The Chart is wrong!!! Outside Chichagof -- Part 3

Tuesday 17 July (written by Urs)

We waited until high-water slack before leaving Lake Anna.  Judy was uneasy about going back into the Gulf of Alaska because we had not been able to receive weather forecasts and because the wind was relatively strong.  But it was no problem. We first wanted to go to Waterfall Cove, a beautiful anchorage with a spectacular waterfall and nice bear-friendly grassy flats. However, we found it very tight and the wind was blowing strongly into the anchorage. What really scared us off was also the fact that the chart did not seem correct. The GPS showed us very close to land when we knew we were in the middle of the space between the two sides and chart depths didn’t match what our instruments read. We were already well anchored when we decided to abandon the spot, and go on to nearby Falcon Arm, a side bay to Slocum Arm and behind the Khaz Peninsula.

Black arrow shows where we are on the chart.
Cross in middle of bay is where we were

For Falcon Arm, our guidebook talked about a rock in the middle of the outer bay being shown incorrectly on some older charts. This was corrected in all three of our charts, one paper and two digital. They should be correct since they have only recently been downloaded from NOAA, the US marine chart authority!

At center left is the rock which was 350 meters away

However, when we were deeper into the bay and close to the tip of a peninsula, the GPS showed us still quite a ways off. Something was clearly wrong here too but the discrepancy was larger. We proceeded cautiously into the inner bay by sight and depth sounder and bow watch, ignoring the GPS position shown on both of the two independent plotters. Visually, we were anchored in the middle of the bay; however, the GPS digital charts had us sitting on top of a rock at the entrance to the bay, some 385 yards away (350 meters)!!! This is an example of the chart being clearly wrong relative to the GPS position. I also checked the chart to see what horizontal chart datum (a base line position reference for charts) had been used. The chart says they used NAD83, which is practically equivalent to WGS84. Our GPSs are set to WGS84, hence, we are talking the same language, and the chart and GPS should be in synch. I conclude that the chart is just clearly wrong!!!

We have learned a long time ago that one must always be alert and consider all the inputs. The following text is printed on the chart in red: "The prudent mariner will not rely solely on any single aid to navigation ...."

So much for the excitement of the day.