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Old 19 August 2016, 15:34   #11
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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...
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Old 19 August 2016, 15:50   #12
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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!
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Old 19 August 2016, 16:04   #13
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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....
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Old 19 August 2016, 16:13   #14
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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
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Old 19 August 2016, 16:15   #15
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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...
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Old 19 August 2016, 16:18   #16
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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
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Old 19 August 2016, 16:20   #17
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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
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Old 19 August 2016, 16:27   #18
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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.
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Old 19 August 2016, 16:39   #19
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>>>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!
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Old 20 August 2016, 04:53   #20
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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.
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