LOOK OUT BELOW
As you contemplate anchoring, it is inevitable that you will end up in blue tinted regions of a chart, whether you're using a NOAA chart or a CHS chart. If the anchorage is regularly visited, it is unlikely you will encounter an unexpected rock.
But, if you're like us and are attracted to less well-known spots, some extra caution is needed. We have found that the Interphase Outlook forward-scanning sounder that we installed on Pacific Voyager in 1999 and on Pacific Sapphire (our Krogen 42) in 2006 has become indispensable. Observers may wonder why we do a complete circle in unfamiliar anchorages, but this has become our normal approach to assessing the bottom contours and sizing up the distances to shallows, rocks, or another anchor rode. If you are considering forward-scanning sonar, keep it simple. (A single transducer is adequate.) This keeps the cost down and simplifies both the installation and the interpretation of the output.
Learn how to use the forward-scanning sonar in familiar settings, and you will become addicted to the security of knowing something about what lies in front of your vessel. With practice, you will be able to interpret the echoes, differentiating the bottom from schools of fish and kelp. We recently met a boater who had an Interphase forward-scanning sounder identical to ours. He said he didn't use it because the screen images were "too confusing to follow." The problem was simple to rectify by taking the instrument off the automatic ranging (setting it for manual range adjustment) and reducing the sensitivity. Too high a sensitivity will cause double bottom echoes, particularly over rock, and kelp will look like solid rock!
THE EYES HAVE IT
The WAAS differential GPS positioning used here provides positional accuracy to less than 30 feet, and this screen capture shows the vessel located "onshore" when it is actually situated midway between the rock and the shore on chart no. 17404.
On several occasions, we felt that the horizontal datum on NOAA charts was incorrect by as much as 0.25 nautical mile (about 500 yards). This was most evident in narrow passages and along the shoreline. Although the charts are generally correct and the electronic charting using raster-scanned charts has been properly registered using NAD 83, it is important to use visual cues to position your vessel. More than one vessel has run aground while relying solely on the accuracy of a GPS.