Charting Differences at Jekyll Island

When I first started out in 1985, I assumed that everybody’s charts all agreed and any reference to the magenta line would be a good framework for passing information on the best path. Now I know that not to be true, an opinion forced on me by reality. However, I am still guilty at times of using the magenta line in referencing a path through a hazard. I’ve tried to avoid that in my latest postings but old habits die hard. In the future, I’ll double down on changing my posts to only reference routes relative to visual data (e.g., visual center of channel, relative to markers, etc.) I’ve taken to using Navimatics Charts and Tides which does not incorporate a magenta line. I have written permission from the owner of Navimatics to use his charts in my book so there are no magenta lines shown there except for a few examples of what not to do. 

In one famous area, I plotted a route through Jekyll Island where I found 5.3 MLW (5.3 ft at 0.0 tide). I converted that path to a GPX route and imported the route into Navimatics Charts and Tides, The Garmin app, and NOAA ENC charts as displayed by OpenCPN on my PC. The results were enlightening. The route I took did not follow the same “channel” as shown in the three apps. Jekyll is an area of the ICW where there appears to be a shifting of the datum for the charts between vendors. All this brings into question advice like, “Favor the green side of the channel (or red side).”

The Navionics app does not allow for the importing of GPX routes much to my dismay. I called Navionics on this and they just said they had no plans to implement that feature. The app does allow communication of GPX waypoints from a partner chartplotter but not directly into the app via a GPX import function. It will let you create a route by tapping on the display but the waypoints so created don’t show up with Lat/Long coordinates. In order to get a common waypoint for comparison purposes, I took the shallowest part of the route (just north of G19) and manually added the shallowest waypoint in my trip through Jekyll Island on 4/20/2017. That took about 20 minutes for that one waypoint since the app does not allow you to edit the Lat/Long of the waypoint. You have to tap, look at the Lat/Long (at least they show that much!), erase the waypoint if not accurately placed for the shallow spot and then repeat the process until you get a waypoint Lat/Long that matches the shallowest waypoint of the route I took for 5.3 MLW. The waypoint then shows up on all the charts shown below. 

Now for the charts:

The first one is the Navimatics Charts and Tides app showing the GPX route I entered for Jekyll Island based on my spring trip up the ICW where I found 5.3 MLW minimum. I’ll start with the Jekyll Island channel just north of G19. The “Common Waypoint” will appear on all the following charts so we can watch how different vendors display the same waypoint on their charts. You can see that the route is pretty much in the middle of the channel by G19 but diverges to the green side at the Common Waypoint. As an aside, one way ask, “How did you come up with this and how do you know it’s the best path?” Well, on the way north I was muddling at my usual slow pace in the shallow waters and I noticed the locals whizzing by me at high speed but quite a bit east of me, outside the charted channel. On the way south in the fall, I was more in the center and found the water shallowing to 3.0 MLW so I knew the answer was not to favor the red side. So I said “what the heck” and wandered over to the green side where I then discovered why all the locals were using that path, it was deeper (it’s also the outside of the curve). Perhaps it’s even deeper than 5.3 MLW farther over but I was happy at what I found so I didn’t go farther east. I also knew that you needed to standoff G19 by 80 ft or so. I had visited a sailboat that was hard aground that afternoon when he used G19 as a turning buoy, not good.

Next up is the Garmin BlueChart Mobile app. The first thing you might notice is that the waypoint of the route near G19 is outside the channel as shown in their chart (I have the v2017.0 version of their charts installed). The route waypoints are the same in both cases. It’s just that the underlying chart is shifted compared to the NOAA ENC chart used by Navimatics. In the case of the Common Waypoint, being in the center of the channel as shown by Garmin would be a good thing for the shallowest area of the route. However, It would not be such a good thing when turning around G19. If you stayed in the center of the channel there, you would be too close to G19 and would find shallow water.

Next is the Navionics USA HD app. It costs $49.99 for a one-year subscription which has to be renewed annually. The yearly price seems to vary somewhat so you may find it different. There are numerous updates during your term that usually run 300 to 500 MB each if you do them monthly. For the chart below, I didn’t go to all the trouble of plotting every GPX waypoint since it was so time intensive. I just put the shallowest waypoint with the blue pin that the GPX route passes through. In the example below, the Navionics chart shows the shallowest spot right in the middle of the channel. That’s good. I didn’t plot what happens by G19, maybe next week. The dotted, red line by the blue pin is labeled, “Centerline of a recommended route”, not bad guidance. The solid, red line to the west of the blue pin has two labels, “Navigation line – leading line bearing a recommended track” and “Recommended Track.” I have no idea what they mean by that. Which one should you follow? In this case, it’s the dotted line but how would you know without more input?

Next up is the Navionics SonarChart. It takes input from boaters with Navionics compatible sonar units that upload readings through local WiFi in real-time (or later if WiFi is not available immediately). I have one of the compatible units, the portable SonarPhone T-Box for $199.95. You will notice right away that the blue pin (location of 5.3 MLW path) sits on top of what the chart shows as 0.0 MLW. If one follows the chart which shows the channel farther to the red side, shallow water will be found. In many places on the ICW, the SonarChart gives accurate information, just not always. They are dependent on receiving updates from those that have their sonar compatible units installed. Unfortunately, they also do not time stamp their sonar charts so you don’t know the age of the soundings.

However, just to confuse the issue more, the same two guidance lines shown on the Navionics chart above are also shown on the SonarChart. So if you’re passing by the area just north of G19 in Jekyll Island, which line do you follow? If a boater is following the SonarChart contours, it would take a brave boater to continue on the dotted, red line with the chart showing disaster ahead – but that would be the correct thing to do in this case, at least by the 5.3 MLW waypoint! I’m not speaking for the area of the chart (or dotted, red line) by G19, that’s a story for another day. I would have to manually put in a bunch of waypoints into Navionics to discover if their dotted, red line is good by G19.

Next up is the Navionics rendering of the NOAA ENC chart of the same area. The first thing you might notice is the absence of a magenta line. However, it compares closely to the Navimatics chart as it should since they are both based on the same NOAA ENC chart. The advice to favor the green side of the channel above G19 would still make sense. The turn by G19 also looks the same as expected.

Finally, we come to the OpenCPN display of the NOAA ENC chart. The GPX route is displayed and looks identical to the GPX route displayed by Navimatics. It’s a comforting thought that the various charting programs display the Jekyll GPX route the same as long as NOAA ENC charts are used. I would give up if even they were different.

So, what to make of all this? Both Garmin and Navionics must use the NOAA ENC charts as base charts but I have no idea why they should be different (except, of course, for the Navionics SonarCharts which are user modified). The Navionics app is certainly capable of displaying accurate charts per Lat/Long as demonstrated by their rendering of what they call the “Govt Chart.” I am now challenged on how to describe the 5.3 MLW path I found 4/20/2017. If I use favor green or red, the boater would have to have the same charts I do to find the same path. The charts even vary between different charts supplied by the same manufacturer (in the example above for Navionics Govt vs their native charts vs SonarChart).

So in the future, I’ll always reference the NOAA ENC chart but try to avoid all references to a magenta line and attempt to reference visual info like “visual center of the channel, more to one side or the other, 100 ft off a marker, etc. But that’s not good enough for some areas, especially Jekyll Island where the markers are far apart and the deep water is not obvious.

The other option is to publish GPX routes. Boaters could if they chose to load the route, follow the exact route I took for the water depths I found. That would get you along the same path with good accuracy but there’s a caveat even with that! Jekyll is known to be very sensitive to wind tides. If there’s a strong (>20 kts) wind out of the east for a few days, you’ll wonder what all the fuss was about. Such a wind will add at least 1 ft to all readings. On the other hand, if there’s a strong wind out of the west (up to 1 ft less water), you’ll be able to discover all the pleasures of a shallow water passage through Jekyll Creek.

Note: although the routes I publish are ones I have personally taken, except where noted for Army Corps of Engineers (ACOE) routes, Jekyll is my own experience, Do not speed through at 20 kts! Things change on the ICW. Barges may come through and plow up mud and change the channel, especially in Jekyll Creek. I’ve seen a pair of barges go through the creek at dead low tide with a 1000 ft of dredging pipes loaded. They plowed mud!! They are probably the only reason Jekyll doesn’t silt in all the way.  Just look below, that’s the two barges going around G19 at dead low tide. It took about an hour to get all the way through with lots of mud flying behind the barges. Perhaps the sailboat had more water after that than he expected?

If you want to have the exact waypoints I used, download BJekyll. You can see all the GPX routes at GPX Routes on my blog site at fleetwing.blogspot.com. I also published the 2017 ICW Cruising Guide if you want similar information for other shallow areas. Ann and I will be leaving Poughkeepsie YC, New York, around September 15 for our eighth cruise from NY to Key West. Please join us for the trip south. I’ll be updating what I find in my blog.