The NOAA Office of Coast Survey (OCS) produces two kinds of digital charts;  Raster Navigational Charts (RNCs) and Electronic Navigational Charts (NOAA ENC®). 

A Raster Navigational Chart is an accurate digital image comprised of pixels, displayed on an electronic screen.  Each pixel has a unique color or it has no color.  The pattern of the colored and the empty pixels gives shape to the individual features of the chart.  To better understand the nature of raster charts, we examine the process that creates them.

Like all color printed materials, charts are printed from color separated negatives.  In 1994, OCS began to scan all of the negatives used for chart printing.  From this library of digital files, cartographers are able to update charts efficiently utilizing Computer Aided Design (CAD), a software commonly used for drafting.  From these files, paper charts continue to be printed.  The digital files make it possible to provide navigational information to the public through two new products: Print on Demand (POD) paper charts and raster navigational charts (RNCs).
 
To become a raster navigational chart, cartographers add a system of geographic reference to the digital image.  The system enables the cartographer to update charts accurately.  Using the formatted images in electronic display systems called Raster Chart Display Systems (RCDS), mariners receive accurate real-time vessel positions. 
 
However, raster charts are mere images.  There is no digital difference between pixels in a pattern that represent a buoy and that which represents a submerged wreck.  Therefore, using an RNC, mariners must rely on human intelligence to interpret the images for navigation decisions.

An Electronic Navigational Chart (ENC) is navigational features in a vector format. That is achieved by digitizing each feature’s geometry into a specific object. NOAA cartographers collect the vector features according to a data standard, called the S-57 format, recognized by international treaties obtained through the International Hydrographic Organization (IHO).  The navigational objects are maintained in a database, with additional information about their real world characteristics: geographic position, shape, color, the age of the data, etc.

Mariners navigate with an ENC, using an Electronic Chart Display and Information System (ECDIS) that interprets the stored data.  In addition to providing real-time vessel position, the ECDIS allows the mariner to select features to display that enhance navigating with his particular vessel characteristics, and in specific weather and marine traffic conditions.  Mariners program these parameters into their ECDIS system that can produce visual and audible alarms to warn of dangers.

The NOAA Office of Coast Survey, the United States Hydrographic Office, is exclusively responsible for producing and authorizing ENC data in U.S. territorial waters.  An ECDIS must use authorized ENC data.  However, ENC data may be used by any system that interprets S-57 format.  The federal government authorizes private electronic chart vendors to produce other vector products using NOAA ENC data, and to convert it to their own proprietary format.

The federal government authorizes private firms to scan NOAA paper charts and download NOAA raster files to produce other navigational products. The public is invited to download raster and electronic navigational charts for free.

The following exhibit portrays examples of the two types of charts, with a bulleted list of characteristics that distinguish them.

RASTER NAVIGATIONAL CHART (RNC)	RNC Characteristics
	Looks like paper chart; familiar to paper chart users RCDS software integrates real time GPS with chart image Can be updated with weekly raster patches No inherent safety warning capability Does not have the capability to show denser data when zooming in. Cannot surpress specific charting features Cannot rotate text