Wednesday, September 1, 2010

Accuracy and reliability of charts versus confidence

The Island of California (Map, circa 1650) refers to a long-held European misconception, dating from the 16th century, that California was not part of mainland North America but rather a large island separated from the continent by a strait now known instead as the Gulf of California.
One of the most famous cartographic errors in history, it was propagated on many maps during the seventeenth and eighteenth centuries, despite contradictory evidence from various explorers.

From Mike Prince, Director of Charting, Australian Hydrographic Service

How accurate are nautical charts?
How much faith can be placed in them?

The Australian Hydrographic Service proposes a valuable guide which any skipper or navigator of a sailing vessel should be aware with.

Unfortunately, the answer is quite complex – far more complex than simply saying one chart is accurate whilst another is not.

However, having the necessary skills should be essential for any mariner venturing into unfamiliar waters.

All charts, whether paper or electronic, contain data which varies in quality due to the age and accuracy of individual surveys. In general, remote areas away from shipping routes tend to be less well surveyed, and less frequently, while areas of high commercial traffic are re-surveyed frequently to very high levels of accuracy, particularly where under-keel clearances are small.

It is quite accurate to consider a chart as a jigsaw of individual surveys pieced together to form a single image.
These surveys vary in age and quality, particularly due to changes in technology.

However, one fundamental truth remains – a hydrographic surveyor can typically only physically see a very small percentage of their survey area – the parts which rise above the sea surface; for the remainder they must have confidence in their systems and long-standing practices to accurately and confidently chart the seabed.

Because priority for surveying is given to the major shipping routes, an essential skill for mariners venturing into unfamiliar waters away from these routes is the ability to interpret the various quality indicators that are, or should be, on every chart.

These are the best guides available to mariners, whether on commercial vessels or cruising yachts, to help them decide how much confidence should be had in past and current surveyors and the technology available to them when surveying the different areas of each chart.

Indeed, a prudent mariner should be wary of any chart that does not show these indicators, irrespective of whether it is a traditional paper chart, a Raster Nautical Chart or one of the new Electronic Navigational Charts.

Finally, if in doubt, post a lookout, make your approach in daylight and good conditions, or go somewhere else – there is no such thing as a good grounding.

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