The accuracy of GPS/Navionics just blows my mind sometimes...

There are places, eg in the Pacific, where it is out by as much as two nuatical miles. Please don't get over confident.
 
There are a number of OS markers around the country with published VERY accurate lat and long. When you place a handheld Garmin device on them and get a reading of +/- 2meters you do wonder.

The daft thing is I actually understand the maths behind it all!
 
tillergirl said:
It's a shame it isn't good enough. In my area I can see two buoys completely missing and another buoy over 1nm in the wrong place!
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If you report them they do actually amend the chart.
I suggested a minor amendment near my home port, two cardinals guarding a wreck, only one of which - the least important and smaller one- was shown in higher zooms. After a couple of weeks the chart was duly amended. It now shows the logical buoy in higher zooms.
I feel a small satisfaction every time I sail past.
 
Andrew_Trayfoot said:
The accuracy of GPS/Navionics just blows my mind sometimes...

https://boating.page.link/9hJtCgsBDuThNToM8

The track starts about 10 minutes after I left the mooring, but the interesting bit is when i got back to the mooring, into the tender, then back to base, and around the car park.
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It isn't the accuracy of GPS or any electronic charting system that 'blows my mind'.

What impresses me is how accurate charts and maps in general were before the coming of the aeroplane and the artificial satellite...

'Frinstance the fact that when first seen from space Australia was in fact the same shape and in the same place as when cartographers had mapped it in the 19th century.... that is what impresses me... the chart of Bass Strait today is essentially the same chart as was produced after the Beagle's survey of 1840 ish.....

I recommend 'The Great Arc' by John Keay to anyone interested in how they did it ashore and 'Mariners are Warned http://www.chartandmapshop.com.au/16690/Mariners-are-Warned/9780522850444 for a bit about HMS Beagle's time in Australian waters.

And people complain when they hit some remote atoll which is , according to GPS, a few miles adrift...... sigh....
 
Andrew_Trayfoot said:
The accuracy of GPS/Navionics just blows my mind sometimes...

https://boating.page.link/9hJtCgsBDuThNToM8

The track starts about 10 minutes after I left the mooring, but the interesting bit is when i got back to the mooring, into the tender, then back to base, and around the car park.
Click to expand...

I'm in the south pacific , bits are amazing (french polynesia), but tonga isn't good, according to navionics I'm anchored in the middle of an island which is actually 1.5nm off my port side. So it's only as good as the map it's based on!
 
Billyo said:
I'm in the south pacific , bits are amazing (french polynesia), but tonga isn't good, according to navionics I'm anchored in the middle of an island which is actually 1.5nm off my port side. So it's only as good as the map it's based on!
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There are 2 issues, one is the map and the accuracy and the detail which it displays, the other is the position calculated by the device.

If you set "improved" accuracy on a handheld device, it will use WiFi information, Cellular Towers, Bluetooth Signals and GPS to try and work out the position - handheld devices are used a lot indoors where there is no GPS reception which is why they do it.

If using for navigation outside, it is essential that all this is switched off and the device is forced into GPS only mode - otherwise your position may not be accurate. The apps on the device will also continuously ask you to change the settings back so be vigilant. When an innacurate position is transposed onto even the most accurate map, the result is that you will appear to be somewhere you are not.

In areas of poor GPS reception or when there are not enough visible satelites, or the device doesn't have a GPS fix, it will guess based on other information. Dangerous when there is a scant spattering of WiFi/Cellular info in the vacinity.

This is not to say that your position on an island is not a mapping error, they happen too, but switching the device to GPS only will eliminate the innacuracy of WiFi etc. derived positions.
 
stingo said:
There are places, eg in the Pacific, where it is out by as much as two nuatical miles. Please don't get over confident.
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According to GPS and UKHO charts (via MX Mariner) I sailed down the middle of one of the islets outside the Treshnish Islands anchorage last week. On the way out I sailed clean over another one.
 
JumbleDuck said:
According to GPS and UKHO charts (via MX Mariner) I sailed down the middle of one of the islets outside the Treshnish Islands anchorage last week. On the way out I sailed clean over another one.
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Last week my Garmin car satnav told me I was drving through some woodland and fields about a hundred metres from the M3.
The following and previous days, the M3 was in the right place on the map as I drove along it.
 
Another point is that although the accuracy of GPS is usually amazing, it CAN fail without warning - the signals are incredibly weak, and any ionospheric disturbance will either distort the position or (more likely) stop it working at all. I've seen research which used the errors in position to determine the properties of the ionosphere! Further, the extreme accuracy we are used to in the UK is not generally available in less populated parts of the world.

And I echo all those people who warn of charting errors - I spent my career working on Antarctica where we were delighted to get the average error DOWN to 100m - and there's very few areas with any bathymetric survey at all! In places where we'd been able to use terrestrial position fixing and aerial photography, we got down to centimetre accuracy but in many paces we relied on satellite images with no fixing better than the orbital parameters (which are good to 100m or so).
 
westhinder said:
If you report them they do actually amend the chart.
I suggested a minor amendment near my home port, two cardinals guarding a wreck, only one of which - the least important and smaller one- was shown in higher zooms. After a couple of weeks the chart was duly amended. It now shows the logical buoy in higher zooms.
I feel a small satisfaction every time I sail past.
Click to expand...

There are far too many to correct - buoys missing, soundings, inaccurate lights, improper user amendments. All of the first three could be right when based on the UKHO charts.
 
The Russians have spoofed GPS signals so that they're 100 miles or so out. Interestingly, it was Russians geocaching near the Kremlin, and then Merchant Vessels in (IIRC) the Baltic, which first reported it.

I suspect that the most obvious forms of *serious* error with GPS is using the wrong chart datum settings.
 
AntarcticPilot said:
Another point is that although the accuracy of GPS is usually amazing, it CAN fail without warning - the signals are incredibly weak, and any ionospheric disturbance will either distort the position or (more likely) stop it working at all...
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From memory 30dB below noise. Clearly this is impossible.

I'm old enough to remember when you sailed to Cherbourg you aimed off to one side so you knew which way to turn when you hit land.

I use a GPS watch for open water swimming. It manages to get a fix between my wrist breaking the water at my waist and entering the water in front of me. The old Garmin on the boat can take 10 minutes.
 
lw395 said:
Last week my Garmin car satnav told me I was drving through some woodland and fields about a hundred metres from the M3.
The following and previous days, the M3 was in the right place on the map as I drove along it.
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Some land based GPS devices have option "snap to road" that eliminates inaccuracy in map drawing and positioning. If the device finds you within a reasonable distance from a road, it assumes you are ON IT and puts the cursor there. Didn't you change the settings the second day by any chance?
 
jiris said:
Some land based GPS devices have option "snap to road" that eliminates inaccuracy in map drawing and positioning. If the device finds you within a reasonable distance from a road, it assumes you are ON IT and puts the cursor there. Didn't you change the settings the second day by any chance?
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No, I didn't even actively switch it on. I knew where I was and where I was going.
I think most car satnavs are permanently 'snapping to the road' and assuming that if you were on the M3 a moment ago, you probably still are.
 
Some of the less visited Greek islands haven't been surveyed since the 1850s and, although the cartography is correct, they can be a good way out in relating to the datum now used. WGS76 ?
 
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