Garmin 188C Waypoint Inaccuracy

J

Jazzmanic

New member
When setting a course in my Garmin 188C using waypoints from Wetherley's, I've noticed there is at least a 500 yard variance on the GPS compared to where I should be. In other words, according to the waypoints plotted in the GPS, many times the courseline is showing the boat going directly over land rather than being offshore by 500 yards or more. Obviously, I have to adjust. What this tells me is some kind of differential adjustment must be made in the Garmin.

Has anyone experienced this problem and is there some way to correct this variance? Are the Wetherley waypoints inaccurate? Pat (Daydream), I know you have good experience with the Garmin 188C so chime in at any time.
Peter
 
Wetherly's has always seemed pretty close for me, but so has Garmin. Check the datum setting in the Garmin and see if the coordinates are on the same datum as Wetherly's. I think Garmin comes preset with WGS84. A lot of Wetherlys are older and might be using an older datum.
 
I guess my first question would be: have you checked the Wetherley waypoint coordinates against plotted coordinates on a chart? I always thought that waypoint books had some sort of disclaimer on the accuracy of their coordinates.

Dave
 
We had a 188C on our last boat, and loved it...fantastic unit. I never had any waypoint accuracy problem. Maybe a call to Garmin?

Rick
 
We did...I have a couple of thoughts: (A) zoom level affects where you see a route going - I often see a route going over land on the base map with Coastal Explorer until I zoom in to get chart-level detail (I assume that is not what you are experiencing, though) and (2) I would suspect Wetherly's coordinates before I would fault the Garmin 188C chatplotter. But of course I really don't know, just guessing here.

c-dancer":3440vi01 said:
Pat (Daydream), I know you have good experience with the Garmin 188C so chime in at any time.
Peter
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I have the 188 and experianced the same thing using the internal base chart. I recently purchased a "Blue Chart" cartriage and well see if this makes the differance. With the base chart I was motoring in the middle of the Hudson Rive and it was showing ..at times...that I was on land.
 
Thanks for all the quick responses guys. Sailor-d, I'll check the Wetherly waypoints vs the charts and see if they're accurate. Pat, I know what you mean about the zoom effect but I'm pretty sure it was showing our courseline on land either zoomed in or out. Lloyds, good idea about checking the datum setting. Stanley-J, I do have the BlueChart chip and wasn't using the internal base chart. 416-Rigby, I love the Garmin 188C as well, very user friendly for a newbie.

What a great site, thanks C-Brats!
Peter
 
While useing a Garmin hand held in the Gulf of Mexico near Belize I had the same expierence. We were in deep water but the GPS showed us right on top of a reef. Luckly we had a pilot on board and he told me the charts were incorrect as they were made before GPS and the Lat and Long were incorrect. My little Garmin was doing a fine job- the chart was at fault.
 
Based on my understanding, the problem lies in the datum disparity between your GPS and the inherently less accurate, land-based surveys used to compile the charts.

Have you noticed that when you follow your return route track, you are right where you should be, even if it shows you running over land?

I've fiddled with the datum offset on my GPS only to find that if I set it for one area of our cruising area, it now is way off in another. So I've learned to just chock it up to the nature of the beast. I'd trust the GPS coordinates more than the underlaid chart.

Al
 
teflonmom":3jo9ux1m said:
While useing a Garmin hand held in the Gulf of Mexico near Belize I had the same expierence. We were in deep water but the GPS showed us right on top of a reef. Luckly we had a pilot on board and he told me the charts were incorrect as they were made before GPS and the Lat and Long were incorrect. My little Garmin was doing a fine job- the chart was at fault.
Click to expand...
Similar experiences in Barkley Sound when off the outside of Gibraltor Island. Some uncharted rocks, also, but that's another problem!

In the Columbia River, muddling along the Prairie Channel, my Garmin shows me traveling over islands which clearly have eroded since the time of original charting, maybe some 70-80 years ago (?).

Charting done today using GPS technology would correct these systematic errors in the chart. The old charts, however, seem to do a good job showing landforms and overall shoreline configuration -- just offset a bit in lat/lon. Many of us navigated "by sight" along tricky shores in the old days, and did OK. Now we know the chart is off, because we have a GPS to tell us that. It would be a mistake to blindly follow the GPS.
 
We've never used the GPS without the Blue Chart chip installed, so maybe it does make a difference...more accurate with the chip? I did notice that a couple of buoys here were not in the right place on the GPS...one of them was almost 200 yards off from where the GPS chart said it should be...talked to the buoy guys at work and they said they occassionally move them a bit and you should always look out the window to compare the chart as the charts get updated, but the GPS chips may not have the most current data...good idea.

Rick
 
416rigby":1o363utw said:
We've never used the GPS without the Blue Chart chip installed, so maybe it does make a difference...more accurate with the chip? Rick
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I think the error in my case (Barkley Sound) was in the original charting from which the BlueChart is taken, so the BlueChart can't be any more accurate than that. Mind, the errors were on the order of a couple hundred feet or less, such that anybody running a hundred yards off the shoreline would be OK ... except for the odd uncharted rock, of course.
 
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