Chart plotter shows your on land?

[Ward H]

I recently bought a Garmin chart plotter. I like the unit but the track going into and out of my channel shows I am on running on land instead of in the channel.
I also run Navionics on my iPad and the track on that program shows me exactly in the middle of the channel.
I tried running Garmin Blue Chart Mobile on my iPad and found it showed the same thing, my track was off about 50' on the map and I was on the hard. GPS accuracy is 10'
I checked the coordinates of my tracks and they matched up on both the Garmin track and Navionics track.
I also compared the coordinates of a ICW marker and Garmin appeared to have located that correctly on the map.

I guess my question is, Is it common to have a chart plotter to show you on land when you are clearly not? Does that show chart cannot be trusted or is it probably just a local thing in that area small area close to shore and on a larger scale it will be fine.

I know I am fine where I sail and really don't need a GPS Chartplotter except for those few times I may be out after dark and I do have an accurate depth sounder.

I just like see where I am and was wondering if this is common with all chart plotters in some areas or if it was a Garmin thing.

 

[agprice22]

You could read Nigel Calder's "How To Read A nautical Chart." In it he explains why these errors occur as well as the fact that they are quite common.

Thanks,

Andrew

 

[dscribner]

I have found my free NOAA GPS software to be much more accurate that the "industry leader", Garmin. But much less convenient and not as waterproof. In a narrow channel, we always have Garmin, paper and our tablet going. Open water, usually just the Garmin to make The Admiral happy.

 

[alexco38]

WGS 84 (aka WGS 1984, EPSG:4326),

There is a setting in the Garmin (on mine) that need to select WGS 84.

"The latest revision is WGS 84 (aka WGS 1984, EPSG:4326),"

https://en.wikipedia.org/wiki/World_Geodetic_System

If set wrong you may be off by a bit.

 

[shemandr]

Check the datum.

 

[Jackdaw]

First selecting the 'wrong' datum will NOT show you in the wrong position. This simply formats the output (and expected inputs) of lat/long coords to different standards. The map and the GPS always stay in sync. The only time problems can occur is if you INPUT a coord in the wrong datum. That will show in the wrong position.

Now to the issue at hand:
It is all too common for cartography to be off from what is called in the industry 'ground truth'. This can happen to government agencies (NOAA, UK ordnance survey) as well as private companies like Garmin, Navionics, C-map, etc.

Private companies like Garmin will often start with a government source and then try and 'improve' the data and sell it. The initial conversion of the map data is done part via automation and part by hand. Its common for errors to be introduced, and they are always interested in knowing about them.

You can report garmin map errors here:

https://my.garmin.com/mapErrors/report.faces

 

[kloudie1]

I have had the same problem around Slidell, La. on the North shore of Lake Pontchartrain. I reported to Garmin on both locations and got no reply.. Seems to be a very localized area, but one is a critical channel that has the marks displaced enough to put ya out of the channel..

 

[Meriachee]

The factory charts in my a65 put me squarely in the parking lot when on the mooring. Being in the trees is a good thing when the weather is not so nice. The Navionics+ charts show me on the mooring, even when the weather is bad.
The gps feed is the same. Remember that the gps is not as exact as they'd have you believe, and as Jack mentioned, the placement of that data against the cartography is not always very exact either. couple those two things, and a narrow channel can suddenly get very narrow. Navionics also has a facility to feedback and their "sonarlogs" includes gps data, which helps remove errors too.

Rely on the data to get you to the channel, and then your eyes to work in the channel if it's that tight.

 

[Stu Jackson]

Funny, I was never off the chart when I used just paper.

 

[Meriachee]

Funny, I was never off the chart when I used just paper.

There's a marketing opportunity Stu. 3d printed coasters with contours, and close to gps current location. Well, ok, something nearby, current location, a few hours ago.

 

[Jackdaw]

A local company makes and sells placemats featuring an image of the local NOAA chart. Laminated in plastic they are very handy. During raceweek we tape one in the cockpit so the mostly-new-to-the-area crew can maintain situational awareness.

We call it our GPS. Global Placemat System.

 

[Maine Sail]

A local company makes and sells placemats featuring an image of the local NOAA chart. Laminated in plastic they are very handy. During raceweek we tape one in the cockpit so the mostly-new-to-the-area crew can maintain situational awareness.

We call it our GPS. Global Placemat System.

Jack,

That looks like a later model 36.7. What plotter is that and what size screen? I have a 36.7 I'm doing a ton of work to a full electronics package is part of it.. Owner just wants a temp plotter for the rest of this season but he's got an older boat with an Edson not the pyramid. Jus curious as to screen size on that one..?

Sorry for the hijack...

 

[Jackdaw]

Jack,

That looks like a later model 36.7. What plotter is that and what size screen? I have a 36.7 I'm doing a ton of work to a full electronics package is part of it.. Owner just wants a temp plotter for the rest of this season but he's got an older boat with an Edson not the pyramid. Jus curious as to screen size on that one..?

Sorry for the hijack...

Maine,

Yes, it’s a 2006 First 36.7, the first year with the recessed wheel on the pyramid. For a bunch of reasons it’s a much better setup then the older pedestal, and it is a common (if not cheap or easy) class-legal 367 upgrade.

The plotter on the helm is a Lowrance HDS-5 Gen2. It is networked with our HDS-7 touch downstairs and our 3G radar and sat weather. The screen size is just fine, and I like using the RAM mount arm because I can turn the screen and have it face beam-to when steering from the rail which I do most of the time. My 53 year old eyes are pretty OK still and I find it easy to read. I'm not a 'stare at the plotter' kind of guy; it gets most use when we have to round a GPS phantom mark. When navigating we do most of our 'heavy' work downstairs on the bigger touch screen or via a connected iPad.

The Lowrance in general does not have much in the way of sailboat-specific functions, but I'm OK with that. When we bought the setup neither did the more expensive but basically the same Simrad line, and the new B&G stuff was not out. But I wanted broadband radar. We would probably do B&G now.

The cable run is from the main panel around the upper notch in the bulkhead, and down the wiring channel in the headliner next to the hull deck joint. Once back in the transom area, they go along the forward wall to and through the rudder chain-drive cutout. After opening the roof panel under the wheel, up the wheel post. Then it’s just going through the pyramid wall to the plotter. Used BSS clams because of the complex cable heads.

I once feared that we eventually would tear the thing off in a particularly nasty gibe, but that has never come close to happening. Maybe my main trimmer is just that good! And she own the boat! ;^)

 

[Allan12210]

It is probably related to Plate Tectonics. The ground is moving. We get a lot of that out here.

My Loran had me in the parking lot while in my slip, but the GPS not so much. One thing I have liked about the Garmin systems is the ability to save my tracks on my laptop and then re-upload them when returning to some tricky areas. There are a few channels around here where the chartplotter position is off. Since I made it in and out alive using traditional methods, that breadcrumb trail is valid, regardless of where it appears to be on the display. So, on return visits I have followed the track and made it repeatably. Just have to use the tried and true when transiting new places.

 

[Cruising Spirit]

What a great thread! So much information for newer sailors...and old salts in this world of fast moving technology.

My 2 cents...back in the mid 90s, we got onto a lap top loaded with Nobeltec navigation software connected to a GPS. What a boom!

So we kind of turned into GPS junkies for a few years--till one day, cruising the south coast of Puerto Rico using Van Sant doctrine...I think it was the bay where Gilligan's Island (as the locals call it) was. We nearly ran on a reef in bright daylight sailing flat, in shore protected waters with 15 to 20 knots of trade wind.

We had a conflict aboard. The lap top was below at the nav station and Van Sant' book (with it's chartlets) was in the cockpit near the companionway. But we were hooked on one of us being below at the 'live' progression of ourselves across electric charts, while the other responded to commands of "plus or minus, 1, 5, or 10 degrees" on the autopilot to blindly navigate as though it were dark outside.

I sadly say, I allowed ourselves to fall into this malady...it was I below. My wife, Angela, called me saying there was a reef shortly ahead in the area I just told her from below we would be going into. Wondering what was up, I jumped up the companionway and looking forward saw a beautiful reef just 200 feet dead ahead sailing at some 5+ knots! Hard starboard about, throw the sheets and halyards--start the engine in a flash...there was current as well. By grace, we saved our boat and ourselves from a peril we put ourselves in.

Later that day at anchor. We realized through 'stitching' (as that early era program was set to do) of charts, we had brought in a chart (of that bay) whose datum was based on 'Zimbabwe 1953'. I've never found the answer to that...but I can tell you, it does not match WGS 84!

So, to this day we remind ourselves on journey that navigation has a 'piloting' element to it in confined waters. In open areas, clean navigation using rhumb lines takes the day...but introduce any near shore complexity and we move toward practical comparisons of information. Such as looking at the depth of the sounder versus the charted position. Many call it situational awareness. We also assign a value of 'confidence' to information. You can be pretty sure, if a GPS sourced position laid on an electric (or paper) chart conflicts with other real world information (like showing me on land when I am afloat)...my confidence in the electric information will be pretty low. Just my 2 cents. All the best, Tim at Cruising Spirit.

 

[JamesG161]

My C-Map chart card did same. My new LighthouseII chart is perfect. My a98 does allow adjustment in the chart if needed.
Jim...

PS:All markers are not shown on Raster charts (or paper) the Vector charts are updated often, but still no certainty.

 

[Joe]

Kind of side comment here... but I'm always a little nervous when drivers stare at the pedestal mounted chartplotter..... and don't take a look around for real life indicators. Don't get me wrong.. I love the electronics... but I have found myself staring at my large tablet screen's chart sequence.. watch the track lay down... rather than following my compass or gps course indicator that was on a more horizontal sightline.

A few years back in Tampa Bay... we were working into the Manatee River on our way to Bradenton YC. It is guarded by a reef that can be detected by the disturbed water it lays under. Following the channel markers precisely will insure safe transit... so it's important to check the boat's position between marks.... like you would if using range markers. The wife loved to drive, but she had a tendency to stare at the chartplotter screen.... which made it easy to drift off course even if you're pointed in the right direction. ESPECIALLY if you have it magnified to see more detail. Needless to say the rest of us watched the nav aids on the water and let her know if she was drifting..... However, during this time we cam upon another IP that had stuck himself in the sandy bottom... fortunately on the opposite side from the reef.... as we passed he refused assistance, but stated that his chart plotter showed plenty of clearance. We pointed out to him that he was not in the channel.... he looked at the markers ahead and behind and said.... oops.. should have been paying more attention.

The lesson is "stay vigilant".. it's okay to rely on you senses.

 

[gettinthere]

There are several places on the Intracoastal Waterway where we are well inland on our garmin plotter. Its the nature of the beast.

 

[Ward H]

I guess my question is, Is it common to have a chart plotter to show you on land when you are clearly not? Does that show chart cannot be trusted or is it probably just a local thing in that area small area close to shore and on a larger scale it will be fine.
I just like see where I am and was wondering if this is common with all chart plotters in some areas or if it was a Garmin thing.

From the replies, the answer is yes, all charts can have the same type error.

I did call Garmin and they assured me my settings were correct. They too, as Jack Daw suggested, asked that I report the error with a screen shot so I did that.

By doing some more comparing of other Garmin and Navionics tracks in other areas of the bay I found the error is only in my local channel. Most other areas appear to be spot on.
Now, knowing it is common and most of the chart for the bay I sail in is accurate, except for some areas near shore, I feel more comfortable with my chart plotter selection.

Who knows, maybe the chart was drawn when the shore line sat back much further?

In any case,
I mainly use the chart plotter to look ahead at my COG to help avoid the shallows and to check my tacks, things like that.

I'll never be fully reliant on the chart plotter unless maybe I get caught out in heavy fog and if I do I have multiple back ups including my older chart plotter which runs on AA batteries and I do have a separate depth sounder.

Thanks for all the replies.

 

[Stu Jackson]

Tracks

I'll never be fully reliant on the chart plotter unless maybe I get caught out in heavy fog ...

That's where the TRACK function of the GPS comes in handy.

You can make a track on a bright sunny day, save it and/or use it to make waypoints/route and then it never matters what "pictures" are on the screen.