capnsensible
Well-known member
Blue job. Mrs S steers, I do the sounding. Normally because our good old fashioned rotating neon sounder had gone breasts up.
Always trust your charts???
- Thread starter john_morris_uk
- Start date
dunedin
Well-known member
Why do you think Navionics is “awful and error prone”?
Unless you choose to switch on the crowd sourced sonar data (which I don’t), all the leisure chart suppliers use largely the same data, as supplied by the various national Hydrographic Offices (such as UKHO for UK and Ireland). This is supplied already in vector format and generally untouched by Navionics or others - though they do add additional sources (such as harbour and marina layouts not on IHO data) and recent buoyage changes not yet on the IHO feeds.
So not sure why you think Navionics is error prone.
Clearly in places which are unsurveyed, or the surveys are not to modern GPS accuracy, ALL charts will be incomplete or wrong.
capnsensible
Well-known member
Because the places I've used it that weren't UK it's been awful and error prone.
john_morris_uk
Well-known member
Good plan but Mrs M doesn’t like steering in confined waters with lots of expensive boats around.
dunedin
Well-known member
But again that is probably due to the local Hydrographic Offices and their surveys - which would be exactly the same on C-Map or any of the other chart options, including the paper charts.
As noted way back, in such areas most people are using satellite images alongside chart data to help mitigate this.
noelex
Well-known member
Interestingly, if you activate the sonar chart option on Navionics the results look reasonably accurate although the rock is not present.
Roberto
Well-known member
No, they add any sort of data that has no relationship with actual surveys.
I won't copy all the images again, take a tour with their "webapp" to North Korea, flooded areas of Bangladesh, Cape Horn island, Canadian sea area usually covered by ice etc etc all of these show contours with 50cm spacing... who in his right mind might think that corresponds to reality? Re Sonar charts, they also show them for North Korea, notoriously full of local anglers sending their user data to Navionics/Garmin US, or Cape Horn, where apparently a lot of people turn around the island sending depths to them.
RunAgroundHard
Well-known member
Interesting.
Many organisations are contributing to data used by Navionics to build their “sonar charts”.
Acknowledgements
Navionics using an algorithm to interpolate between soundings is no different to a depth contour drawn on a paper chart by a hydrographic office.
The risk of interpolation is part of the RYA syllabus (or was on my Day Skipper shore based). The diagram of the unsounded rock between soundings is common in text books, usually associated with a lesson on the source chart. There is no reason to assume Navionics is any more accurate because they show more contours. Navionics though leverage the density of soundings on their charts for marketing purposes suggesting it gives an advantage. It is also clear that modern leisure based sonar systems are providing a lot of data.
Its not just a case of leisure users sonar data being shared. There are EU, USA and other country agencies established to share sonar from different sources and make it available.
Never mind the quality, feel the width.
Many organisations are contributing to data used by Navionics to build their “sonar charts”.
Acknowledgements
Navionics using an algorithm to interpolate between soundings is no different to a depth contour drawn on a paper chart by a hydrographic office.
The risk of interpolation is part of the RYA syllabus (or was on my Day Skipper shore based). The diagram of the unsounded rock between soundings is common in text books, usually associated with a lesson on the source chart. There is no reason to assume Navionics is any more accurate because they show more contours. Navionics though leverage the density of soundings on their charts for marketing purposes suggesting it gives an advantage. It is also clear that modern leisure based sonar systems are providing a lot of data.
Its not just a case of leisure users sonar data being shared. There are EU, USA and other country agencies established to share sonar from different sources and make it available.
Never mind the quality, feel the width.
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Birdseye
Well-known member
Used to go the UKHO quarterly as a rep on a liason group. Fascinating to talk to them about chart accuracy. OK this was some time ago when they were first going through the process of redrawing charts to WGS 84 but in doing so they had found areas of the UK that were as much as 5 miles adrift on the then current charts - errors that only became obvious with satellite data. They also revealed that they were now using foreign state data as the base for foreign countries rather than the previous RN data where available. So some of the source data for small foreign locations might not be quite the quality you expect.
As I said this was some years ago but I wouldnt see chart data as absolutely accurate even now. How much of the UK coastal waters has for example been surveyed fully with modern survey kit? Most tidal flow data is interpolation between a small number of points
As I said this was some years ago but I wouldnt see chart data as absolutely accurate even now. How much of the UK coastal waters has for example been surveyed fully with modern survey kit? Most tidal flow data is interpolation between a small number of points
Roberto
Well-known member
"Algorithm" is often a word for hiding dubious or at least unclear practices. A hydrographic office will have a grid of actual measured data to say a scale of 1:X000, they will output a chart to a scale of 1
X+something)000, the "+something" being a sort of quality insurance, the chart will be to A1, A2, B etc Catzoc standard depending on the density of the data. The contours they output is based on measured, actual data. Navionics takes the final data, whatever it is, and interpolates into purely theoretical .5m contours, irrespective of the type of original data (existing, not existing, catzoc category, etc, who cares just add contours they are nice)
How can an original CATZOC A2 or B chart, with a few "2", "5" and "10" soundings a few hundred/thousands meters away, produce an electronic chart with .5m spaced contours between the two measurements?
Very appealing though, oh look at the contours it's a lot more detailed
westernman
Well-known member
Look this up on Google Earth and see what happens to route 10 as it goes from Hong Kong to Shenzhen.
Google Earth
Google Earth
westernman
Well-known member
When sailing on a tall ship in Polynesia and the Cook Islands I was told that when going through narrow passes in the reefs, that there was absolutely no point in using the GPS as the chart was last surveyed well before GPS was invented and the coordinates could easily be up to mile or more off. It was essential to use the marked transits. And not deviate in the slightest as some of those passes are extremely narrow.
In some areas, apparently the last survey was done by Captain Cook. But some one might have been pulling my leg.
In some areas, apparently the last survey was done by Captain Cook. But some one might have been pulling my leg.
john_morris_uk
Well-known member
No leg pulling involved. I understand that what you say is true.
Resolution
Well-known member
Fascinating how much the Chinese side of the HK border has grown. Back in 1980 I drove up from Hong Kong to what was then a fishing / farming village called Donguan to inspect one of the first open China projects, to build a dozen modern yachts using a Doug Peterson 30 ft design. I ordered production number one, being AFAIK the first glassfibre yacht made in China. Production was very difficult and slow, but some months later "my" yacht floated down river and joined all the other yachts in HK. Quality was always a problem - there was a soft patch on the deck, the stern gland leaked massively , the rudder broke due to inadequate internal welding, the stainless steel wasn't. etc etc. But she was very cheap! The project folded soon after.
dunedin
Well-known member
That’s one of the real issues with all (AFAIK) leisure electronic vector charts - Navionics, C-Map, Lighthouse et al - is that they don’t show you the sources (survey date, CATZOC etc) to allow you to see how confident the chart maker was.
This is one of the specific concerns raised with leisure chart publishers by the RIN and the Cruising Association.
Plenty of areas in Scotland with very old survey dates - or simply still unsurveyed.
dunedin
Well-known member
Out of interest, do you know what a full on commercial ships ECDIS would show in this situation? I assume it would have the correct spot depths, but would it show any contours? And if not, how does it decide how to colour the sea area?"Algorithm" is often a word for hiding dubious or at least unclear practices. A hydrographic office will have a grid of actual measured data to say a scale of 1:X000, they will output a chart to a scale of 1
Navionics takes the final data, whatever it is, and interpolates into purely theoretical .5m contours, irrespective of the type of original data (existing, not existing, catzoc category, etc, who cares just add contours they are nice)
How can an original CATZOC A2 or B chart, with a few "2", "5" and "10" soundings a few hundred/thousands meters away, produce an electronic chart with .5m spaced contours between the two measurements?
Very appealing though, oh look at the contours it's a lot more detailed
boomerangben
Well-known member
Sounds to me that if navigating in areas of uncertain bathymetry, a forward looking sonar might be a worthy investment
capnsensible
Well-known member
A powerful echo sounder with the correct charts can be exceedingly useful in submerged submarine navigation but it does tend to also signal your position to 'listeners'.
Fr J Hackett
Well-known member
Not really a problem for yachties unlessss they are trying to blow up gas pipelines perhaps.A powerful echo sounder with the correct charts can be exceedingly useful in submerged submarine navigation but it does tend to also signal your position to 'listeners'.
westernman
Well-known member
In the case of Polynesia and the Cook islands, the bathmetry in the passes in the coral reefs is well known.
However, the precise location of those very narrow passes - often no more than 15-20metres wide, does not correspond to the charted location.
May be Captain Cook forgot to set his GPS to WGS84.