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Author Topic:   Garmin charplotter error at Isle Royale
kmev posted 09-03-2012 01:28 AM ET (US)   Profile for kmev   Send Email to kmev  
Just returned from a week of circumnavigating Isle Royale while hiking and diving. While there, my Garmin 440 chartplotter was always approximately 300 feet off - so that I was usually running over the top of islands and rocks according to the unit.

Elsewhere, including western Lake Michigan, eastern Florida, and even the Apostle Islands this unit has been dead-on accurate. Only at Isle Royale and along Minnesota's north shore (Grand Marais and Two Harbors) did it show this error.

I attempted to change datums in the chartplotter to correct the error but could not find that option in the Garmin. Has anyone else experienced this in western Lake Superior or know how to correct it?

swist posted 09-03-2012 09:09 AM ET (US)     Profile for swist  Send Email to swist     
Always off in the same direction?

How many satellites are being tracked when it is off, as opposed to when it is correct? Any significant difference?

jimh posted 09-03-2012 09:17 AM ET (US)     Profile for jimh  Send Email to jimh     
An error of 300-feet in the computed position of a GPS receiver is a rather large error in the modern era of GPS navigation with Selective Availability (SA) removed and precision fix augmentation from the FAA's wide area augmentation system (WAAS) available. You don't mention if your GPS receiver was using a precision fix with WAAS, but I would assume it was, as most modern receivers typically take advantage of the augmentation system. My expectation for position error on a modern GPS receiver with WAAS would be more like 30-feet, not 300-feet.

It would also be unusual for the error to be consistent and repeat for a long period of time. If there were an error in the GPS position solution, I would expect the error to be more random.

Having faith in the global positioning system and the accuracy of modern GPS receivers, I tend to think an error of 300-feet is probably more likely due to the chart data being used. What sort of electronic chart was being used?

We were around Isle Royale for ten days in 2010. I still have the GPS trails from that trip. As I recall the position fix we were getting was very accurate. I remember the afternoon I accidentally maneuvered toward a dock and into three-feet of water. When I reviewed the trail on the electronic chart, sure enough, it showed me going right over that shoal.

6992WHALER posted 09-03-2012 10:37 AM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
There is an error in the Garmin Chart plotters on Isle Royal.

I have not found it to be a consistent error.

The first time I experienced it, it was with my Garmin 172c in 2006.

As I went into Washington harbor the unit showed me driving on shore about a 1/8 mile the entire length of Washington Harbor. The map was the Lake Superior BlueChart probably version 6.5 or so.

The second time I experienced a problem was 2010 with the Garmin 3206 using the built in Maps, Bluechart G2. This time as we approached the entrance to Washington Harbor in the dark in the rain and fog, the Radar that was overlaid on the chart plotter map shifted off the map. It was clear even in the limited visibility that the radar was correct and the chart plotter was wrong. After we passed the narrow channel of the entrance the chart plotter was accurate again. So it definitely had improved over the 2006 experience.
The only other time I noticed an error was when we were entering Chippewa harbor, the chart plotter had us cross over the little island in the entrance.

I think the error is in the chart data not the accuracy of the GPS. This summer I purchased a Garmin Bluechart G2 chip of the great lakes, because the internal maps do not have any Canadian detail on them. It will be interesting to see if the new maps have improved.

kmev posted 09-03-2012 01:49 PM ET (US)     Profile for kmev  Send Email to kmev     
The error did appear to be in the chart overlay and not the GPS accuracy. All my waypoints and track lines were accurate and repeatable - they just were in the wrong location on the chart.

I created several waypoints in Google Earth beforehand for entrances to harbors. All those were accurate, as well as all the shipwreck mooring buoy coordinates. I did have WAAS enabled and the GPS accuracy stated less than 10 feet most of the time. The error most often shifted the chart to the northeast, but seemed to change direction in different locations about the island.

It was never really a problem as we had clear weather and mostly calm winds. The only time it created a problem was when we were going into Fishermans Home near the southwest end of Siskiwit Bay. The GPS position started skipping north to south so rapidly my track line was a zigzag even though I was idling along. The sun was getting low so we couldn't see the rocks very well (and there were a lot of rocks in that area) so we decided to bail out and head for the west end of the island.

kmev posted 09-03-2012 01:55 PM ET (US)     Profile for kmev  Send Email to kmev     
I should have mentioned that the charts were the basic charts that come loaded in the Garmin unit. I have loaded the latest Garmin update from 2011, which will probably be the last update they provide given the age of this unit.
6992WHALER posted 09-03-2012 03:29 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
The build in maps are Blueschart G2, the same as my 3206. For a lot of money you can get Bluechart G2 vision maps for your unit.

Looks like there is a July 2012 software update on the Garmin web site? I have found that even with my discontinued units they still have software updates. I missed one last summer and my weather system stopped working. I now check for updates a few times a year and when ever I am going on a long trip. cID=168&pID=8055

kmev posted 09-03-2012 03:53 PM ET (US)     Profile for kmev  Send Email to kmev     
Thanks for pointing that out - downloading the new update now.
6992WHALER posted 09-03-2012 05:36 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Do you live in MN, WI or MI?
kmev posted 09-03-2012 05:56 PM ET (US)     Profile for kmev  Send Email to kmev     
Madison, WI - just updated my profile.
6992WHALER posted 09-03-2012 08:11 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Just checking to see if you were on the MN side. We use to do a lot of diving.
David Pendleton posted 09-04-2012 01:08 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I didn't not experience any chartplotter anomalies while in the Isle Royale area with my Raymarine plotter and C-Map electronic cartography.

I have a Garmin Nuvi automotive GPS in my truck. Every day I drive across the "High Bridge" is St. Paul as part of my commute. Whenever I pass over the downtown side of the bridge, the map on the gps rotates about 90 degrees and then back to the correct orientation a hundred feet or so later.

My previous Nuvi was not without map problems too. When driving to the 2009 North Channel rendezvous along Highway 17 in Canada my position shown was almost always off the highway in the woods by several hundred feet.

Buckda posted 09-07-2012 07:44 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Note to KMEV-

If you're interested, I can send you all the waypoints that the Voyageur uses to circumnavigate Isle Royale. They're helpful for sure when planning a trip up there. You should consider joining us if we have another trip in the area.


David Pendleton posted 09-07-2012 08:40 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
What do you mean "if?"

I'm going back next year.

K Albus posted 09-07-2012 09:01 PM ET (US)     Profile for K Albus  Send Email to K Albus     
Me too.
kmev posted 09-07-2012 10:56 PM ET (US)     Profile for kmev  Send Email to kmev     
Dave, thanks for the offer, but I don't think I'll need those numbers. We (my girlfriend and I) had a blast exploring and discovering places on our own.

We intended to go with another couple on a second boat, which would have been nice as we were diving and one boat could have dived while the other sat topside. The other boat cancelled on the trip, and since this was the third year in a row we had our "other boat" cancel we decided to go and explore on our own and solo dive while the other watched the boat.

It turned out to be a lot of fun, and I don't think we would have seen as many different places if we have been with another boat. We purchased a hiking/canoeing guidebook and a detailed USGS map at Windigo on our arrival and had a blast poking about whatever interested us. It was nice to be able to make spur-of-the-moment decisions and do whatever we wanted without worrying about what anyone else thought. Needless to say, our actual itinerary was nothing like the proposed itinerary we recorded at Windigo. We made some great dives, too.

6992WHALER posted 09-07-2012 11:05 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Dave did you find any mistakes in your Garmin maps?
kmev posted 09-07-2012 11:06 PM ET (US)     Profile for kmev  Send Email to kmev     
This is my favorite pic of the whole trip: Whaler?authkey=Gv1sRgCN7h2fOJ0Jm8Mg#5784143527163485858

And I will say my Outrage did fit through the Keyhole by Amygdaloid, but watch out for currents!

Buckda posted 09-09-2012 11:42 AM ET (US)     Profile for Buckda  Send Email to Buckda     

I'm running the basemap in my 2010C which is a 2008 or 2009 version, and BlueChart V5.5.

Here are three images. Red tracks are from 2010, and green tracks are from 2007 (using my GPSMAP168Sounder).

First up Washington Harbor:
My only visit to Washington Harbor and Windigo was on my most recent visit, in 2010, using the 2010C unit. There was no apparent error in the map data: WashingtonHarborChartdata.jpg

Next up: Chippewa Harbor. As this is one of my favorite locations on the island, I've visited it several times, but the tracked data compares the 2007 visit with the 2010 visit (using two different units). ISROcharterror1.jpg

Finally, entering Passage Island in 2010: PassageIslandCharterror.jpg

Note that in this one, the data definitely shows us driving over a reef at the entrance to Passage Island.

Some of this is why I like having the waypoints that I mentioned boat does not have radar - so in a foggy or even nighttime situation, those waypoints are a good lifeline to get you from point to point around the island to help you navigate to a deep water entrance with few shoals to find safe harbor or help/assistance in an emergency.

6992WHALER posted 09-09-2012 12:18 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Dave, can you blow up the data. The spot that my unit in 2010 went over the land was the island between your two way points T PT INTO and North Gap. It looks like your track is close if not on shore there.

The island in the entrance to Chippewa Harbor, that I drove over does not show on your map. I just pulled up my charts and it is not on mine either.

Buckda posted 09-09-2012 12:36 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Actually, it shows as a rock (awash).

Even though it appears more like an island "in person".

In my experience, if your chart data shows "overzoom" in the lower right corner, then your map data won't always be very accurate.

Here's the image you requested in very close detail. NorthGap.jpg

In my memory, I don't recall passing that close to shore, however since the depth was 18' or deeper, I probably didn't notice too much - I was trying to decide if I was going to try to overnight in Huginnin Cove or Tobin Harbor, if I recall, and there was a decent swell coming up the lake from Duluth, so I may have hugged that shore for a little bit more protection before running into the open lake.

Buckda posted 09-09-2012 12:43 PM ET (US)     Profile for Buckda  Send Email to Buckda     
Here is my 2005 data for Chippewa Harbor: 2005ChippewaHarbor.jpg

Note that the dock is drawn further down the harbor than where it actually is. Also note that I apparently drove right over a rock near the dock.

That rock labeled uncovered rock in the channel near the red bullseye is how the map represents the island in the entrance to the harbor.

Also note how similarly the tracks match for all three years that I visited the area.

6992WHALER posted 09-09-2012 01:09 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
For ease of explanation I will use right and left in my description. In 2010 my chart plotter showed the (North Gap) island further to the left then it really was. That might explain why you look like you drove so close to the shore. I bet you were dead center in the channel in reality.

I am sure it jumped out more for me because it was dark and the radar so obviously disagreed with the maps.

6992WHALER posted 09-09-2012 01:12 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I said that backwards, the actual Island was further to the left than the chart plotter showed it.
Buckda posted 09-09-2012 01:12 PM ET (US)     Profile for Buckda  Send Email to Buckda     
That is true - I'm sure it would have screamed at me if I was in the dark, or in the fog and relying on instruments to guide me in....

...again, which is why I jumped at the chance to get those GPS waypoints from the ferry when I had the opportunity. Those alone would likely keep you off the rocks in all but the tightest passages in the fog or in the dark...but RADAR would be a great comfort as well...gonna have to look up that as an addition for the future.

6992WHALER posted 09-09-2012 01:54 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Aside to kmev. Why did you decide to dive solo as apposed to leaving the boat alone and diving together?
kmev posted 09-09-2012 08:18 PM ET (US)     Profile for kmev  Send Email to kmev     
I once left a boat unattended during a dive. A storm blew over during the dive and when we surfaced it was gone. I vowed that would never happen again.

We did leave the boat unattended on the America, but it was moored to both buoys and I tossed the anchor overboard just in case. It would have been very hard to lose the boat in that situation, and if so we could have just swam ashore.

Adrift offshore in Lake Superior is not a situation in which I intend to find myself.

6992WHALER posted 09-09-2012 08:55 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
That is a good point, but some of those dives are pretty deep. My gut tells me there is more risk during the dive than the risk of the boat not being there when I resurface.
So I personally would rather have my partner beside me on the bottom.

We all weigh risks as we see them and plan accordingly.

kmev posted 09-11-2012 12:21 AM ET (US)     Profile for kmev  Send Email to kmev     
It all depends on your experience and comfort level of diving. I can deal with most any problem on the bottom by myself. There isn't much I can do if I'm adrift in the open sea and no one knows to look for me. At least that is how I weigh my risks.
Chuck Tribolet posted 09-11-2012 12:26 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
kmev, there are several things you can do if adrift, but they
require preparation:

Acme Thunderer ref's whistle
DiveAlert whistle powered by your BC inflator.
A couple of boat flares in a six C-cell dive light case.
A narrow-beam flashlight.

I carry all but the last which I'm considering.


6992WHALER posted 09-11-2012 01:18 AM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
Almost all the wrecks on Isle Royale are close to land.
Personally I think I can live a much longer time on the surface in a dry suit than I can under water out of air.

Again , you have to asses your risks, I have never had a boat not be where I left it diving or not. But I have been on dives when things did not go as planned under water. From equipment failures to nitrogen narcosis.

David Pendleton posted 09-11-2012 02:24 PM ET (US)     Profile for David Pendleton  Send Email to David Pendleton     
I was under the impression all the wrecks had mooring buoys.

I took a look at the Lifeline on their web site. It looks like an interesting design. They retail for $299.00 locally. You can get a pretty nice DSC handheld for that.

Chuck Tribolet posted 09-12-2012 03:59 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
But can you take that DSC handheld diving?


6992WHALER posted 09-15-2012 11:21 AM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
All the wrecks that the typical recreation diver, dives have mooring buoys.

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