Navigation anomolies

Brian D · Aug 16, 2019

Since you did not identify the GPS unit you were using, my first guess is software anomaly. There recently was a change made to the GPS 2019 Rollover that can actually put you somewhere else other than where you think you are. But mostly that Rollover was related to the date, not the Lat/Long.

Did you have a cell phone with you that could also be used as a GPS receiver. Most cell phones these days have that ability, or even AGPS (Assisted GPS).

I am sure you were aware of the Rollover and Assisted GPS, but just in case I went ahead and mentioned them.

JohnShannon · Aug 16, 2019

Might be format, No expert but... the coordinates can be expressed in degrees/minutes or in degrees-decimal minutes. They are different

126' 30" 126 degrees 30 minutes or 126.50 same physical location but two different numbers.

dlochner · Aug 17, 2019

JohnShannon said:
Might be format, No expert but... the coordinates can be expressed in degrees/minutes or in degrees-decimal minutes. They are different

126' 30" 126 degrees 30 minutes or 126.50 same physical location but two different numbers.

Not too long ago there was a thread about this. A sailor in Ohio kept finding himself landlocked. Turned out he was reading one format and plotting the other.

The Shadow Knows · Aug 17, 2019

To me the problem was that you don't have a plot charter. I've been in "The Soup" many times here in Florida where the was no visibility but yet, my chart plotter never failed me.

It's a WIZE decision to have one pal..........

CrispyCringle · Aug 17, 2019

higgs said:
I did use google maps on the cell phone which was not always accurate. It showed us 3 miles out when the Garmin said we were 1.5 out. The GPS was a Garmin - old model hand held- and it led us in on the nose. It was the same Garmin that gave us the lat/lon that put us on land.

If Im understanding you correctly, you were using and older GPS to get your position. This could very well be an issue with the leap second/GPS rollover. Especially if you have not used this GPS in a while. If you have used it recently and it was good, then never mind. I would check Garmin to see if there is an update available.

Kings Gambit · Aug 17, 2019

So, the OP’s post helps demonstrate the need to check/confirm one’s equipment at the beginning of every cruise. Deep in fog nearshore with an unreliable depth finder is not a good time to discover that one’s only GPS receiver is evidently not reporting accurate positions. By all means, track down the problem. But consider also having two GPS receiver units aboard, and test plotting the positions they give you early, at the start of a cruise, etc. The back-up for the installed depth finder might be an over-the-side hand-held unit. These are not expensive.

Kings Gambit · Aug 17, 2019

JohnShannon said:
Might be format, No expert but... the coordinates can be expressed in degrees/minutes or in degrees-decimal minutes. They are different

126' 30" 126 degrees 30 minutes or 126.50 same physical location but two different numbers.

Yes, but the notation is as follows. The two notations are deg (126 o), min (30‘), sec (00“), versus deg (126 o) min with sec expressed as decimal minutes (30.0’).

Johnb · Aug 17, 2019

I think John Shannon et al have most likely got it. Between

Decimal degrees
Degrees and decimal minutes
Degrees minutes and seconds there is much confusion

As far as gps reliability goes I have something like 9 of them, from an ancient Gecko to our smart phones and they all work with amazing accuracy. There is no substitute for knowing how to use them before you need to.

Kings Gambit · Aug 17, 2019

I’ve sometimes had to convert seconds to decimal minutes to plot a GPS position as displayed but not the converse except on high-resolution charts, where seconds appear on the chart scale but where the GPS display is reporting decimal minutes, etc. USCG typically reports in degrees, decimal minutes.

twalker H260 · Aug 17, 2019

Possibly a change in chart datum? I.e. NAD27 or NAD83? Is gps set the same as your chart?

Kings Gambit · Aug 17, 2019

Johnb said:
I think John Shannon et al have most likely got it. Between

Decimal degrees
Degrees and decimal minutes
Degrees minutes and seconds there is much confusion

Some of the confusion may derive from the intersection of two different math systems. The decimal system familiar to us all, versus the sexagesimal system which is intrinsic to time and navigation divisions, etc. The latter system is one covered in basic navigation courses, esp. celestial. BTW, I note we rarely report elapsed time, or time otherwise, in decimal minutes.

Brian D · Aug 17, 2019

I am sure the OP is checking right now how his GPS is setup; dd.ddd° or dd° mm.mmm' or dd° mm' ss''. And I am sure he will be back and tell us. We all make mistakes. I now I change my setup when checking against something that cannot be easily changed. It happens to the very best of us.

jssailem · Aug 17, 2019

I like @Kings Gambit ' s statement about checking your equipment at the start of the cruise.

I further encourage all of us sailors who want to rely on electronic sensors to play with them in the day light on a calm day when you can see for miles. Put a blanket over your head, have a friend be your eyes on the surrounding waters, and try to navigate to a nearby buoy with out hitting it or anything else. Try doing this with your GPS and plotter. With out the plotter just you and a chart. Testing yourself to guide the boat safely if some or all of your electronics fail.

Sure it is not as much fun as jumping in the boat and cruising around drinking a beer, but it is a whole lot better then suddenly finding your self in the middle of a fog bank trying to break out the manual and figure out which button to push to turn on the automatic horn signal.

Airplane pilots do this all the time. Perhaps they know something the vast number of boat owners do not. Perhaps we could steal this thinking from them. Fly - Sail under a blanket so when that fog bank hits and the bow of your boat disappears you don't have a cow...

John Nantz · Aug 17, 2019

Just thought I'd pass this story along. I remember reading about a cruiser sailing in the Indian Ocean toward Madagasdar (or that area) and they almost ran aground one night. They could hear the breaking surf but knew they were several miles from land. Tacked out of there and sorted things out.

Turned out they were using a used French chart they picked up earlier in their cruise and it had Paris as the 0º meridian.

Ummm.... what language was the GPS set in? Canadian waters, eh? (sorry, couldn't help it)

rgranger · Aug 17, 2019

To quote the great Yogi Berra

"You’ve got to be very careful if you don’t know where you are going, because you might not get there."

shemandr · Aug 17, 2019

Have we reached the point of personal experiences yet? Hope so. I once transited the Watch Hill passage with what I would call zero visibility. We had a hand held GPS on which I had put the Watch Hill R"2", I think and the Napatree R"4." Nothing after that. I went to my Chart Kit and the chart for Fisher's Island Sound was in minutes and seconds while the GPS read in decimals. After a while of seeing nightmarish visions of jetties, towering lights and hearing surf nearby I anchored. While moving I was way to stressed to interpolate. Anchored I could sit down and do the math finding a position on the chart (Yikes) and plotting a course back Academy Rock and nose into the harbor. Which I did. Our problems weren't over but at least they were more under control. My long suffering wife bought me a hand held chart plotter for my birthday.