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Charts and GPS

With the advent of the Navstar Global Positioning System (GPS) and, in United States waters, the U.S. Coast Guard's Differential Global Positioning System (DGPS), mariners are now capable of navigating ships with much greater accuracy than previously possible, especially now since May 2000, the GPS signal is no longer being degraded. As a result, it is very important that mariners understand the inherent limitations of nautical charts when plotting vessel positions using GPS or DGPS.

Before GPS, mariners realized that their geographical positions could be over a nautical mile in error when ship's positions were derived using various electronic instruments such as LORAN and celestial observations. With this error of uncertainty, mariners gave a 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 chartmaker to position the hazards were more accurate than the navigational 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, to save steaming time, mariners 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 navigational means than GPS and, if repositioned using GPS, could be charted in a different position.

Chart datums can also play a role in plotting GPS positions or transferring positions from one chart to another. When transferring positions from one chart to another navigators have always had to determine if a correction to adjust for different chart datums was necessary. To plot correctly, GPS positions must be on the same datum as the chart being used. Most GPS receivers default to the World Geodetic System 1984 (WGS 84); if the chart being used is on another datum, the GPS receiver must be set to that chart datum or a corrector applied before plotting the position. Ninety-nine percent of NOAA nautical charts are on the North American Datum of 1983 which, for charting purposes, is considered equivalent to WGS 84.

Absolute Accuracy with GPS and DGPS

The U.S. Coast Guard's DGPS has a stated horizontal accuracy of +/- 10 meters (95%). In other words, 95 percent of the time a position determined using DGPS will be within 10 meters of its true position on the earth. Under certain conditions, mariners may observe better than 10-meter accuracy.

GPS without differential is less accurate. Estimates place accuracy as good as 10-15 meters depending on satellite configuration and atmospheric conditions. Therefore, a vessel can be more accurately positioned using DGPS.

Chart Accuracy For the chartmaker, the accuracy of the final product must take into account the limitations manifested by the chart user's acuity of vision, lithographic processes and plotting techniques, and the symbolization of features (e.g., line widths).

The accuracy of a nautical chart is also dependent upon the accuracy with which its underlying data was collected and plotted on the final drawings and manuscripts used to print the finished chart. NOAA has specified accuracy standards at each step in the data collection and chart production process.

Specified Chart Accuracy. NOAA has specified the accuracy for its nautical charts in terms of the accuracy with which features are plotted on the chart from their original surveyed position. The plotting positional accuracy of most features is approximately 1mm at chart scale. To put this into perspective, at 1:20,000 scale 1mm is 20 meters and at 1:80,000 scale 1mm is 80 meters.

Cartographic Presentation. The chartmaker's cartographic license may also be a factor. When attempting to display two or more significant features very close together on a chart, the chartmaker may displace one feature slightly for best presentation. For example, a sounding may be displaced slightly in order to show a buoy in its published position.

Positioning of Survey Data. The methods used to acquire the underlying survey data will also affect accuracy. NOAA has specified stringent accuracy standards for collection of data by its survey vessels. Currently, surveys are being conducted to DGPS accuracy. However, for surveys performed prior to the mid 1990's, the accuracy requirement was only 1.5mm at the scale of the survey as few surveys of years past were able to match DGPS accuracies. For example, on a 1:20,000-scale harbor approach survey, an accuracy of 1.5mm equates to 30 meters. NOAA makes every effort to produce the most accurate chart possible given the available data. The prudent mariner should pass shoals or isolated dangers with utmost caution, no matter what navigation method is used.

Line Width. While seemingly trivial, line width becomes a significant source of error at some chart scales. At 1:20,000 scale, a line 0.5mm wide on the chart equates to 10 meters on the earth. At 1:80,000, the same line width equates to 40 meters on the earth. Thus, if a dull pencil were to be used by the mariner, it could become the accuracy base in the use of the chart.