How Can Garmin BlueChart Be So Wrong?

[Scuffy]

I would wonder if there are ways to adjust the GPS unit/antenna for known inaccuracies? I also wonder at the next Tugnut gathering if someone (another Ranger T) sitting right next you in the next slip over would the two GPS units come close to showing the near same position? Just wondering. I guess I'll have to see if my independent AIS unit shows the same position as the Garmin does.

 

[baz]

My understanding is.... The public accessible GPS gives an approximate accuracy of around 50 feet.

 

[hughesdave123]

I have raymarine e125 . when I have a "that should be there" , or why is the marker missing" , I make an edit and also submit to raymarine , through the iPhone.

My raymarine e125 I installed , they let me download all new updates that are from user edits and also submit corrections . I use the navionics app on the iPhone to do all the syncing . I also use the iPhone to do the auto routing and transfer it to the e125 through wireless.

 

[baz]

hughesdave123: Can that also be done using an Apple Watch as well. :lol: Seriously though.... It would be nice to have AIS and obstacle/danger ahead alerts on the Apple Watch.

I can pair my Apple Watch to my 1st Mate's Bluetooth capable Hearing Aids and using the watch as a Mic can communicate with 1st Mate directly into her ears. Wonderful technology.

I agree that Raymarine has several advantages over Garmin. Both have good and bad, but Raymarine IMO is better with the free updating and the user interface (UX). Gamin's Tech help is awesome as well.

 

[Binare]

they let me download all new updates that are from user edits and also submit corrections

Are those users smart enough to realize when their GPS is off and it's not just a missing feature? There has to be more to that or RayMarine's charts would be way off and littered with garbage.

 

[hughesdave123]

they let me download all new updates that are from user edits and also submit corrections

Are those users smart enough to realize when their GPS is off and it's not just a missing feature? There has to be more to that or RayMarine's charts would be way off and littered with garbage.

read this , you can turn on or off. I like it as you have to be registered to use it . I use it all the time and have not had any issue with it.

http://www.navionics.com/en/community-edits

I also like the iwatch watch idea , but the boat is so small I just turn and talk to the wife .

I use the phone / ipad to look up a marina or destination I want to go , and the app builds the route out for me , then I just hit the upload route button .

Now I purchased a 2008 model year boat that was already outfitted with raymarine , and my previous boats were raymarine , its just what I was used to .

Isn't navionics available for garmin ? post wasn't meant to be a raymarine vs garmin . rather a bluecharts vs navionics .

 

[Pilotnavigator]

I posted a picture in my album that show the chart image with the satellite overlay for one island. Notice that the south west side is distorted on the chart. The actual land is larger that the chart. The shape is still accurate. As mentioned before GPS and plotters some would have stayed well away. Now the plotter could auto route you too close to the actual land while showing save passage on the plotter.

 

[CAPTCRUNCH]

Those are some dramatic discrepancies between chart and satellite images in Pilotnavigator's gallery, particularly the last image. No wonder Garmin's warranty quoted above disavows the accuracy or completeness of the charts it sells and warns against using them to determine where you are or where you're going. [insert "snide" smiley here.]

 

[Pilotnavigator]

The charts they use are the exact ones we buy in paper form. Can you image the legal issues they would be in if they drew their own charts and made a mistake! Eventually charts will be corrected to GPS coorinates. But until then use your eyes and always double check when things look wrong, or when you are close to dangers. And as I tell everyone in our power squadron courses "be stingy with advantages, Don't give them up" depth, distance from shore, weather, fuel, rested crew, day light, etc

 

[knotflying]

I would wonder if there are ways to adjust the GPS unit/antenna for known inaccuracies? I also wonder at the next Tugnut gathering if someone (another Ranger T) sitting right next you in the next slip over would the two GPS units come close to showing the near same position? Just wondering. I guess I'll have to see if my independent AIS unit shows the same position as the Garmin does.

If I am not mistaken I believe there is a page where you can see how many satellites your GPS is locked in on. When I was flying my GPS would automatically come up with a warning if it was locked on less than 4 satellites. The issue may not necessarily be the location of your boat, but rather the accuracy of the charts.

 

[SGIDAVE]

My understanding is.... The public accessible GPS gives an approximate accuracy of around 50 feet.

It's better than that, according to our own government:

Real-world data from the FAA show that their high-quality GPS SPS receivers provide better than 3.5 meter horizontal accuracy.

SPS = Standard Positioning Service Performance Standard (not military system or standard).

Even worse-case they quote as 7.8m.

dave

 

[baz]

Well that's certainly better than 50-feet. Thanks for the update. 🙂

 

[SGIDAVE]

I didn't realize until I was researching this just now, but 'civilian' and 'military' GPS HORIZONTAL accuracy are the same since May 2000 (Prez Clinton made them basically equiv). There IS a difference (civilian v military) in GPS signal redundancy and VERTICAL accuracy.

dave

 

[Binare]

What you all seem to be forgetting is that those numbers are what is possible, not what you will get all the time. Your GPS works on triangulation, all your gps chip is doing is trying to figure out where in space it is, relative to its references.

Its why as mentioned before in avionics an alarm would go off if it wasn't locked on to four satellites. Three should be suitable for a marine environment. My question is...

When people feel their chart plotters are lying, how many satellites where they locked onto at that moment in time?

I would suspect that you would need atleast 6 or 7 to hit those accuracy numbers mentioned, blue water... no problem, coastal... Could be a big issue.

I love gps as much as the next guy, but at best they are just another tool to use, at worst they are probably responsible for the vast majority of groundings becuase believe to be more than what they should be, a supplementary tool to proper marine navigation.

 

[CAPTCRUNCH]

On my 7212, I think the GPS status is shown as a number of bars (just like a mobile phone) in the upper right corner of all screens. (I say "think" because I have yet to find any reference to that symbol in the documents I have.) It's available in more detail at Configure|Sytem|GPS which shows the number of satellites in view, their locations, and the signal quality of each. GPS error is also shown. A few minutes ago, I had 5 bars overall, 9 birds at 5 bars, 1 bird at 4 bars (the one that was low on the western horizon), and a GPS error of 9.84 feet (1/3 of a boat length). I'll take that.

 

[knotflying]

And where did it show your boat on the chart as compared to reality?

 

[CAPTCRUNCH]

The chartplotter showed the boat to be in my driveway, pretty distant from any shorelines or navaids. It does know which side of the road it's on.

 

[baz]

Today I reviewed the accuracy of GPS while my boat was on its trailer in my driveway. The calculated position was IMO spot on, had the boat located on the correct side of the road and pointing in the correct direction.

The dashed/track lines presumably are for when I was maneuvering the boat+trailer and backing it up into my driveway. 🙂

I used my iPhone to determine what it was calculating as well (last photo).

 

[RonB]

For more on this subject, check out Nigel Calder's book, "How to Read a Nautical Chart (Second Edition), A Complete Guide to Understanding and Using Electronic and Paper Charts", published in 2012. Here's a quote from the section "The Limits of Accuracy" (pg. 20-21):

"With the advent of satellite-based navigation systems, mariners can precisely locate themselves any where in the world in terms of latitude and longitude. What few of them realize is that this position is with reference to a particular ellipsoid (for GPS, it is WGS 84), which may differ markedly from the underlying ellipsoid or datum of the chart they are using. Other than confusing the navigator and providing a false sense of confidence, it serves little purpose to know exactly where you are (latitude and longitude) relative to WGS 84 if your chart is based on ellipsoid and datum that result in the lines of latitude and longitude running through substantially different real world locations than those given by GPS."

A highly recommended book.

Ron

 

[scross]

My very old Garmin 496 GPS, which I use for both as an aviation and marine moving map, has it's horizontal location accuracy information displayed on the screen. This position accuracy changes based on several factors with the most critical (I assume) being the number of satellites currently acquired. Typical horizontal accuracy showing on the 496 in the airplane is between 12 to 25 feet. The GPS accuracy is also enhanced in the aviation mode by WAAS (Wide Area Augmentation System) around airports and important navigational points.
It would be nice to see the horizontal location accuracy displayed on all GPS devices!