Garmin GPS position error

Fenlander · 11 August 2016, 21:18

This is a weird one...

For years (sibbing) I've used a pocket Garmin Etrex with Bluechart (2008 version not updated) mapping and been most impressed with its accuracy when fixed points were encountered that enabled a check. Last year on holiday in the Scottish Loch Sween area I noticed it was plotting about 50m north of actual which caused some concern near hazards.

Used it earlier this year down south and accuracy seemed fine so assumed last year was just a blip.

However this year in the Loch Sween area it's just the same... shifted about 50m north of actual.

The Etrex is now just the backup to the new Garmin 45DV I bought for this season... and the 45DV is spot on in the locations that throw the Etrex.

I wonder what's going on?

willk · 11 August 2016, 22:07

Were you under/near cliffs at the time?

Fenlander · 11 August 2016, 22:46

Not really the error is present throughout this whole area... hilly but not under cliffs...

Poly · 11 August 2016, 22:50

Can you check what error the GPS thinks it has? You can usually get it to tell you estimated error - If it is consistently 50m N though I think it is systematic rather than random error.
What makes you sure that the GPS is wrong rather than the map? Might be interesting to compare to either a chart and/or OS map?

Personally I've never tried to navigate that precisely in Loch sweet by gps and use mk1 eyeball.

Erin · 12 August 2016, 06:23

I had something very similar a few years ago and managed to demonstrate to Garmin that their chart of Braye harbour was out by a certain amount. Obviously their response was the stock reply that Gps is subject to errors and not for navigation. However they did correct it on subsequent updates.

Fenlander · 12 August 2016, 08:03

Thanks guys. I must admit I'd locked onto the thought it was a GPS positioning error rather than a chart error but that's a fair possibility... particularly given Erin's experience.

Next time we're out (waiting for the rain to finish!) I'll compare what the two devices are showing more carefully.

Agree re actually using your eyes Poly... after all chartplotting might be thought of as a bit over the top for a smallcraft described by a kid on the pontoon the other days as a "dinghy". However there are circumstances when I'd like to think it was accurate and it's a bit unnerving in a very narrow section of loch to find it shows you are proceeding at 14kts on land!

jeffstevens763@g · 12 August 2016, 09:58

i use a Garmin Oregan 650 for work to find structures by grid reference sometimes its 200 ft out i just put it down to what satellites it can pick up to get a fix. funny last week it has a compass on the main page which disappeared now its back technology glitches i suppose

jyasaki · 12 August 2016, 15:00

Fenlander; did you bounce the numbers on your two units against each other, or work straight off the map? Curious if the units thought they were in different places, or if they thought the same location was a different spot on the map.

Doesn't Garmin's display show a circle denoting the probability of error (an old GPSMap 76 I had did; don't know about their other models)?

As an aside, I had an early Lowrance NMEA2K antenna that would go nuts around radar sources. Any largish vessel running radar or shore based ship tracking would cause very strange errors (and in at least a few reported cases, killed the antenna, though that never happened to me.)

jky

Fenlander · 12 August 2016, 15:49

jyaski no I didn't check the numbers... was just looking at the maps... still raining but tomorrow when the heatwave starts (?!) and we get out again in the evening I'll do that.

chris123 · 19 August 2016, 20:29

It would be interesting to confirm if the reported lat and long is the same on the 2 GPS. I suspect it is, and the issue is a mapping error.

I suspect Garmin are the same as most leisure chart suppliers, and source their UK data from UKHO. Last official survey of Loch Sween was 1936 - 1937 so not going to be super accurate but better than you are seeing.

Bit of a fundamental error but I wonder if it is a chart datum error. The latest UKHO chart for Loch Sween is WGS84, at some point it would have been to a different datum.

If the old Garmin chart was actually referenced to a non WGS84 datum any GPS position (which by default is referenced to WGS84) would appear in the wrong position. Depends on the datum (OSGB 1936?) but I know that WGS84 positions on charts to Irish National grid will be 50 to 75 metres out (a lot of UKHO charts of West coast of Ireland are still Irish National grid not WGS84).

Don't rely on it for navigation but if the unit will let you what happens if you change the GPS datum from WGS84 to something else (eg OSGB 1936)?

willk · 19 August 2016, 20:34

Quote:

Originally Posted by chris123

Depends on the datum (OSGB 1936?) but I know that WGS84 positions on charts to Irish National grid will be 50 to 75 metres out (a lot of UKHO charts of West coast of Ireland are still Irish National grid not WGS84).

I've certainly been surprised to glance at my plotter only to find the vessel icon gliding along the shore above MHWS when I had 10m under my keel! Mulroy Bay springs to mind...

chris123 · 19 August 2016, 20:50

Quote:

Originally Posted by willk

I've certainly been surprised to glance at my plotter only to find the vessel icon gliding along the shore above MHWS when I had 10m under my keel! Mulroy Bay springs to mind...

Official charts are gradually transitioning to WGS84 (several charts around Lough Swilly changed in an update last year IIRC) - some of the survey data still dates back to the 1850s though!

willk · 19 August 2016, 21:04

Quote:

Originally Posted by chris123

some of the survey data still dates back to the 1850s though!

Absolutely - and we're acutely aware of it. Any cove, bay or inshore stretch we decide to get to close quarters with prompts us to think: Would wet, tired, half starved HM surveyors, knowing a warship would NEVER enter this area, put in that extra hour on the measuring chains?

So we stand off that wee bit....

chris123 · 19 August 2016, 21:13

Quote:

Originally Posted by willk

Absolutely - and we're acutely aware of it. Any cove, bay or inshore stretch we decide to get to close quarters with prompts us to think: Would wet, tired, half starved HM surveyors, knowing a warship would NEVER enter this area, put in that extra hour on the measuring chains?

So we stand off that wee bit....

I'll have to ask him where it was, but an ex grey funnel line work colleague showed me a bay surveyed in the 1850s that was incredibly well detailed..

Turns out the lieutenant responsible for the survey had a mistress near the bay

Fenlander · 19 August 2016, 21:15

Sadly I didn't manage to get the position comparisons. Just after I'd posted the Garmin Etrex (one with the error) started going blank screen the moment the brightness was brought to readable. A new set of batteries from my stock seemed to bring it back then the next time I looked at it dead again... happened through three sets of "new" batteries.

Thought oh well it's end of life... but then tried some new batteries from the village shop and it was perfect for the last outing... seems previously I had a whole batch of dodgy poundshop batteries.

But it was our last day out of the two weeks and family had better things to do than indulge my GPS obsession!

So until next year...

willk · 19 August 2016, 21:18

Quote:

Originally Posted by chris123

I'll have to ask him where it was, but an ex grey funnel line work colleague showed me a bay surveyed in the 1850s that was incredibly well detailed..

Turns out the lieutenant responsible for the survey had a mistress near the bay

The Swilly would have been extremely well surveyed as it was a major deep water "Grey Funnel" port (up there with Scapa Floe) but the nearby areas not so much. I have a growing number of uncharted reefs and isolated hazards marked up on my plotter for personal reference

Amazing what a few months at sea will do to a man's standards

willk · 19 August 2016, 21:20

Quote:

Originally Posted by Fenlander

but then tried some new batteries from the village shop and it was perfect for the last outing... seems previously I had a whole batch of dodgy poundshop batteries.

Tuts!

False economy - buy a small quantity of fresh Lithium AAs

chris123 · 19 August 2016, 21:27

Quote:

Originally Posted by willk

The Swilly would have been extremely well surveyed as it was a major deep water "Grey Funnel" port (up there with Scapa Floe) but the nearby areas not so much. I have a growing number of uncharted reefs and isolated hazards marked up on my plotter for personal reference

Amazing what a few months at sea will do to a man's standards

Apparently it was a bay on Lough Ree, and one Lt Beatty RN.

Fenlander · 19 August 2016, 21:39

>>>Tuts!

I know. When it was my main device I would buy Duracell in 10 packs... now it's been demoted to second fiddle I tried poundshop Sonys. Daft really as a backup needs to work!

Iron Dials · 20 August 2016, 09:53

I don't know how many of you are aware of this.

GPS jamming exercises | Ofcom

Basically, your gps quality can be adversely interfered with by (generally military) interference.
Normally it's localised but worth having a look at every now and then if you are planning on relying on your gps.