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the dangers of chartplotters

With GPS and electronic charts, navigation for cruisers has never been easier. Or more dangerous.

Most of the cruisers leaving today have extremely accurate GPS units that provide real time latitude and longitude information. Many cruisers also now utilize electronic versions of paper charts, displayed on dedicated LCD or PC displays. It's commonplace now to marry these two technologies - GPS and chartplotter - together, resulting in your boat's position being precisely indicated "on the chart", in real time.

With this precision, it's easy to be lulled into thinking that the need for dead reckoning, visual verification and taking bearings is no longer necessary. But precision and accuracy are two different things.

As the photos below illustrate, our "precise" location on the chart did not agree with our actual location reported by our radar. Since it was still daylight, we recognized the discrepancy between what we were seeing, and what the GPS and chartplotter were reporting.

The green boat shows our reported position according to our chartplotter and GPS

The same viewpoint, as reported by our radar

Had we been making this approach to Islas Benitas, on the outside of Baja Mexico, during the night, and not utilizing radar. Trusting only the GPS/Chartplotter we would have driven our vessel hard aground as we attempted to thread the passage.

Later that evening, when securely anchored for the night, we again checked our location on our GPS/chartplotter. It indicated we were located high and dry in the middle of the island!

This was a wakeup call for us, and hopefully for other cruisers. While charts, paper and electronic, are very accurate in the US, that is not the case in the rest of the world. Many of our Mexico charts are based on surveys made in the late 1800's. Electronic charts are just digitized versions of these charts.

We find the GPS/Chartplotter combination great for passage making. We utilize the handy tools of bearings vs. headings, speed vs. "velocity made good", time to go, and many other features to help us manage the journey. But as we approach landfall, we revert back to the more traditional skills of navigation. These include visual checking, verifying depths, taking compass bearings, observing navigation lights (when they are working!), and confirming position with our radar. If all agrees with our position indicated on our chartplotter, great. If not, we ignore the GPS/chartplotter information. After securely anchoring, we look to see if our anchorage is again reported as 3 miles inland!

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