Cruising

Datum Shift

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Here’s a chartplotter photo that shows the boat plunked into the local parking lot. In fact, it was tied alongside the pier northwest of number 4. The distance to the apparent position was over 200 yards. The fault lay in a discrepancy between the GPS lat-long data and the electronic chart itself. In other words, it’s a classic datum shift. Think about what could have happened to the boat entering this harbor in fog if her pilot had relied on total GPS accuracy. Need I say more? Tom Cunliffe

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