Nautical chart

Lore & Background

Nautical charts are based on hydrographic and bathymetric surveys, which are laborious and time-consuming, so data for many areas may be dated or unreliable. Historically, depths were measured with a sounding line; modern methods include echo sounding and sweeping with a horizontal wire to check minimum depth over obstructions. All depths are measured relative to a datum or reference level to ensure safety. Charts are issued by national hydrographic offices and are considered 'official,' as opposed to commercial publishers' charts. The International Hydrographic Organization coordinates an international chart series (INT) to unify chart systems worldwide. Chart corrections are provided via systems such as Notice to Mariners, Local Notice to Mariners, and Broadcast Notice to Mariners; print-on-demand charts can include corrections downloaded as recently as the night before printing. As stated in the Mariners Handbook, 'No chart is infallible. Every chart is liable to be incomplete.'

Reader's Guide

Nautical charts are foundational to maritime safety, providing mariners with critical information for route planning and hazard avoidance. Their significance is underscored by legal requirements in many countries for commercial vessels to carry them. The development of electronic navigational charts and print-on-demand paper charts has improved timeliness of corrections, but the inherent limitations of survey data mean uncharted dangers remain a risk, as demonstrated by several notable groundings and collisions. The Mercator projection, used on most nautical charts, allows direct use of plotted courses as steering bearings, though it distorts distances at high latitudes. The international coordination through the International Hydrographic Organization aims to standardize charting worldwide, but national hydrographic offices remain the primary authorities. The legacy of nautical charts is their role in enabling global maritime commerce and exploration, while their limitations remind users that no chart is infallible and that continuous correction and prudent seamanship are essential.

Did You Know?

• Nautical charts may be printed on paper (raster navigational charts) or as computerized electronic navigational charts.
• Depths on charts are measured relative to a datum or reference level to ensure safe clearance over obstructions like masts.
• The Mercator projection is used on the vast majority of nautical charts because it is conformal, allowing bearings to be used directly as course-to-steer.