New update method for NOAA raster charts

A NOAA paper chart is an impressive visual database; rich with information, it instantly conveys a sense of authority. And the durable, high-quality paper means one can use the same chart for decades. Unfortunately, however, a chart has an accuracy lifetime: sandbars dissolve and re-form in a different position, bridges are built, and buoys are moved, changed, and have their light characteristics altered.

So no chart remains 100% correct indefinitely. At some point, mariners need to either update or replace every paper chart in their inventory. Since electronic charts are subject to the same changes as paper charts, they also need a correction method. Now NOAA and Maptech (the raster electronic chart partners via a Cooperative Research and Development Agreement or CRADA) have announced a service for correcting their raster electronic charts via the Internet. Maptech’s Notice to Mariners Chart Updating Service allows you to update electronic charts based on changes published in the Coast Guard’s local Notice to Mariners, the National Imagery and Mapping Agency’s (NIMA) International Notice to Mariners, and the Canadian Notice to Mariners. And in addition to fixing electronic charts, the Chart Updating Service will also allow you to update paper charts more quickly than before (more on that below).

In the days when paper was the only display method for charts, mariners had to correct charts using pen and ink based on the published corrections in the Notice to Mariners. If one has a large chart collection, this can be a slow, labor-intensive process for those professional mariners and the few recreational mariners who keep their charts updated. Various tools and techniques have been available to facilitate the process: rub-on symbols of buoys, daymarks, etc., and plastic templates for helping one draw chart symbols. But nothing changed the fact that, in order to keep charts updated, one had to look through the Notice to Mariners, find corrections that applied to one’s own charts, and then make those corrections. Paper charts, of course, have always been updatableall it takes is a pen or pencil. One drawback to raster electronic charts, as far as chart corrections are concerned, is that raster charts are bitmapped graphic files, and changing them can open a can of worms. Bitmapped graphics files are certainly capable of being edited (that’s the raison d’être, after all, of digital image editing programs such as Adobe Photoshop), but the problem in changing such a nautical chart could be the loss of geographic and navigational accuracy. In the paper world, users made their own corrections. However, allowing an electronic chart user to edit a digital image file could lead to some problems. What if, when erasing an old buoy and drawing in a new buoy, a user obliterates a few dangerous rocks? Or puts in a new buoy in such a way as to cover an obstruction? Plus, once the file has been changed and then saved, there is no way to get the information back. With a paper chart, even if you ink over chart features, you can, to some extent, erase the correction enough to see what is beneath it. So the risks of letting users modify electronic charts seem to outweigh the advantage of having corrected charts.

With this in mind, the NOAA/Maptech CRADA partners had to devise a system that worked automatically, correcting charts via a standardized process that didn’t rely on users making the corrections by hand. Also, the partners wanted a weekly update cycle and a Web-based distribution system. However, before such a system could work, NOAA’s chartmakers first had to change their method of updating charts. Previously, charts were updated in batch modeneeded corrections were allowed to accumulate, and then all the corrections were applied and a new chart produced. “We had to develop the technology for continual maintenance, instead of batch maintenance,” said Dave Enabnit, technical director, Office of Coast Survey. But first the chartmakers had to make all the corrections that accumulated in the system. This meant the cartographers at NOAA took a year to apply the more than 40,000 pending corrections. “And that was in addition to our normal workload,” said Enabnit. By working extra hours and farming some of the correction work out to contractors, NOAA got its chart catalog up to date.

This change to continual maintenance is part of another change in the way NOAA maintains its chart database. Previously, the prime source of charts was the film separates (one piece of film for each color on a printed chart) used to produce metal plates for printing paper charts. These films were scanned, combined into one digital file, and offered as the original NOAA/BBA Chartkit/BSB raster charts (BBA Chartkit/BSB has since been purchased by Maptech). Now, NOAA has transitioned to a digital file cartography system. When a chart needs updating, a cartographer at an Intergraph workstation loads a digital file of the old chart, changes the digital image, and then saves the updated chart as a new digital file. For producing paper charts, he or she prints out the film separates for making printing plates.

Thus, the digital files are now used as the base for making the films for paper charts, rather than, in the old system, the films being used to make the digital files. The raster digital files are now the source for making and updating both the NOAA/Maptech raster charts and the paper charts. (We hope NOAA has a good computer backup system!)

Once NOAA had switched to continual maintenance mode, which means weekly updating of all the charts in the database, it was ready to implement the clever updating scheme developed with Maptech, its CRADA partner. After NOAA cartographers update a chart with several corrections, Maptech technicians use proprietary software that compares, pixel by pixel, the new version of the chart with the old version of the chart. This comparison process results in a difference filea digital file that is composed of only the changes from the old file to the new file. It is this difference file that forms the basis for updating the raster charts on a user’s computer. Users are e-mailed a file transport protocol (ftp) address at which they can access the correction files. Employing either Netscape Navigator or Microsoft Internet Explorer, or an ftp client, a user downloads a correction file. When the downloaded file is clicked on, it launches the updating application and the corrections are merged with the old chart files. This results in the corrections being written as patchesindexed to the chart’s lat/long gridonto the old chart. A new, merged chart is created, and that updated chart is saved to the user’s hard drive (the chart can’t be saved to the CD because, of course, CDs are a read-only medium and can’t be written to).

This NOAA/Maptech system also allows you to quickly update paper charts. You can use the NTM software to output the correction patches to a color inkjet or laser printer. These paper patches can then be cut out and glued down onto each of the paper charts that require correction. The result is that all the corrections to an entire CD region (say, Maptech’s region 2, which includes all the charts from Block Island, R.I., to the Canadian border) can be done with simply a few clicks. This eliminates the need to update each chart by hand. For commercial mariners, who are required by the Coast Guard to keep all their charts up to date, this system is potentially a big time saver.

To use this Web-based updating system, you need to have a connection to the Internet and at least 500 megabytes of free hard-disk space. The cost for the Notice to Mariners update system is $499. This entitles you to one regional CD of charts and a one-year subscription to the chart updates.

While this product is available to anyone, Maptech expects the primary users will most probably be commercial mariners. According to Ted Shanstrom, who coordinates Notice to Mariner sales at Maptech, a smaller, less expensive version of this system that appeals to recreational sailors may eventually be offered by Maptech. “Down the road we hope to have the corrections for individual charts available on the Web site,” said Shanstrom.

This more recreational mariner-friendly chart-update approach might be set up a variety of ways, including the option to just pay for the corrections to those charts you’re interested in keeping up to date, rather than having to buy all the corrections for an entire chart region. If this automated system is offered by NOAA/Maptech and is actually adopted by recreational mariners, it could mean that, for the first time ever, a sizeable percentage of recreational mariners will have corrected, up-to-date charts. And that fact should be good for everyone who is out on the water, recreational and professional alike.

By Ocean Navigator