Ocean Navigator | The magazine for long-distance offshore sailing and power voyaging
The extent of bottom coverage is one issue. The level of accuracy with which data is collected is another. Pre-GPS inshore surveying had a customary positional accuracy within 1.5 millimeters times the scale of the survey. Offshore (out of sight of reference points on land), positional accuracy was far less.
What this means is that given a survey at 1:20,000, the level of inshore accuracy in charting soundings and features would be within 1.5 x 20,000, which is 30,000 mm or 30 meters. It was common practice to survey at twice the scale at which the data was used — e.g., a 1:20,000 survey would be used to construct a 1:40,000 or smaller-scale chart. So now we have a level of accuracy of +/-30 meters on our 1:40,000 chart. Thirty meters at this scale is 0.75 mm on the chart. Given traditional plotting techniques, no navigator could plot with a precision greater than this, so all the pieces fit together.
With the introduction of GPS and electronic charts, the pieces started to fall apart. Any modern off-the-shelf GPS has a positional accuracy of 10 to 15 meters. If you add wide-area augmentation system (WAAS) corrections, this goes to 2 meters. So long as plotting is done on paper charts, the fact that the GPS is more accurate than the placement of details on the chart is more or less irrelevant because the accuracy of a fix would continue to be determined by the accuracy with which it could be plotted. However, electronic charting with electronic position plotting provides a degree of precision that the underlying chart data simply does not support.
Current International Hydrographic Organization survey standards call for a positional accuracy of +/-2 meters for all critical soundings (harbors and channels), and +/-5 meters for most other inshore soundings, with less accuracy required offshore. Although these standards are no better than a WAAS-corrected GPS, the pieces fit once again. Unfortunately, this is somewhat irrelevant, because hydrographic offices worldwide have shifted resources into digitizing existing databases without much new survey work!
Compounding the problem
The picture with the existing Databases is a bit more complicated than this. The printing process used to manufacture paper charts
results in an image display resolution equivalent to around 1,000 dots per inch (dpi). Modern computer and charting display hardware
has a screen resolution that varies from a low of 40 dpi (some cheap chart plotters) to a high of around 100 dpi.
Because of the much lower screen resolution as compared with a paper chart, if you display the fine details of a paper chart on a screen at the same scale as they are shown on the chart, you won't be able to read them. At the electronic chart display and information system (ECDIS) level, which sets the standards for electronic charting on big ships, the industry seems to have settled on a zoom level of 1:1.7 for making electronic chart displays readable. This is to say a paper chart at 1:20,000 is displayed electronically at a scale of 1:20,000/1.7, or 1:11,765. At the recreational level, for reasons I do not have the space to explore in this article, the relationship between a paper chart and its electronic display is often 1:6.25. That means a 1:20,000 paper chart is now displayed at a scale of 1:3,200.
Let's consider an area of an original survey that was done at 1:10,000, with a positional accuracy within +/-15 meters. This area was found to be foul with rocks. The chartmaker, working at a scale of 1:20,000, showed a couple of these as close together as possible using traditional drafting techniques, which is to say about 2 mm apart (representing 40 meters in the real world). No sane navigator plotting on this paper chart would try to take a vessel between these displayed rocks. But now this chart gets digitized and displayed at 1:3,200. The space between the rock symbols on the display has just increased to 2 x 6.25, or 12.5 mm. All of a sudden, it looks like I can take my boat between these rocks, especially when its position, based on my WAAS-corrected GPS, is displayed with pinpoint accuracy. Unfortunately for me, there is no gap between these rocks — just another rock the chartmaker could not show at the original chartmaking scale of 1:20,000.
Overzooming a chart — i.e., using it at a scale for which it was not designed — is one of the cardinal sins of navigation. All electronic charts are overzoomed to some extent, with those found in the recreational marketplace typically grossly overzoomed. To compensate for this, in theory, any navigator using electronic plotting should place an imaginary circle of possible error around the boat's plotted position. This circle should have a radius equal to the allowable error that was used in plotting the features on the chart. Note that this circle of error does not represent errors in plotting the position of the boat, which will be phenomenally accurate, but instead represents the extent to which the features around the boat may be out of place on the chart. All potential hazards should be kept outside the circumference of this circle, or, put another way, if any comes inside this circle, we are clearly into the realm of eyeball navigation.
So how do we determine the radius of this circle? We need to know two things:
# The level of accuracy of the survey on which the chart is based.
# The scale at which the chart is being displayed.
Unfortunately, neither is readily available for most electronic charts supplied to the recreational marketplace!
Many (but by no means all) paper charts have the first part of this information included in a source diagram, and the scale is, of course, clearly stated beneath the chart title. A source diagram is typically a small inset chart that breaks up the area covered by the chart into sections according to the age of the data on which each section is based, the scale at which this data was collected, and the method used to collect it. When navigating in any of the waters covered by the chart, a quick glance at the source diagram will provide a good sense of the accuracy and reliability of the underlying data for that section of the chart.