Thanks for the information, Bob. Whenever I switch over to the Navionics Sonar chart on my Raymarine chart plotter, I'm faced with a "Not for Navigation" warning. Do you know why they have this warning and what the deficiency of the Sonar charts might be?

Editor's Note: Almost all navigation platforms, including NOAA charts, include the "Not for Navigation", or "Not to be Used for Navigation" disclaimer in their charts, maps, digital offerings, etc. The underlying concept is that piloting a vessel is the responsibility of the skipper, who must employ various and numerous resources to arrive at safe transit of any waterway. We are to never depend solely on one source for navigation advice. There may be more concise explanations, but this is the baseline reason for why all publishers of charts and guidebooks include a disclaimer. As well as to protect from uninformed and inexperienced persons who believe someone else is to blame for their lack of experience and/or mistakes. Specific to Sonar charts, it's accepted knowledge that the contours on the sonar charts are not all mapped, sounded, or charted lines. The contours are drawn between depths that have been sounded. The space between the depths may not be as shown. And, depths are always subject to change, as are conditions in and along all waterways.

I would caution members against blindly trusting the USACE surveys when they are really old, especially in areas of frequent shoaling, near ocean inlets, or areas near a river. Choose no older than 1.5 years option in the USACE survey display. How to do that: In the Aqua Map app, in Settings, under the "exclusive US data" is where you find the option to enable/disable the "USACE surveys". Once you enable the overlay, click on the small font "settings" just to the right of the USACE surveys blue toggle switch. In that submenu select the max age of the survey. You may choose from All / 3 yr / 1.5 yr / months. I recommend NEVER rely on the "ALL" or the "3 yr" data. Those surveys are just way too old--lots of movement in the bottom can occur in 3 years. Even showing survey data 1.5 years old or newer has inherent risk, but if you do use this overlay, I recommend only using the 1.5 yr setting, or to be most conservative, go into the months setting and enter "12" so you know you have only very fresh survey data displayed. Toggling back and forth between 1.5 yr and 12 months gives you confidence that the data is fresh.

Robert Norman, NOAA has improved the location accuracy of their charts over the past five years. There are many fewer places like the one in the article now but there are still there in spots. Aqua Map with USACE surveys and Navionics SonarChart match the GPS lat/long output by your GPS receiver - so it's always wise to have one or both running and treat the NOAA charts with caution when it comes to location sensitive areas. Another area with an even greater offset is Steamboat Creek, a popular anchorage south of Dawho River at MM 497, where following the NOAA chart can lead you into shallow water. Both Aqua Map with USACE surveys and SonarChart will show you the safe path.

Captain Gene of Galene, I agree with you on every point! The only observations I would add are: 1) The USACE surveys are usually run once per year from Norfolk to Hilton Head but less often south of there. Areas that shoal more frequently are surveyed more often such as Lockwoods Folly and Browns Inlet and others. There are areas of the ICW in that stretch that haven't changed in over a year, even with hurricanes - which do not affect ICW depths as one might expect per my study. The date of the survey is available in Aqua Map with a tap on the survey. 2) Navionics SonarChart is a great resource but must be used with caution in areas that shoal rapidly. In areas such as Lockwoods Folly and Browns Inlet, it can lead you directly into a shoal. In places where rapid shoaling occurred and a buoy was moved to direct boaters away from it, Navionics SonarChart never gets updated and in one place (by G65A at MM 244) shows 12 feet MLLW where boaters are enjoying a sandy beach. The reason here is that boaters have been diverted from updating the depths in that area by the buoy relocation - so SonarChart never gets new data and displays the old data. The areas shown in the example has not changed in years so SonarChart has had plenty of time to digest many boaters' soundings inputs.

Captain Gene of Galene, I agree with you on every point! The only observations I would add are: 1) The USACE surveys are usually run once per year from Norfolk to Hilton Head but less often south of there. Areas that shoal more frequently are surveyed more often such as Lockwoods Folly and Browns Inlet and others. There are areas of the ICW in that stretch that haven't changed in over a year, even with hurricanes - which do not affect ICW depths as one might expect per my study. The date of the survey is available in Aqua Map with a tap on the survey. 2) Navionics SonarChart is a great resource but must be used with caution in areas that shoal rapidly. In areas such as Lockwoods Folly and Browns Inlet, it can lead you directly into a shoal. In places where rapid shoaling occurred and a buoy was moved to direct boaters away from it, Navionics SonarChart never gets updated and in one place (by G65A at MM 244) shows 12 feet MLLW where boaters are enjoying a sandy beach. The reason here is that boaters have been diverted from updating the depths in that area by the buoy relocation - so SonarChart never gets new data and displays the old data. The areas shown in the example has not changed in years so SonarChart has had plenty of time to digest many boaters' soundings inputs.

Good article and examples thank you kindly. But the USACE surveys can be out of date by months or maybe a year or two. Unfortunately, after storms things sometimes change and a new survey needs to be redone again. I run Aqua maps with the surveys overlaid but I also run Navionics sonar on my other devices as the crowd sourced information is especially helpful to see the latest almost current conditions. The Navionics sonar information also covers areas that the surveys when off the channel the COE surveys do not cover. Between the two applications you have much better information to guide you than using only a government chart or your normal chart plotter without the updated information. If you use the shading options on the Navionics sonar it makes staying away from the shallow side of the channel straight forward and lowers the odds of running a ground. Of course, you need to update often to have the latest information, or the Navionics sonar chart is not the latest information just like the surveys are out of date. I update at least weekly to have the latest. When I used Charts in the old days you could do everything in the channel as on the chart but still run aground as the waterways are dynamic and not static as the charts. Change is always happening. Captain Gene Of Galene

Pays to read the disclaimer that appears on your plotter screen each time you turn it on.

Hunh? I thought we had come a long way from the old British Admiralty survey charts but apparently not far enough. I find it hard to believe that any published charts today are not slaved to gps. How else can we make sense of it?