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Old 20 August 2016, 05:09   #21
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Originally Posted by Iron Dials View Post
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.
security threats change things as well depending on severity
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Old 01 September 2016, 16:14   #22
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security threats change things as well depending on severity
Aye, they know when Jambo is in the area!

Joking aside tho' I regularly get that very effect on my phone - which I tend to keep driving whilst thinking "noooo, we're over here!" .......

In what perceptively are certain weather conditions it always dumps me about 1/2 mile to the left. I know it's GPS 'cos if I switch from Google (mostly for the traffic ) to , say, Viewranger or Memory map it also dumps me 1/2 a mile to the left.

One of these days I'll remember to chuck in the Garmin V and when Google puts me in a field to the left I'll see what the Garmin thinks.



On the subject of nav bby Garmin, I had a long debate with myself as to whether I needed to cobble together a SW data receiver for my backup bick - A Garmin 12. (main reason for this is it has a >24hr battery life and a clutter free screen, but I digress...) It doesnlt have the extra "correction" channels, so is , on paper about 10m accuracy. I decided not to worry because if I needed that level of accuracy 1) I won't be doing 30 knots through the channel and 2) I'd probably be using the Poly MK1 eyeball & /or my wooden end of paddle depth sounder!
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Old 01 September 2016, 16:47   #23
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Old 01 September 2016, 19:51   #24
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It'll be the mapping that is out. The Garmin mapping for a large area is made up of various charts and normally you can display the chart boundaries on your plotter screen. Garmin literature tells you about possible inaccuracies of the charts relative to each other - and occasionally passing from one chart to another you'll find a stagger in the mapping - and also the possibility of certain patches of a given chart having inaccuracies. My Garmin plotter has a function for off-setting the latitude and longitude of the map relative to the gps fix so if you know where you are, you can move the map to suit. However, their literature warns against relying on this once you move position and I've certainly seen an inaccuracy of the chart but not many meters away it regains its accuracy. I put it down mainly to the digitising accuracy (or inaccuracy!) rather than the original chart accuracy.
As an example, in the detail chart of the Sound of Harris there is a leading line marking a narrow deep(ish) channel and also leading markers on the shore which the chart shows but the leading line and markers on the chart are about 20 meters out of position - wider than the channel! However, less than a 100 meters away there are posts in the water which mark another, and important, deep channel and they are spot on. The harbour is within 400mtrs of these features and the map is off position to the north west by approximately 10mtrs.

I also know that the harbour light on the chart for my local harbour will have you on the reef if you followed it in the dark!

I don't know whether it still exists but there used to be a section on Garmin's web site for reporting mapping errors.

There's a reason for you getting a warning at start-up that the charts are not to be solely relied upon for navigation.

Use the chart plotter but always keep an open mind to what it shows you and what you can see around you. And, always keep an open mind as to what you think you can see around you!

All IMHO, of course.
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Old 02 September 2016, 09:43   #25
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It'll be the mapping that is out. The Garmin mapping for a large area is made up of various charts and normally you can display the chart boundaries on your plotter screen. Garmin literature tells you about possible inaccuracies of the charts relative to each other - and occasionally passing from one chart to another you'll find a stagger in the mapping - and also the possibility of certain patches of a given chart having inaccuracies.
Not specific to Garmin. Using my LMS-335C with their supplied basemap, after launching the boat in Monterey, it would show me several hundred yards up on the beach, approximately in the middle of Lighthouse Ave.

Tracking was spot on; just the map itself was crap. Navionics charts seem to have remedied that. I haven't looked at the HDS-7 native basemap, so I can't say whether that's any better.

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Old 03 September 2016, 16:47   #26
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Yeah, if you go land for a mo and do a "where am I " on various land maps it will put you a good few yard different to the last one.

What you got to remember is the world is round. Maps are flat. I am a couple of glasses of red down, so Google this yourself before believing my numbers but the OS 1:50K maps of the UK are a case in point - I forget the "zero location" of the grid, but when you get to the likes of sheet 3 or sheet 201 you are so far off centre that if you walked along the bottom & top of the sheets there can be up to 1/4 mile difference in the real distance compared to the "square on the paper".

Anyone with OS Landranger on their phone will have experienced this!
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