Garmin 188C Waypoint Inaccuracy

Jazzmanic · Posted: Fri May 19, 2006 4:51 pm Post subject: Garmin 188C Waypoint Inaccuracy

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

lloyds · Posted: Fri May 19, 2006 5:05 pm Post subject:

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.

sailor-d · Posted: Fri May 19, 2006 5:08 pm Post subject: Waypoints

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

416rigby · Posted: Fri May 19, 2006 5:14 pm Post subject:

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

Pat Anderson

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.

Stanley-J · Posted: Fri May 19, 2006 7:13 pm Post subject:

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.

Jazzmanic · Posted: Fri May 19, 2006 8:24 pm Post subject:

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

teflonmom · Posted: Fri May 19, 2006 9:11 pm Post subject: Garmin & Waypoints

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.

MOOSE · Posted: Sat May 20, 2006 10:29 am Post subject:

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

AstoriaDave · Posted: Sat May 20, 2006 10:48 am Post subject: Re: Garmin & Waypoints

416rigby · Posted: Sat May 20, 2006 12:36 pm Post subject:

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

AstoriaDave · Posted: Sat May 20, 2006 2:03 pm Post subject: