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Using Nautical Charts with Global Positioning System (Edition 2)

by National Imagery and Mapping Agency (NIMA)

With the advent of the Navstar Global Positioning System (GPS), mariners are now capable of navigating ships with much greater accuracy than previously possible. This presentation focuses on the inherent limitations of nautical charts when plotting positions from GPS.
For the chartmaker, accuracy of the final product must take into account the limitations manifested by the chart user’s acuity of vision; the lithographic processes and plotting techniques; and the symbolization of features (e.g., line widths).

For the GPS user, concerns are focused on the need to ensure that latitude/longitude datum shifts are made when plotting GPS-derived positions on a chart with a different datum. This problem is exacerbated in the near term because National Imagery and Mapping Agency (NIMA) nautical charts currently are compiled on different datums. Full conversion to the standard datum, World Geodetic System 1984 (WGS 84), for over 4,000 NIMA nautical charts is not expected until the year 2002. Accordingly, the user could be required to apply several different datum shifts to the acquired GPS position, which is based on WGS 84, as the ship’s track is progressively plotted on subsequent charts with possibly different datums.

Before GPS, mariners realized that their geographical position could be over a nautical mile in error as ship’s positions were derived using various electronic instruments and celestial observations. With this error of uncertainty, mariners gave wide berth to hazards depicted on charts, including shoals and obstructions. There was general acceptance that the available navigational information and cartographic processes used by the chart maker to position the hazards were more accurate than the navigation means available to the user of the chart.

With GPS providing such an accurate fix, the mariner now needs to pay closer attention to the reliability of the chart because accuracy limitations of charts will be critical to ship safety when GPS is used. For example, mariners, to save steaming time, may become more daring and rely on their GPS to pass hazards depicted on charts much closer than prudent. However, the plotted hazards may have been positioned by less accurate navigation means than GPS, and, in fact, may be significantly mispositioned. In other words, the chart being used may contain all the "errors", but from the marine perspective, the chart has always been "accurate"!

GPS ABSOLUTE ACCURACY
Military User Under Selective Availability (SA) and Anti-Spoofing (A/S). The military user has access to full GPS accuracy, the Precise Positioning Service. Accordingly, with selective availability corrections applied and the ability to decode the encrypted signal, the horizontal accuracy is 21 meters 2drms (2 distance root mean square). In other words, a 95% probability exists that the derived position is within 21 meters of the true position on Earth. This accuracy equates to approximately 0.01 minute of latitude.

Thus, the best accuracy that a GPS fix could be plotted in this mode is only 0.01 minute, regardless of the number of decimal places the GPS receiver displays. This accuracy is suitable for plotting needs.

Military User with Real-Time Differential GPS. A military user of GPS in a differential mode may reach an accuracy of 2 to 7 meters.

Commercial GPS User. For the commercial GPS user, only the Standard Positioning Service, which is limited by the engagement of SA, is available. Thus, a real-time accuracy of 21 meters is not obtainable under normal circumstances, since when SA is engaged the result is a degradation to 100 meters horizontal (2drms) position accuracy. This "degradation" still results in a position accuracy of 0.05 minute, but this reduced accuracy would become apparent when plotting these positions on larger scale charts (1:100,000 and larger). With charts 1:30,000 scale and larger, the commercial GPS user should use extra caution when piloting in restricted waters. An accuracy of 2 to 7 meters (same as the military user) can be achieved, however, when GPS is used in a differential mode.

CHART ACCURACY
Specified Chart Accuracy. The NIMA specified accuracy for harbor, approach, and coastal charts is that features plotted on a chart will be within 1 mm (at chart scale) with respect to the preferred datum, at a 90 percent confidence level. For a large-scale chart of 1:15,000 scale, a 1 mm error equates to ± 15 meters (16.2 yards), which is the same order of magnitude as the absolute GPS error. For a smaller scale chart of 1:80,000 scale, the chart error is ± 80 meters (86.4 yards), which will become the limiting factor in position plotting accuracy. The reverse can be true for large-scale (small area) charts, such as a harbor plan inset at 1:5,000 scale. In this case, the navigator’s plotting accuracy is limited by the absolute accuracy of GPS, rather than the chart; however, features on this chart should be accurate to ± 5 meters.

Cartographic Presentation. The chart maker's "cartographic license" also may be a factor. When attempting to display two or more significant features very close together on a chart, the chart maker may displace one feature slightly for the best presentation. This adjustment will normally keep the feature within 1 mm plotting accuracy discussed above.

Positioning of Survey Data. Errors in the underlying survey data will also affect accuracy. While NIMA makes ever effort to produce the most accurate chart possible given the available data, the prudent navigator should pass shoals or isolated dangers with utmost caution, no matter what navigation method is used. Few coastal surveys of years past were possible to differential GPS accuracies.

Pencil Width. Although seemingly trivial, pencil line width becomes a significant source of error at some scales. At 1:15,000 scale, the 0.5 mm line width of a mechanical pencil lead equates to 7.5 meters (8.1 yards) on the chart. At 1:80,000 scale the same pencil line width equates to 40 meters (43.2 yards) on the chart. Thus a dull pencil width could become the accuracy base in the use of the chart.

DATUM TRANSFORMATION
World Geodetic System 1984 (WGS 84). GPS receivers operate on the WGS 84 global geocentric reference system (datum). Most military and commercial receivers allow the user the capability to select the reference datum, but the receiver will default to WGS 84 if none is selected.

World Geodetic System 1972 (WGS 72). WGS 72 is an earlier, less accurate global geocentric reference system; transformation from this datum to that of WGS 84 would only be required for navigation purposes at chart scales 1:50,000 and larger. Positions can be read directly from the GPS receiver for charts based on WGS 72 at scales smaller than 1:50,000.

Other Datums. Among other major datums, Tokyo datum is an example of one that requires significant adjustments to conform to GPS positioning. Many charts in the Japan and Korea area referenced to the Tokyo datum, for which positions must be shifted more than 700 meters horizontally to convert to WGS 84 datum.

Isolated datums, such as those used to reference many islands in the Pacific Ocean, can be in error by a half mile or more (see figure). Thus, the logical datum shift to WGS 84 can be quite large, depending on the area of the world and the local datum in use, as in the case of the Tokyo datum.

GPS RECEIVER LIMITATIONS
Some GPS receivers may not have a selectable datum feature, and as such, extra care must be taken. Thus, navigators should ensure they understand the limitation of their equipment before using the output for piloting. However, accuracy should not be a problem if the chart in use is based on WGS 84 or NAD 83 (the WGS 84 equivalent used by the National Ocean Service for U.S. waters) and either a Precise Positioning Service receiver or differential GPS method is used. Some NIMA charts that are not currently based on WGS 84 datum include a note that specifies the necessary adjustment in position to correctly place a WGS 84 position on the chart.

USE OF TEMPLATES
The use of templates for plotting is an easy way to get reasonably accurate and repeatable plots. However, each template must be tied to the chart for which it will be used. Unlike topographic line maps, nautical charts are not published with consistent scales (such as at 1:50,000 or 1:100,000), but are scaled individually for the best use in navigating a particular area. This practice leads to a range of scales from 1:5,000 to 1:180,000, with odd steps in-between. Additionally, because of the projection used to display the curved surface of the earth on a flat sheet of paper (e.g., Mercator Projection), the latitude scale will vary with changing latitude. Care should be used to ensure that any locally produced templates are for the correct scale of chart in use, and are restricted to the latitude intended.

ELECTRONIC CHARTS
Commercial and military vessels are now using various versions of electronic charts together with GPS for voyage planning and situational awareness. With an established maintenance system, electronic charts used with valid display system will be the navigation method of choice for most mariners.

For the U.S. Navy and U.S. Coast Guard, NIMA’s Digital Nautical Chart (DNC®) forms the basis for paperless ship navigation. The Digital Nautical Chart (DNC®) is a comprehensive, vector-based, geo-relational database containing maritime features essential for safe marine navigation. The database, on WGS 84 datum, is compiled from a global portfolio of over 5,000 NIMA and National Oceanic and Administration Agency charts (U.S. waters) that supports marine navigation between 84° North latitude and 81° South latitude.

However, all paper chart accuracy cautions apply to the DNC® (or other electronic charts based on paper charts) when used with GPS for navigation. In fact, extra care should be taken by mariners when using electronic charts because the normal caution statements and datum notes readily visible on paper charts may not always be "on screen" with electronic chart display systems.

SUMMARY
GPS-derived positions often exceed the accuracy of charts used for navigation. Navy and civilian navigators should be aware of all the factors that may affect the use of GPS positions when plotting these positions on nautical charts. All mariners should continue to give wide berths to charted hazards until charting worldwide can be brought to the required new accuracies.
For further information, please contact:
National Imagery and Mapping Agency
Customer Support Office
4600 Sangamore Road
Bethesda, MD 20816-5003


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