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Old 07-25-2021, 10:58 AM   #1
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Ran Aground Using Navionics.

First...I'm new at this, and I fully expect my bad experience with the Navionics chart plotter to be my fault.

I have a 2021 Barletta LA-25 tritoon and use it on Lake Norman in NC. The boat came with SIMRAD and I bought a Navionics chip and installed it in the SIMRAD. I believe it was the inland lakes southeast version, something like that.

First outing with the new chip and within an hour I ran aground. The chart said I was in 12 ft or water but the sonar said 1.5 ft. Churned up plenty of mud but was able to get out of it without getting wet. Since then I have seen depths on the chart all over the place... saying 4' but depth was actually 18', saying 18' but was 4', etc etc. Most of the time it is close to the sonar reading but most doesn't count, because most of the time I don't want to run aground.

Why is the chart off by so much? Am I possibly not using the correct chip? Any suggestions?
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Old 07-25-2021, 11:06 AM   #2
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Call Navionics. Lakes donít change that much. Perhaps itís the Simrad not giving steady position data. Is the antenna clear?
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Old 07-25-2021, 11:09 AM   #3
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Welcome aboard. We had Navionics in our last boat. Never saw anything like thar though.
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Old 07-25-2021, 11:11 AM   #4
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My first thought was to call them but I've not been successful in finding a number other than a local dealer.

Antenna? What antenna??? LOL! If it has an antenna it must be within the unit. Nothing external. Cell sigs on the lake are strong so if that's what it needs, it should get it. Instrument panel is plastic so if it has an internal antenna it's not likely shielded by anything metallic.
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Old 07-25-2021, 11:18 AM   #5
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May not be Navionics...could just be the charted depth data everyone has.


Many, many areas of the US has very badly charted data due to when and how it was recorded/estimated.


Back in the older days much depth info was interpolated between spot soundings and was wildly inaccurate.
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Old 07-25-2021, 11:25 AM   #6
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Quote:
Originally Posted by Wingsy View Post
My first thought was to call them but I've not been successful in finding a number other than a local dealer.

Antenna? What antenna??? LOL! If it has an antenna it must be within the unit. Nothing external. Cell sigs on the lake are strong so if that's what it needs, it should get it. Instrument panel is plastic so if it has an internal antenna it's not likely shielded by anything metallic.

??Are you saying this device uses cell phone data?! If it is a GPS, it does need an antenna. Most likely looks like a little plastic mushroom.
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Old 07-25-2021, 11:27 AM   #7
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Consider the noted depths for inland waters and the AICW a suggestion. Rely more on your depth sounder.
There are times I talk back to the 'lady' in my car GPS too.
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Old 07-25-2021, 11:38 AM   #8
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Oh, I think you mean the GPS antenna. I don't think there is a problem there. The boat track is smooth and the GPS speed and heading always seems spot on.
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Old 07-25-2021, 11:40 AM   #9
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Use sonar for depth readings.
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Old 07-25-2021, 11:42 AM   #10
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Use sonar for depth readings.
That is what I use, but it would be nice to know that I can go over THERE and not have to tip-toe.
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Old 07-25-2021, 11:46 AM   #11
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The SIMRAD evo 3 series of MFDs (as well as some of the older NSS's) have an internal GPS, no need for an external antenna.
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Old 07-25-2021, 11:48 AM   #12
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Our last boat had Raymarine and it had a built in antenna in the MFD.
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Old 07-25-2021, 11:55 AM   #13
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Many charts, no matter where they present, were done MANY years ago. That means some inaccuracies. When we used charts only we allowed for those discrepancies, still sometimes goofing up.

What has shown up since the advent of GPS is that most charts are actually not exactly right. The original plottings of the shores are off somewhat. They were done with the best technology at the time but there are so many of them that it will still take many years to catch up.

In B.C. , where I live I see this sometimes.

On my GPS as I approach my marina I run right through a rock breakwater.


Tablets and especially cell phones often use cell towers to HELP location determination. But get away from settled areas with cell towers and that cell function is less usefull to useless and the device can only work on the GPS. Good as it is even GPS has limits of accuracy depending a lot on how many satellites it can see.

Internal antennae of cells and tablets are not as good as a dedicated exterior antenna properly located.

Do NOT depend implicitly on any chart program especially f you are running close to a shore line OR in shallow waters OR in a rock littered area.

And shore lines change as silting builds. We also have that and MILES away from the river that creates it. Ocean currents carry silt many miles from the river mouth and ocean currents can move silt and sand to new areas which the charts cannot keep up with.

Use everything such as the sounder, maybe a second Navionics equipped tablet as a double check.


Glad you got away with no damage.
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Old 07-25-2021, 11:58 AM   #14
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While aground did you record the Lat/Long from Navionics and then compare to another chart?
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Old 07-25-2021, 12:02 PM   #15
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Years ago, before I was born, Bermuda was purposely mis-charted to prevent the British from finding it when it was used as a pirates' hangout.
I wonder if it is sometimes the explanation for the shortcomings of today's charts. LOL
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Old 07-25-2021, 12:05 PM   #16
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In addition to what the others have posted. When running in shallow waters it is good to know what your sounder's offset is. If there is no offset then the depth indicated is depth below the transducer. To know depth of water or depth below the keel you will need to know the depth of the transducer relative to the keel or waterline. What I mean by keel it is the lowest part or the boat. That could be keel, rudders or props. The offset can be used to indicate depth of water, depth below the keel or anything you want.

Also double check that your sounder and charts agree on the dept unit. Feet, fathoms or meters.

Next look at the settings. Is gain set to auto or manual. Same for range. You'll have to learn the best way to run yours for your area.

Then look at frequency. 50 and 200 are common. 200 will find things in the water. 50 will find hard bottoms but can penatrate soft muddy or silty bottoms.

To roughly calibrate the accuracy in shallow water drop a small weight to the bottom abeam of the transducer on a string. Measure the wet length. Compare to your sounder. I have 2. The Furuno is very accurate. The Garmin is not to be trusted in shallow water.

My practice running in unfamiliar skinny water is to use both chart and sounder. Go with smallest reading until I know for sure.
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Old 07-25-2021, 12:49 PM   #17
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Quote:
Originally Posted by OldDan1943 View Post
Years ago, before I was born, Bermuda was purposely mis-charted to prevent the British from finding it when it was used as a pirates' hangout.
I wonder if it is sometimes the explanation for the shortcomings of today's charts. LOL
When I was there read a plaque saying the many ships that sank after grounding in heavy seas as they did not see land ahead due to low elevation
of land 249 feet.
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Old 07-25-2021, 12:59 PM   #18
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Have you read the opening page on just about every MFD. to escape liability they all say it’s an AID.
We generally have two chips, micro-sd or whatever in the MFDs using two different sourced charts. Usually supplemented with a free standing chart on a laptop and/or IPad when in coastal waters. Some look at crowd sourced information which has been notoriously inaccurate at times but on other occasions clued us into stuff that wasn’t on any other chart. So, yes they are all excellent aids to navigating but don’t supplant your senses and a depth. I’m pretty anal so at least once a year use a drop line and check the depth as well. I make sure everyone on the boat knows what shows is depth at waterline not depth at bottom of my keel/running gear. They also know my draft. Some captains like it the other way so good to know on a new to you boat which one it is. . Similarly you can check your gps positions generated by different sources (Internal gps in each MFD, iPad, hockey pucks AIS hockey puck, gps from handheld etc.) and compare them. If ones off it’s bad for some reason. Also check how many satellites you’re getting. That’s easy to do on any MFD. lastly know if the military has transiently degraded it and you were just off for a brief time.
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Old 07-25-2021, 01:02 PM   #19
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What I've done is to measure from the water line to the bottom of the prop (2.5 ft) and from the water line to the transducer (1 ft). So if the sounder reads 1.5 ft I know i'm about to churn up something. When cruising slowly along the shoreline looking for For Sale signs and admiring the houses I use the chart to give me a hint that this inlet is probably OK but keep an eye on the sounder as I go. Sometimes I am surprised at how quickly the depth changes. And, if I suddenly see the sounder showing 2' I assume the bow is at 3' or less. That's not what I expected to have to do when I ordered the Navionics chip. I assumed it would be accurate. I'd like to know what the depth is before I get there, not when I'm already in the mud. And judging from the replies I've seen I guess it's just something I'll have to live with and do what those of you with experience do.

I appreciate everyone's advice on this.
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Old 07-25-2021, 02:18 PM   #20
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It sounds like Navioncs has crappy charts in that location. Whether that's uniquely a Navionics issues would require comparing against some other chart source. There are a lot of places where the charts suck, and different vendors suck more than others.


I'd start by checking with them to be sure you have the latest.
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