Ran Aground Using Navionics.

[Wingsy]

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?

 

[High Wire]

Call Navionics. Lakes don’t change that much. Perhaps it’s the Simrad not giving steady position data. Is the antenna clear?

 

[Comodave]

Welcome aboard. We had Navionics in our last boat. Never saw anything like thar though.

 

[Wingsy]

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.

 

[psneeld]

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.

 

[AlaskaProf]

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.

 

[OldDan1943]

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.

 

[Wingsy]

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.

 

[alormaria]

Use sonar for depth readings.

 

[Wingsy]

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.

 

[PJHoffnet]

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.

 

[Comodave]

Our last boat had Raymarine and it had a built in antenna in the MFD.

 

[C lectric]

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.

 

[SteveK]

While aground did you record the Lat/Long from Navionics and then compare to another chart?

 

[OldDan1943]

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

 

[Portage_Bay]

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.

 

[SteveK]

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.

 

[Hippocampus]

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.

 

[Wingsy]

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.

 

[twistedtree]

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.

 

[Blissboat]

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

Another factor is the variability of the lake's level - the water level in most lakes fluctuates. In the case of lake Norman, NC, the water level is affected by periods of drought or heavier rainfall, plus by the management needs of Duke Energy, which has a dam and a nuclear power generating plant in the watershed.

According to Lakesonline.com, Lake Norman has not been at "full pond" since 2019. Sounds as though mariners depending on Navionics chart data need to know the initial lake level from which they are making their depth predictions.

 

[Blissboat]

The Lake Norman data can be viewed graphically here: Lake Norman Water Level

Earlier this year, it was about five feet lower than full pond. Right now it appears to be about three feet below full. In recent years, it has reportedly been as much as eight feet low - enough to strand some residential docks.

 

[mvweebles]

Electronic charts appear deceptively accurate. Behind the numbers, there is a "Zone of Confidence" classification. Depths in shipping channels are very accurate. Outlying areas, not so much. Navionics has a crowd-source bathymetric feature that collects depths from users but it must be enabled. Over time, accuracy will improve.

OpenCPN can display ZOC data for chart soundings. I know of no other charting program that has this info easily available.

https://nauticalcharts.noaa.gov/updates/how-accurate-are-nautical-charts/

Bottom line is the chart plotter is indeed an aid. This is not just a legal disclaimer, but a fact - the data is 99.9% accurate and improving all the time. As a helmsman, you must collect data from all available sources including your depth sounder and your eyes. I can tell you that inlets in Florida shift. If it's not a USCG priority area, you're dependent on local knowledge or visual queues. I have seen channels be significantly off, though the OP is admittedly at a far end

I'm not saying I wouldn't have run aground under the same circumstances, I'm just saying that blaming Navionics is missing the point on a very teachable moment

Peter

 

[mvweebles]

.......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.....

Good post. I've been on a few boats where the owner has a label near the depths sounder or MFD indicating draft, and how the depth sounder is calibrated ("depth beneath keel" is pretty typical). I personally prefer not offsetting for draft as I'd like the depth sounder to be in synch with the charts. But not a strong preference - mostly just need to know the calibration whatever it might be. Good topic for another thread might be preferences of others.

BTW - Bridge clearance (Air draft) is another good number to include prominently at the console. I do neither....but should.

Peter.

 

[koliver]

You are in the SE of the US, an area with which I have no familiarity.
I do, however, use charts, both Navionics and others.
At best, they are a snapshot of the conditions surveyed at some time in the past. Then there is the datum, which means that the charting was referenced to some fixed point that was able to be referenced by other means, such as getting its lat and long and then measuring the offset to any other position. Not all areas have been resurveyed since we have had the advantages of GPS accuracy.
For example, we did a charter in Turkey in 2007. The charts we were using were surveyed by Beaufort, who died in 1857. The warning on the charts advised that the accuracy of the charts could be in error by 1 nautical mile.
Hopefully yours are from a more recent survey.
I use my chart and my depth sounder when setting my prawn traps. this week, in Desolation sound, I have found little correspondence between the charted depth and that reported by my sounder. I have learned to trust my sounder. The differences can be of more than 200' in the depths I want, 200' to 400'.

 

[guy with a boat]

I think Blissboat nailed it. The charts could be perfect (or not), but once the lake level changes then all bets are off and you have to adjust what the chartplotter is telling you. The chartplotter doesn’t know that the lake level changed and can’t adjust its own numbers. That’s why you need a depthfinder and you do need to be aware of water level changes if you are boating in a lake.

As a side note, we are boating in SE Alaska this summer, and the chart accuracy is pretty sketchy in many areas. Like Koliver, we have seen charted depths in places that are frighteningly inaccurate. It makes me appreciate even more the high level of accuracy that we see in most of the lower 48 and parts of BC.

 

[Greg S]

... in Desolation sound, I have found little correspondence between the charted depth and that reported by my sounder. I have learned to trust my sounder. The differences can be of more than 200' in the depths I want, 200' to 400'.

Charts show depth contours with a few spot depths as filler. As long as the depth on your sounder falls between the values of the contours I'd say that the charted depth is correct. The spot soundings aren't necessarily showing high, low, or average depths. They show the depth at that spot and are between the contours.

I'm off track for the OP, sorry about that.

For the OP, I wonder the accuracy of of the GPS fix, as well as data accuracy. How fast are you moving too? When you are moving, GPS doesn't show where you are, it shows where you were a moment ago.

 

[BandB]

Lake Norman is not in terms I would use a charted body of water. The level fluctuates up to about 4.5'. We're talking an inland lake and we don't have the normal attention to charting there by the Coast Guard or NOAA or anyone else. I lived the first 40+ years of my life there and never knew there even were charts. You can generally read by the contour of the land, the color of the water, and the use of your depth sounder.

 

[Jeff F]

Lake Norman is not in terms I would use a charted body of water. The level fluctuates up to about 4.5'. We're talking an inland lake and we don't have the normal attention to charting there by the Coast Guard or NOAA or anyone else. I lived the first 40+ years of my life there and never knew there even were charts. You can generally read by the contour of the land, the color of the water, and the use of your depth sounder.

I wonder whether crowd sourced data might solve at least partially for a place like Lake Norman. My sense is that there is lots of recreational boating there, and modern MFDs can provide data back to vendors to improve chart quality.

 

[High Wire]

If you go to the Navionics website and find the Lakes Hotmap, you can zoom down to their contour map for Lake Norman. It should agree with your chip. So where you ran aground shows good water in both places?