A Tale of Two Charts: Raster and Vector

The electronic chart world in the U.S. is going through an upheaval that will have considerable repercussions for chart users. This upheaval has its roots in the U.S. government’s policy of not applying royalty rights to charts – once produced, they can be reproduced by anyone else without the signing of any agreements or payment of any fees (a highly controversial position within the international hydrographic community).

There are two types of electronic charts: raster charts and vector charts. Raster charts are digital pictures of the Mylar sheets used to make paper charts. Vector charts consist of chart data that is reduced to a series of points. These points are stored in terms of latitude and longitude coordinates and then “tagged” with data appropriate to that point.

Due to the extra work required to make a vector chart, the hydrographic offices have been much slower in producing them than raster scans. As a result, almost all vector charts to date have come out of the private sector. This is now changing as “official” vector charts, known as Electronic Navigation Charts (ENCs), are making their way into the marketplace.

In keeping with its standing policy on copyright issues, the National Oceanographic and Atmospheric Association (NOAA) produces ENCs and puts the files on its Web site for anyone to download. Currently, about 50 percent of the U.S. chart database has been vectorized; the rest should become available over the course of the next five years. On the other hand, all the Mylar-based raster scans are now available for download on the Internet for free (www.nauticalcharts.noaa.gov). Navigators will still need a software package to display and manipulate these chart files, but the availability of the ENC and raster files is changing the relationship between the various electronic chart and software providers in the recreational marketplace.

To get a better handle on these issues – raster versus vector and what the availability of raw chart files may mean to the end user – I took a look at some electronic chart products on a recent delivery trip from Chesapeake Bay to the Bahamas. I used two versions of Maptech’s Offshore Navigator (one released, based on raster charts only; one a beta version at the time with both raster and vector, which has now been released as Chart Navigator Pro) together with Nobeltec’s Admiral (with both raster and vector charts). I ran them side-by-side on two different laptop computers.

When we set sail it was blowing a gale out in the Atlantic. Instead of going outside around treacherous Cape Hatteras, we opted for the inside route, using the Intracoastal Waterway (ICW) from Norfolk, Va., to Beaufort, N.C. The ICW, it turns out, is a great way to highlight the differences between the two electronic chart options.

The ICW via raster

Both Maptech and Nobeltec have numerous “value-added” features inserted as overlays on their raster charts, including tide and current stations (once again derived from free NOAA data), and various kinds of photographic overlays. Some of the photographs are geo-referenced (the photograph is manipulated in such a way that it can be directly overlaid on the chart with all the features matching up), and some of the photographs are traditional photographs of things like marina entrances. There may be detailed information that can be called up on marinas and other shoreside places of interest to mariners. This is all useful stuff with new material being added all the time. If in the market for electronic charts, you should wait until close to the last minute to buy them (give yourself enough time to get familiar with the software), and then compare the “added value” features from one software provider to another.

The ICW via vector

For all their great features, I rapidly found that raster charts, especially on the ICW, take a back seat to vector charts. A typical computer screen has a resolution of between 70 and 100 dots per inch (dpi), as compared to around 1,000 dpi on a printed chart. With raster charts, in order to see the details of the paper chart, you have to blow it up to about twice the scale of the paper version to make it legible on screen. In practice, following a narrow channel such as the ICW is still not that easy, so the temptation is to zoom in another 2:1, so now the chart is grossly overzoomed at 4:1.

Because a raster chart is an electronic photograph, every time you zoom in you proportionately increase the width of all lines, the size of all soundings, letters, etc. Given the relatively low resolution of computer screens, no matter what you do, the raster chart is nowhere near as clear or crisp as the original. Everything’s a bit fuzzy and the screen is generally very cluttered.

Switching to vector charts is akin to waving a magic wand over the picture. The nature of vector is such that regardless of the zoom level, lines, soundings, labels and so on are always displayed at the same (readable) size. So now when you zoom in, the lines, soundings and text don’t change, and as a result, the open areas increase disproportionately. At any given zoom level, the vector chart is inherently easier to read than a raster chart. In addition, you can turn various layers on and off in the vector format. The raster chart, for example, shows all nearby roads, which clutter up the chart. You can turn those off in vector and just save the bridges, which cannot be turned off. The raster chart has all the bridge information printed on the chart. You can turn this off in vector, as well, and opt instead to place the cursor on the bridge, at which point the information is displayed in a dialog box. The same goes for all nav aids (buoys, beacons and so on). And for depth soundings you can, for example, elect to show just those soundings that are potentially dangerous to your boat – in our case, 6 feet or less.


Most electronic chart software now allows you to display charts as north up (the traditional orientation of paper charts) or course up (if the boat is going east to west, east will be at the bottom of the screen and west at the top). In our case, we were headed generally south. With a traditional north-up chart, you have to think twice every time you give directions to the helmsperson: if the channel goes to the left on the chart, the boat must be steered to starboard. Putting the chart into course-up mode is more like driving down the highway: if the channel goes to the left, you steer to port.

If you turn a raster chart course up, all the soundings and labels turn with it. Going south, as we were, they all end up upside down, making the chart hard to read. On a vector chart, they all get turned around so that no matter the orientation of the chart, they are the right way up.

Peripheral vision

No matter in which direction the chart is oriented, one of the problems with all electronic charts comes from the combination of low screen resolution, which requires at least some level of zooming in beyond the scale of the original paper chart (even with vector, the International Hydrographic Organization has settled on an appropriate zoom level of 1:1.7), coupled with the small area of even the largest screen relative to the area of a paper chart; the result is extremely limited onscreen “peripheral” vision.

Maptech, Nobeltec and others get around this to some extent by allowing you to split the screen and put a much smaller scale (larger area) chart on one half, but then in the raster version the details are completely unreadable. Maptech also has another neat feature which projects the chart forward from the boat’s position in a course-up mode, onto a sphere with the scale rapidly decreasing as you move forward of the boat. This has the effect of dramatically increasing the peripheral vision, but with a concomitant loss of detail. I used it a number of times, but in the end couldn’t make up my mind as to whether it was a nifty gimmick or a useful device.

Quilting and de-cluttering

While quilting the charts together makes route planning easy, it raises other issues. When you quilt raster charts at different scales, the displayed soundings, symbols and labels will vary in size according to the original (paper) chart scale, so you can immediately tell which soundings are from small-scale (inherently less accurate) charts and which are from large-scale (more accurate) charts. Given soundings displayed at larger-than-normal size, you know the underlying chart was at a small scale and should immediately recognize the need to exercise greater caution near hazards, which may not be accurately charted.

With a vector chart, given that all soundings, symbols and labels get resized to a common size at any zoom level, you lose that sense of the underlying accuracy of the data. If you retain the data layer that shows the outlines of the original charts you can at least see when you are moving from one chart to the other, but if you turn off that layer the only indication may be a change in the density of the soundings (those from larger scale charts will be more dense), and even that will disappear if various de-cluttering measures are implemented. On high-end vector charts (and all ENCs) you can query any individual sounding or feature to call up a dialog box that will tell you where the data came from and what its accuracy is, but with lower-end products you can’t do this.

The resizing of soundings, symbols and labels with vector charts does, at times, produce other problems. A large-scale (small-area) chart will have a much higher density of soundings than a small-scale chart of the same area. If you zoom out the large-scale chart, the soundings resize at each zoom level to maintain their legibility (instead of them getting smaller as they would on a raster chart). Soon the soundings occupy all the available space, and then at the next level of zooming out they start to overlap one another and become unreadable.

It takes sophisticated software to determine what soundings should be taken out of the picture in order to keep the chart readable. (You can’t simply drop every other one because for safety reasons you have to maintain the ones showing the lowest depths at all times; in essence, the software has to mimic the process that a cartographer goes through in reducing data to fit a smaller scale.) At certain zoom levels, many vector charts in the recreational marketplace become too cluttered to be readable.

Copyright issues

Once you are out of U.S. waters, you sail into areas where charts are copyrighted and the official databases are not freely available (a position that I support; it provides the hydrographic offices with an income to continue their work). This radically changes the rules of the electronic chart game. Because almost all electronic chart providers in the recreational marketplace have done much of their own digitizing, especially with respect to vector charts, the quality of the coverage varies markedly.

For example, we sailed from Miami to the Bahamas, and the official government charts are appalling regardless of whether you buy NOAA, British Admiralty or other charts. For years, the only decent charts have been the privately produced Explorer Charts from Monty and Sarah Lewis, which are outstanding. There are now very similar competing charts from a German company Nautical Publications.

Nobeltec has vectorized the Explorer Charts and Maptech has raster-scanned the Nautical Publications charts. As far as I am concerned, if an electronic chart package does not include one or the other of these sets of charts, it does not cut it for the Bahamas. In addition, Nobeltec includes geo-referenced aerial photographs that can be superimposed on the Explorer Charts, providing vivid representation of sandbanks and land masses – this is exceptionally useful in the clear, shallow waters of the Bahamas.

This past summer we sailed on Sweden’s west coast, over to Norway and around Denmark. The government charts here are excellent. I ran Nobeltec on one computer and C-Map (running on Raymarine RNS software) on the other. Both use vector charts privately developed from the same government paper charts, so the look and features are very similar. The choice of which to use became a subtle one that was partly subjective – I preferred the display characteristics of Nobeltec, especially the soundings, while my wife Terrie liked C-Map – and partly objective based on considerations such as which supplier has the best update service.

In-store testing

Before committing to any one software and chart provider, I’d go to a store like Blue Water Books and Charts or a large West Marine that will let you play with competing software packages side by side. There is a wide selection of charting software available, including packages from Furuno-MaxSea (MaxSea Navigator), Nautical Technologies (The Cap’n), Raymarine (Raytech RNS) and others. I’d run my tests using charts from the area in which I intend to cruise, especially if this is a more remote and poorly charted region (this includes the Bahamas, all the Caribbean and most of the Pacific).

One way or another, the availability of the NOAA’s entire electronic chart database ensures that there are numerous electronic charting packages for U.S. waters. This is also increasingly the case for nearly anywhere else in the world you choose to voyage. I’m a long-time paper chart lover, but I have to confess that it’s been quite a while since paper charts were my primary mode of charting.

By Ocean Navigator