The Old Ways of Navigation

[prv]

Forget all this guff about modern plotters being unsafe, but if you enjoy plotting your position on a chart then that is a very good reason to do so, is it not?

I certainly find the hourly fix punctates a passage nicely.

And the record of the voyage with it's highs and lows so you can enjoy recounting it only really works on paper.

I dunno; my nearest equivalent is the iPhoto album of pictures from a particular trip.

Pete

 

[ffiill]

Absolutely, when using GPS to navigate one still needs paper charts to plot the latest position and when in sight of land it is wise to take a look around to confirm the electronics are giving a believable fix. I well remember sailing in Greece when a Nato exercise was taking place. There was no navigational warning given, but each time a plane hove into view the handheld GPS advised us to change our course by 180 degrees! I wonder whether similar signal corruption is to be found in the firing ranges that make up half the south coast cruising area?

Rob.

They do it on the Clyde when trident is coming in as its at its most vunerable when surfaced.

 

[AntarcticPilot]

They do it on the Clyde when trident is coming in as its at its most vunerable when surfaced.

I've been on the Clyde with submarines in sight, with no change in GPS availability. AFAIK, they always issue a NTM when they are conducting GPS jamming.

 

[Hydrozoan]

... I have never seen the GPS go ape whilst I've been sailing. What happens to the 'track' on the chartplotter when the GPS gets degraded for whatever reason. Does it suddenly jump to another location or does it gradually wander off course? ...

Once, having gone below for some time, I happened on my way back up to glance at the laptop PC plotter and found it showed us sailing north rather than west. I called to Mrs H at the helm, then dashed up to find a surpised and mildly indignant spouse – and an unchanged course.

The plot showed no obvious sign that it had gone awry – not the sudden track jump to screen edge that occurs if signal is briefly lost, but a definite turn some time previously and a number of points recorded along the new track. We were close in to a well marked coast, and on such a point of sail that I do not believe that we had at any time altered course by 90 degrees.

As were were about to enter a busy port, I did not check further. The problem never recurred, and I assume it was a fault in the plotter system (which normally agrees closely with the other GPS, or is occasionally quite obviously awry). I imagine that PC plotters are somewhat more vulnerable to such glitches than dedicated ones. Or perhaps it was interference.

The salutary lesson was that it all appeared quite plausible on the screen.

 

[knuterikt]

Jamming by the military or someone in for a "practical joke".
The "Norwegian Defence Research Establishment" discovered that that cars equipped with GPS jamming equipment where passing through one of the most trafficked areas in Norway .
Link to article in Norwegian http://www.tu.no/it/2014/03/09/her-passerer-flere-gps-jammere-hver-dag
Probably a not so likely scenario but..

Solar activity is a Phenomena, that is the most likely to cause inaccuracies in GPS position data, most pronounced at higher latitudes.
Explanation found here http://www.swpc.noaa.gov/impacts/space-weather-and-gps-systems

 

[yachtorion]

When I started the thread I wasn't really trying to say that GPS is unreliable. Just that it's more seaman like to have two different methodologies for confirming position and that i'd been surprised how accurate my really simple fix was.

I still think that's valid, and if anyone is really saying they would 100% rely on one source of data and not consider checking it, that seems odd.

 

[BrianH]

When I started the thread I wasn't really trying to say that GPS is unreliable. Just that it's more seaman like to have two different methodologies for confirming position and that i'd been surprised how accurate my really simple fix was.

I still think that's valid, and if anyone is really saying they would 100% rely on one source of data and not consider checking it, that seems odd.

I think most experienced mariners have a subconscious checking procedure running, absorbing a collection of data, from instruments and visual observation. I am aware of my start point, wind, direction, leeway and progress with the resultant mental picture of where I should be ... coastal details or navigational lights confirming my position on plotter and paper chart when closing my destination.

Many years of GPS usage have given me confidence in the coordinates supplied - what I do not necessarily accept as definitive is the ship's representation of them displayed with reference to the chart on the screen, either on the plotter or the netbook below running OCPN. I have seen electronic chart errors enough to treat them with a certain caution. Either errors when transcribed from the original or application of the chart datum. My ageing Garmin BlueChart folio seems particularly error-prone on that score - indeed I have a local Italian series where my position jumps 500m when zooming in and invoking the more detailed chart.

I cannot claim a formal process of double-checking but it does happen - somehow ... or I think the Darwinian Process would have curtailed my cruising long ago.

 

[Daydream believer]

I'm working away from home at the moment so stuck in a hotel in Swansea. I just got a position fix out of the hotel window using a hand bearing compass to a nav buoy 2 miles out and adding in roughly how far I am from the harbour wall (not very) - overlaid onto google maps the resulting fix was more accurate than the GPS position on my tablet.

Admittedly the GPS would have been better if I wasn't in a hotel building with a lot of floors above me - but I still found it educational

What surprised me was Navionics seemed to have no way of providing compass variation data. Found it in another app (a dedicated compass) - but it seems an odd thing to be missing. Also if I'd wanted to draw more than one bearing line for a cocked hat I'd have been out of luck.

Paper charts still have some uses then.

This is why the yeoman plotter was such a good piece of kit

 

[prv]

This is why the yeoman plotter was such a good piece of kit

Still in regular use on Ariam, installed under the lid of the chart table :encouragement:

Pete

 

[jimbaerselman]

This is why the yeoman plotter was such a good piece of kit

+1

Also +1 for backup . . . and perhaps one set of circumstances where GPS does not do the trick.

Strong cross tide when approaching a narrow channel - marked with shore leading marks.

I can think of two or three passages in N Brittany and CI where you approach in 3kt cross tides, pointing 30 degrees off track. Then you hit the back eddy and you're pointing 15 degrees the other way to stay on track. Easy to notice this when you're visual with the marks; very different if you're relying on GPS and plotter.

 

[Daydream believer]

..

I can think of two or three passages in N Brittany and CI where you approach in 3kt cross tides, pointing 30 degrees off track. Then you hit the back eddy and you're pointing 15 degrees the other way to stay on track. Easy to notice this when you're visual with the marks; very different if you're relying on GPS and plotter.

Set the XTE alarm???