tides.
- •Tidal diamonds (lettered diamonds on the chart) reference a table that gives hourly tidal stream rates and directions.
- •Always cross-reference tidal information with a current tide table for the date of your voyage.
- •Remember that depths shown on the chart are at LAT — actual depth will be greater at high tide.
Pro Tip
Always carry the most up-to-date chart for your area and check Notices to Mariners before each voyage. Electronic charts (ECS/ECDIS) are useful, but paper charts remain an essential backup if electronics fail.
Disclaimer: This guide provides general information only and does not replace formal navigation training or official chart publications. Always consult official charts (Land Information New Zealand for NZ waters) and undertake a recognised boating safety course. The skipper is responsible for all navigational decisions.