Maps and nautical charts serve distinctive purposes. Knowing these differences are key to safe marine navigational practices.
A sure sign of a novice mariner is shown when they inadvertently refer to nautical charts as maps. While it may seem like a fine point to many people, there are important differences to be found between the two publications. We can see the variations in their level of detail, information provided, supporting surveys, and practical use.
Characteristics of Maps -
Level of Detail - Maps represent landforms shown in relief using elevations normally delineated from a datum based on mean sea level. Terrain information is seldom detailed and is shown only in rough orders of magnitude. Topographic data shown has no effect on driving safety.
Static Documents - Maps are not useful for determining a new course. There is no provision to plot a course line and routes are predetermined by existing infrastructure. They show only the existing highways, roads, and trails that can be followed. Intersections are fixed and a vehicle operator has limited choices in the direction that can be followed through the junctions.
Quality of Navigation Data - Roadmaps offer no information regarding the condition of a highway, outside of capacity or surface material. Survey data can be quite dated and maps don't tell a driver about obstructions or rough road conditions. It is up to the driver to visually determine the state of the roadway and find other information to determine road conditions.
Characteristics of Charts -
Coastwise Navigation - Nautical charts are meant to facilitate pilotage and coastal navigation. Pilotage is a form of navigation that relies on correctly identifying natural features and landmarks to establish a safe course of travel. Nautical charts depict the coastline in high resolution accounting for the effects of tidal changes and the character of waterways.
A Working Document - Charts are meant to be used to plot courses. Vessel course lines are plotted directly on the publications. Waypoints are plotted to mark relative vessel position or mark course changes. Charts provide a record of previous journeys and room to record trip notes.
Detailed Information - Mariners depend on charts to provide current and timely information about hazards to navigation. These can be areas of reduced water depth, protruding rocks and shipwrecks.
Invisible Features - Marine charts provide a window into the realm beneath the water surface. Knowledge of underwater topography is vital to avoidance of hazards and safe navigation.
To learn more about nautical charts, download or purchase a copy Chart No. 1 published by the Office of Coastal Survey.
The copyright of the article Map Versus Chart in Boating & Sailing is owned by Alan Sorum. Permission to republish Map Versus Chart in print or online must be granted by the author in writing.
