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The art and science of navigation and piloting has not been immune to the effects of technology. The ever-increasing pace of technological change has started to dramatically affect the way in which the mariner navigates. The most obvious new technology in navigation is Differential Global Positioning System (DGPS). Navigation systems using DGPS for locating a vessel's position on the earth have put mariners in a unique predicament. In many cases, the DGPS position is more accurate than the surveying technology that was used to put the soundings and features on the nautical chart in the first place.
This discrepancy is even more pronounced when chart scale is taken into consideration. The width of a line or symbol on a paper chart is typically 1 to 2 millimeters. A feature's actual position can fall anywhere within that line or symbol. Thus, on a typical 1:40,000 scale chart, a feature could have a potential error of 40 to 80 meters (about 130-160 feet) due to scale alone. This is often the cause of the "ship on the pier" situation, where the vessel tied up at the pier appears on the navigation system to be on the pier rather than alongside. Another source of this type of apparent discrepancy is that symbols on a chart of a given scale may be a representation for multiple real features that are too close together to be shown at chart scale. Similarly, symbols may be displaced from their original location for legibility and presentation. This also applies to products derived from paper charts, such as raster charts and vector charts created by digitizing the paper chart.
The Challenge
The "ship on the pier" situation is causing mariners to realize that DGPS has leapfrogged them ahead of the accuracy of the paper charts. The Office of Coast Survey in the National Oceanic and Atmospheric Administration has recognized that this situation is of particular importance to commercial shipping. Shippers are tending towards using larger, faster ships that are being brought into port closer to the bottom than ever before. Just a few more inches of draft can mean additional thousands to millions of dollars of cargo carried.
The Solution
The potential solution to the situation lies in another technology that has been in the wings for several years, the Electronic Chart Display and Information System, commonly known as ECDIS. ECDIS uses NOAA Electronic Navigational Charts, NOAA ENC®, to draw a chart-like display on a computer screen using vector chart data. Vector data is essentially a database of chart features that intelligently processes the information and draws the display. This display is combined with the ship's characteristics and positional information as well as other information such as RADAR overlays and targets. This information allows ECDIS to warn the mariner of hazards to navigation and situations where the vessel's current track will take it into danger. The potential for ECDIS to solve the accuracy problem lies in how the NOAA ENC® data are created.
Figure 1, Valdez Narrows NOAA ENC®
As mentioned before, products, including NOAA ENC®, which are derived from paper charts are no more accurate than the chart that they are derived from. Coast Survey has developed its NOAA ENC® program by creating the NOAA ENC® database from the source materials that were used to create the chart in the first place. This involves researching where each feature originated and trying to locate the corresponding document in the source archives. In some cases, the original source is no longer available or provides no additional accuracy (e.g., the position was determined by LORAN). In these cases the largest scale chart was used as the only source. In many cases, particularly with aids to navigation and maintained channels, more accurate positions are available. Maintained channels for example are typically created from the U.S. Army Corps of Engineers blueprints that have scales on the order of 1:2400. The goal is to use source material wherever possible so that the NOAA ENC® is created from the largest scale, most accurate information currently available to Coast Survey.
Figure 2, NOAA ENC® Coverage, East Coast of U.S.
Coast Survey is also working to obtain more accurate information so that it can be included in the NOAA ENC®. This new data will be used to replace less accurate information as soon as it is available. Changes to the NOAA ENC®, including weekly Notices to Mariners, will be available to the mariner on a weekly basis. This will minimize any delay between when new information arrives at Coast Survey and when it is available for use by the public.
The fact that the NOAA ENC® is vector data generated from a database also allows it to be used in geographic information systems (GIS) as a background map. Specific groups of NOAA ENC® features can be loaded into a GIS to create a custom view of the data. Coastal managers with systems capable of using NOAA ENC® data will have access to up-to-date chart information in a non-proprietary format.
Figure 3, San Francisco NOAA ENC® used in GIS
Current Status
The current plan calls for completion of the initial collection of features in all 40 major U.S. commercial ports by the end of 2002. As of December 2000, 77 NOAA ENC®s have been completed and are being maintained. New NOAA ENC®s are being produced by contractors and are loaded into the production system for maintenance. Release of the first publicly available data is scheduled for 2001.
Test data sets have been released to the U.S. Coast Guard in June 1998 for use in developing its Automated Identification System (AIS). Another NOAA ENC® covering Tampa, Florida was also created for AIS development.
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Revised Friday January 31 2003by OCS Webmaster