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.