If you understand the fundamental difference between these formats, you may already foresee the implications for file size. In one case the chart is an image comprised of millions of saved pixels. In the other case the chart is a database of navigational objects linked to a chart position. Because raster charts are scanned pictures, and every pixel on the chart must be represented in terms of color and intensity, the files are huge. Vector charts are a database rather than an image, so the information is compressed into much smaller files, taking up less space on your hard drive.

Raster and vector charts look very different on your laptop screen. Raster charts look like traditional NOAA paper charts. Many boaters like raster charts for the simple reason that the chart "looks right." In contrast, vector charts only show land forms and some depths. In fact, they are so sparse that some boaters mistakenly think information is missing.

It's all there. Vector charts are simply displayed in layers. This format reduces chart clutter and lets you select only the information you'd like to view, which is very handy in large harbors and other areas where there is much to note. Simply click on the symbol for an object on the chart, such as the small square icon denoting a marina, and a data window appears with detailed information about that facility.

Because raster charts are scanned images, you cannot "zoom in" to see more detail. Zooming the display past the accuracy of the digitized image, called overscale, results in an unreadable mosaic of colored pixels. Vector charts, because they are created on-the-fly using a database, allow you to zoom in to see more detail.

A similar distinction occurs when you attempt to rotate electronic charts, as you might do to display a chart "course up." Raster charts are large image files and do not rotate quickly. Even worse, the text is fixed and therefore rotates with the image, giving you a course-up chart with depths and other information displayed upside-down or sideways. In contrast, vector charts have the ability to create a display independent of orientation, with text data on a separate layer. Rotating displays are not a problem.

Finally, the most important implication of raster versus vector chart formats lies in their "intelligence." A depth contour of 60 on a raster chart is simply a collection of black pixels to create the numeric characters six and zero. That same depth contour on a vector chart is a value in a database linked to its position in latitude and longitude. Because vector charts are databases of navigation items, they can be intelligently linked to navigation software, which can compute distances or sound an alarm if the vessel crosses a depth contour (say 60 feet).

Raster charts are only pictures, so navigation software is unable to differentiate between a buoy pixel or a water pixel.

FAMILIARITY AND AVAILABILITY

Although vector charts seemingly have the technological advantage, raster charts currently win on two counts: familiarity and availability.

Because they look familiar, raster charts are much easier to use for planning and general chart viewing. Most boaters still have trouble assimilating the sparse display of a vector-format chart. Fortunately, there is an effort underway to standardize paper chart symbols. When complete, there may be less visual discrepancy between paper and electronic chart formats.

Vector charts are based on a database and are organized in layers, that can be quickly drawn at any scale. They are becoming popular for their uncluttered appearance and intelligent storage and display of information. Clicking on a bridge or navigation aid displays a window, as shown.

Raster charts are also more plentiful. Because converting paper charts to raster format is infinitely easier than creating a vector database, a complete U.S. raster catalog is already available through NOAA for free. But only about 60 percent of NOAA's projected chart cells are available in vector format. Creating a vector database of all chart features is an enormous project and the priority for conversion is driven by the "professional navigator" and commercial ship traffic.

In 2001, NOAA initially identified 40 major ports for completion by 2003. After these were done, another 100 or so ports were identified for conversion. NOAA's priority is to begin with major ports, followed by the seaward connections between these ports for commercial shipping traffic. For example, the Intracoastal Waterway between the commercial ports of Norfolk and Charleston is not a priority. Instead, the seaward connection between these two ports, including Cape Henry and Cape Hatteras, is a priority. So far, NOAA has completed about 600 of its roughly 1,000 projected vector charts.

This huge task is hampered by government budgets and further slowed by the demands of simultaneously maintaining and updating an increasingly larger chart library. Alexandra Heliotis, Deputy Chief of NOAA's Marine Chart Division, points out that an average year produces 5,900 individual updates that result in corrections to 16,000 chart products.

Still, there are few who would argue with the idea that the vector format is the future of electronic charting. The files are smaller, more intelligent and make it easier to incorporate frequent updates. Oddly enough, NOAA's long-term vision is the reverse of what it has been doing for decades: the agency wants to print paper charts from an accurate and up-to-date vector database.

NEXT WEEK: How to locate, load and catalog electronic charts.