A chart is a special kind of map, designed for navigation. There is a difference between charts and maps. Maps are representations of land areas showing selected features of the Earth’s surface, political subdivisions and topography.


Charts detail waters areas, showing depths, features of the seabed, details of the coastline, navigational hazards and structures such as harbours and buildings.

In order to build maps and charts, cartographers use projection techniques. As it is impossible to project a three-dimensional object upon a two-dimensional surface without distortion, several methods have been developed to represent the Earth’s surface on a plane, controlling the distortion as much as possible. Different types of projections are used according to the areas to be represented and the purpose of the map or chart.
Istituto Idrografico della Marina compiles charts using gnomonic and Mercator projections.
In the gnomonic projection, all great circles are represented as straight lines. Thus, the shortest route between two locations corresponds to that on the map. The gnomonic projection is used mainly for long range navigation.


The Mercator projection, the most used for nautical charts, shows true angles and true distances. On a Mercator chart meridians are parallel lines of longitude even at the poles, where meridians actually converge. This distortion at high latitudes is a disadvantage of the Mercator projection. This is why the Mercator projection is usually employed up to 66°. The advantage of the Mercator projection is that meridians and parallels cross each other at right angles to form a rectangular grid. This allows mariners to plot a straight-line course as a straight line on the chart.

According to their scale and purpose, charts are usually divided into the following 5 categories:
Overview (up to 1:3.000.000): small scale charts, covering a vast area but providing few details; they are mainly used for ocean crossing and route planning.

General (up to 1:1.000.000): used for off-shore navigation when larger scale charts are not available.

Coastal (1:300.000/250.000 - 1:100.000): used when passing from off-shore to inshore navigation.

Approach (1:100.000 - 1:30.000): having a scale larger than coastal charts, these charts provide details on seabed features, conspicuous points, access to harbours, straits, passages and so forth…

Harbour (1:20.000 - 1:5.000): these charts show harbours and surrounding areas, detailing hazards, signals, berthing and other useful information.

Charts use symbols to represent real-world objects. Knowledge of these symbols, as well as their common abbreviations and terminology, is an important part of using a chart.


For a complete list of the nautical charts published by Istituto Idrografico della Marina, click here.