Toward the end of August this year, Kathy and I were motor-sailing Snickerdoodle from Sandpoint back to Bayview on Lake Pend Oreille, ID. It is a "straight shot" from the Elliott Point light to the Granite Point light. As we motored on a course of 188M (shown on my GPS chart plotter), I began to wonder how accurate the compass rose on the chart for Lake Pend Oreille was (NOAA 18554) . This chart was last updated in 1982.
I drew my course on the chart and then walked my parallel rule from the course line to the chart's compass rose. The compass rose indicated that I was on a course of 192M. So, the chart indicated that I was on a course 4 degrees west of my actual heading. How important is that??? Well, a four degree error amounts to 2/5 of a nm or 800 yards after an hour and a half of sailing at 5 nm/hr. During daylight hours on a lake or other body of water that you are familiar with -- it's probably not a big deal. But at night or on an unfamiliar body of water. 800 yards might be a critical error. On Lake Pend Oreille, an 800 yard error to the east would be the difference between fetching up at the Granite Point light as planned or crashing ashore in Kilroy Bay. I hate loud crunching sounds......
Next time you are out, maybe it's time to verify the accuracy of your chart's compass rose as compared to what your compass or GPS are telling you about the course you are on.
barnaclebillholcomb@gmail.com
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