mick
Well-Known Member
I have Navionics charts loaded on an iPad. If I look at Loch nan Ceall (Arisaig) the chart shows the 2 metre contour well away from where I know it actually is. There are other discrepancies I have noticed. Why is this?
Navionics accuracy
- Thread starter mick
- Start date
pvb
Well-Known Member
Navionics charts are based on paper charts, which can be inaccurate.
AntarcticPilot
Well-Known Member
Basically, there's only one source of chart data in the UK, which is the UKHO, the publisher of Admiralty charts. There are exceptions for major harbours and organizations like Antares on the West Coast of Scotland, but Navionics, Imray and all the rest simply get the data from the UKHO and repackage it to suit their systems and style. The process isn't entirely error-free, but things like a mispositioned contour shouldn't happen (but that doesn't mean it won't!).
In out of the way locations, the latest survey may not be very recent - and in parts of the West Coast of Scotland, the base survey could well be Victorian! There isn't usually anything wrong with the survey; the Victorians used similar techniques (bar GPS!) to those used by the well thought of Antares people, and it will have been adjusted to the current datum (a source of problems in other parts of the world). However, the density of soundings will be nothing like as high as a modern survey, and they didn't take as much care with the depth measurement in water too shallow for a ship or deeper than required for safe navigation. The basic lead line measurement, if done with care, is of comparable accuracy to the acoustic techniques used these days.
Contours are NOT basic data, but are derived from the soundings. A modern chart will only show a sample of the soundings that are used to create the contours, and the contours will be pretty accurate. However, a chart based on sparser soundings, such as a lead-line survey, will a) show all the soundings and b) the contours should be regarded as indicative only. They are usually drawn on the safe side; i.e. they indicate shallower water than may be present.
In out of the way locations, the latest survey may not be very recent - and in parts of the West Coast of Scotland, the base survey could well be Victorian! There isn't usually anything wrong with the survey; the Victorians used similar techniques (bar GPS!) to those used by the well thought of Antares people, and it will have been adjusted to the current datum (a source of problems in other parts of the world). However, the density of soundings will be nothing like as high as a modern survey, and they didn't take as much care with the depth measurement in water too shallow for a ship or deeper than required for safe navigation. The basic lead line measurement, if done with care, is of comparable accuracy to the acoustic techniques used these days.
Contours are NOT basic data, but are derived from the soundings. A modern chart will only show a sample of the soundings that are used to create the contours, and the contours will be pretty accurate. However, a chart based on sparser soundings, such as a lead-line survey, will a) show all the soundings and b) the contours should be regarded as indicative only. They are usually drawn on the safe side; i.e. they indicate shallower water than may be present.
Roberto
Well-Known Member
Besides inaccuracies in original data or errors in copying it, especially with contours Navionics charts often offer totally wrong data, in particular in charted areas with less data density.
They take original data, interpolate a mathematical surface with those data, then they use that surface to determine additional contours (like slicing the surface with horizontal planes).
As an example if your chart only shows say the shore line and the 10m contour, they will add all the contiurs in between, 2m 5m etc which of course is a nonsense. I do not know the place you are talking about, try and check with the available data on the higher scale UKHO chart and see how this is reflected in the Navionics chart.
Other possible source of error, Sonar charts, the work of the devil: say one fisherman passes over that place recording a 2m depth and then sending data to Navionics, they will update the chart with that depth. If the fisherman was there at highish water, think about what happens if you sail there with a lower tide height. (Posted plenty of examples in the past). They eventually agreed "sonar chatrts not to be used for navigation".
They take original data, interpolate a mathematical surface with those data, then they use that surface to determine additional contours (like slicing the surface with horizontal planes).
As an example if your chart only shows say the shore line and the 10m contour, they will add all the contiurs in between, 2m 5m etc which of course is a nonsense. I do not know the place you are talking about, try and check with the available data on the higher scale UKHO chart and see how this is reflected in the Navionics chart.
Other possible source of error, Sonar charts, the work of the devil: say one fisherman passes over that place recording a 2m depth and then sending data to Navionics, they will update the chart with that depth. If the fisherman was there at highish water, think about what happens if you sail there with a lower tide height. (Posted plenty of examples in the past). They eventually agreed "sonar chatrts not to be used for navigation".
benchmark
Well-Known Member
2 meters is nothing you should lose sleep over, the GPS receiver for your chart plotter is accepted accuracy of between 4 to 10 meters margin or error , it should say so in your user manual of you check.
That said, in my case the Navionics charts are the most accurate for the Baltic’s compared to the 2 others I have used , which are C-maps and Garmin charts, the depth contours in any chart er not absolute and only serve as a guide.Nautical charts in some areas of the world are based on old and sometimes outdated sources, form the time the waters were charted by a man leaning over the boats edge with a hand plumb and manual depth recordings.
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GHA
Well-Known Member
As snip and paste into imgur is so easy....
Admiralty from visit my harbour >
Navionics from the webapp >
And cm93 on opencpn >
All look fairly similar...
Admiralty from visit my harbour >
Navionics from the webapp >
And cm93 on opencpn >
All look fairly similar...
prv
Well-Known Member
The OP didn’t say how far wrong horizontally the erroneous contour was - could be half a mile and still consistent with his post.
Pete
West Coast
Well-Known Member
You want to get a set of Antares Charts - amazing accurate detail of places the admiralty will never have time or funds to update their surveys but which those of us with small boats use.
Contest1
Well-Known Member
Charts either paper or electronic are never absolutely accurate. Electronic charts generally carry a warning to this effect.
Anyone who has followed forums such as this one would would see simelar posts on a regular basis.
The moral is never rely on a single source and if it looks wrong it probably is.
Anyone who has followed forums such as this one would would see simelar posts on a regular basis.
The moral is never rely on a single source and if it looks wrong it probably is.
ghostlymoron
Well-Known Member
It's difficult to get information from another source as AFAIK only the national hydrographic office supplies all base data. The various chart suppliers including e charts merely add their own colours and other details. I'm not sure about Garmin though.
I once spotted an error on my Navionics chart of mochras lagoon labelling only - I doubt if the lagoon has been surveyed in recent history - indeed many charts of less strategic importance haven't been surveyed since Napoleonic times (with lead line, quadrant and three cornered hats).But I digress, I reported the error to Navionics who said they would correct their chart and sent me a free update voucher.
I once spotted an error on my Navionics chart of mochras lagoon labelling only - I doubt if the lagoon has been surveyed in recent history - indeed many charts of less strategic importance haven't been surveyed since Napoleonic times (with lead line, quadrant and three cornered hats).But I digress, I reported the error to Navionics who said they would correct their chart and sent me a free update voucher.
ghostlymoron
Well-Known Member
Paper charts have a note of the date of the survey the chart is based on.
AntarcticPilot
Well-Known Member
Can I just clear up some things?
The United Kingdom Hydrographic Office (UKHO) is an executive agency, partially funded by the Ministry of Defence. It is very similar in its set up to the Ordnance Survey. It is a trading fund, which means that it is expected to break even, though part of its costs are met by the MoD. It is responsible for all the UK's international and national marine charting obligations.
Admiralty is (these days) simply a brand name for charts produced by the UKHO. There is no direct connection with the Royal Navy.
The Royal Navy conducts surveys on behalf of the UKHO, but so do other organizations, especially in UK waters. It operates a few specialist survey vessels; notably HMS Endurance and her successor, HMS Protector.
The Hydrographer of the Navy (referred to by some ex-Navy people as "Droggie") is a non-executive director of the Trading Fund. Perhaps worth noting that his position is (these days) advisory only; the Royal Navy has no direct control over the UKHO.
The UKHO is a member of the International Hydrographic Organization (IHO), which coordinates marine charting on a global basis, promulgates standards and oversees marine placenames.
The United Kingdom Hydrographic Office (UKHO) is an executive agency, partially funded by the Ministry of Defence. It is very similar in its set up to the Ordnance Survey. It is a trading fund, which means that it is expected to break even, though part of its costs are met by the MoD. It is responsible for all the UK's international and national marine charting obligations.
Admiralty is (these days) simply a brand name for charts produced by the UKHO. There is no direct connection with the Royal Navy.
The Royal Navy conducts surveys on behalf of the UKHO, but so do other organizations, especially in UK waters. It operates a few specialist survey vessels; notably HMS Endurance and her successor, HMS Protector.
The Hydrographer of the Navy (referred to by some ex-Navy people as "Droggie") is a non-executive director of the Trading Fund. Perhaps worth noting that his position is (these days) advisory only; the Royal Navy has no direct control over the UKHO.
The UKHO is a member of the International Hydrographic Organization (IHO), which coordinates marine charting on a global basis, promulgates standards and oversees marine placenames.
mick
Well-Known Member
Thanks for your comments. All noted.
