Guess what question I sometimes ask YM candidates.

[RunAgroundHard]

We take our accuracy of position for granted these days, rightly so.
I am sitting on my boat just up from Islay and 100 years ago, it was quite frequent that some vessel ran aground. I have a couple of books at home, records of groundings and sinking, FofC and West Coast, there are areas like Islay and Sanda where the wrecks are on top of one another.
Fast forward to today, and the fish farm and wind turbine surveys in close, show that a lot of shoals and rocks need to be moved, as I guess, they were not that accurate when they surveyed.

 

[Neeves]

A. Depth uncertain..

We have charts with big white patches, close to or abutting shore, which usefully say "Unsurveyed".

We have one specific chart that used a local, private individuals, survey in feet, it was made before metrication. The data was transcribed, after metrication, as metres. The original survey is copied in the original form in a relevant cruising guide.

We carry the UK, Admiralty, Pilot books for our cruising guides to supplement anything privately published and find them an invaluable source of information. The surveys were made in vessels similar in size to a, small, private yacht - so have some relevance. The older the volume the more applicable - coast lines and seabeds do not change much (unless you are in HK where the old coastlines have disappeared). Maritime book shops, or dealers, are good source for old volumes.

Jonathan

 

[Dutch01527]

In the 1980’s, as a very young cadet, I crossed the Pacific from California to Fiji. We discovered a picture perfect desert island about 400 metres long compete with palm trees and sandy beaches that was not on the charts on the approaches to Fiji . Miserable old git of a Captain would not let us stop and go ashore for a quick BBQ. Most unreasonable.

 

[john_morris_uk]

We take our accuracy of position for granted these days, rightly so.
I am sitting on my boat just up from Islay and 100 years ago, it was quite frequent that some vessel ran aground. I have a couple of books at home, records of groundings and sinking, FofC and West Coast, there are areas like Islay and Sanda where the wrecks are on top of one another.
Fast forward to today, and the fish farm and wind turbine surveys in close, show that a lot of shoals and rocks need to be moved, as I guess, they were not that accurate when they surveyed.

It’s a bit more complicated than that though isn’t it? I might know where I am on planet earth within a metre of two, but the navigation problem is deciding whether the charted data that I’m relating my position to is accurate in where it is and what it is.
The pecked lines for depth contouring illustrate one part of the problem.

 

[Frank Holden]

You win the prize.



(And the answer is?)

By looking at the source data we can see that inshore the source is a leadline survey dating from 1858/60, further out it is 'f' - misc data and passage soundings.
So imperfectly surveyed - hence the broken contour lines.
I reckon Benjenbav at #2 should get the prize.

Re metric or fathoms. Admiralty ' fathoms' charts had grey land. Metres have yellow land.

Trap for young players - I have some US copies (rasters) of Chilean charts from the early 70's with grey land and soundings in metres. Was a bit confused at first - had to check the title block. Sample is from Ocpn display.

 

[Neeves]

It’s a bit more complicated than that though isn’t it? I might know where I am on planet earth within a metre of two, but the navigation problem is deciding whether the charted data that I’m relating my position to is accurate in where it is and what it is.
The pecked lines for depth contouring illustrate one part of the problem.

Interesting thread, you should conjure them up more often.

You know I would mention anchors

When we anchor in the less frequented locations we do not rely on the charted position by GPS and the 'overlay' of that position onto a chart that was possibly drawn up by hand by a man in a 'long boat' over 100 years ago. We simply use radar to ensure we are equidistant from possible hazards. Depth is an issue - but being a cat we have slightly more wriggle room and aim for a 5m depth anyway.

Jonathan

 

[john_morris_uk]

By looking at the source data we can see that inshore the source is a leadline survey dating from 1858/60, further out it is 'f' - misc data and passage soundings.
So imperfectly surveyed - hence the broken contour lines.
I reckon Benjenbav at #2 should get the prize.

Re metric or fathoms. Admiralty ' fathoms' charts had grey land. Metres have yellow land.

Trap for young players - I have some US copies (rasters) of Chilean charts from the early 70's with grey land and soundings in metres. Was a bit confused at first - had to check the title block. Sample is from Ocpn display.

The official phrase is ‘Inadequately surveyed’. If the contour line is continuous it generally means that the survey established that is more or less where it is. In the UK snd European waters that many of these forum members sail in, the contours will often have been determined by multi beam or sidescan sonar. (I spent a very happy year or so riding on RN Hydrographic Survey Vessels, so I saw it in action.)


In simple terms, the pecked line means it’s a ‘best guess’ by the hydrographic office when they drew up the chart.



One of the chsllenges yachtsmen and women have to rise to when navigating in foreign or distant seas is that the waters haven’t been surveyed half so accurately and regularly as they have in Europe. The coloured chart in paper or on the plotter screen looks so reliable…

 

[Bajansailor]

One of the chsllenges yachtsmen and women have to rise to when navigating in foreign or distant seas is that the waters haven’t been surveyed half so accurately and regularly as they have in Europe.

And in the Caribbean the coral reefs grow - ok, not very quickly, but over 100 + years or so the charted depth can change enough to put an unwary skipper in trouble (and yes, I have 'been there, done that' )

 

[Neeves]

One of the chsllenges yachtsmen and women have to rise to when navigating in foreign or distant seas is that the waters haven’t been surveyed half so accurately and regularly as they have in Europe. The coloured chart in paper or on the plotter screen looks so reliable…

A problem s that now charts, or their electronic versions, for commercial vessels have different specifications to the ones we want. Our needs are not much different to that of Bass and Flinders (or Cook) but we are not a reliable source of revenue (in fact we all want it for free - or as a right). If bits of the world change, Bajansailor's reef, who cares (commercially)? Most commercial vessels are now large and our yachts are the same size, or some of them, as those of Cook who did much of their surveys of interest to us in 'big tenders'. Maersk are not going to pay for charts derived from 'big tenders'.

Different demands need different solutions

Jonathan

 

[Chiara’s slave]

A problem s that now charts, or their electronic versions, for commercial vessels have different specifications to the ones we want. Our needs are not much different to that of Bass and Flinders (or Cook) but we are not a reliable source of revenue (in fact we all want it for free - or as a right). If bits of the world change, Bajansailor's reef, who cares (commercially)? Most commercial vessels are now large and our yachts are the same size, or some of them, as those of Cook who did much of their surveys of interest to us in 'big tenders'. Maersk are not going to pay for charts derived from 'big tenders'.

Different demands need different solutions

Jonathan

Presumably this is what Garmin and others are trying to achieve. They are trying to turn everyone into a surveyor. You just have to tick a box in the terms and conditions, and own a plotter and depth sounder. The community link is via the app on your phone of course.

 

[jlavery]

Similar to St Kilda - on my list of destinations this year.

At least the St Kilda chart says "unsurveyed" in places. But the approach to the main anchorage has broken contours.

 

[RunAgroundHard]

It’s a bit more complicated than that though isn’t it? I might know where I am on planet earth within a metre of two, but the navigation problem is deciding whether the charted data that I’m relating my position to is accurate in where it is and what it is.
The pecked lines for depth contouring illustrate one part of the problem.

I wouldn’t think it’s complicated, just that the risk may be normalised, which access to instant position data could contribute to. In the past, uncertainty, unease, was chronic, so leisure sailors probably managed uncertainty routinely. I wonder if that’s what changed today? Maybe not, what do your Yachtmaster candidates demonstrate, gung-ho, creeping cautious or somewhere in the middle?

I have a depth marked on the UK chart of the Flannan Islands, I think it’s 15m, dotted line, Rep1985. The chart has similar source and contours to your OP. That was derived by a lead line as the risk was of shoals was obvious, when transiting the gap. However, using the lead was not that uncommon to me back then as I explored a lot.
Today, on the west coast, there are a lot of shallow bits and rocks moving as surveys are performed for commercial users, as indicated by chart updates. Then there are Antares charts, which illustrate your point well, as many close in features are discovered by the Antares surveyor, some of which are not even shown on the UKHO chart.

Go cautious!

 

[Chiara’s slave]

Does anyone not watch their echo sounder on coastal waters? Look at that bank that's appeared at Hurst. Even us, drawing 40cm, when creeping into an anchorage, we do 1kn and watch the numbers if there could be any doubt. Fairly sure Newtown creek changes regularly.

 

[Stingo]

Does anyone not watch their echo sounder on coastal waters? Look at that bank that's appeared at Hurst. Even us, drawing 40cm, when creeping into an anchorage, we do 1kn and watch the numbers if there could be any doubt. Fairly sure Newtown creek changes regularly.

mbtile charts made using SASPlanet from recent Google Earth or Bing satellite imagines go a long way to helping solve this.

 

[Neeves]

Presumably this is what Garmin and others are trying to achieve. They are trying to turn everyone into a surveyor. You just have to tick a box in the terms and conditions, and own a plotter and depth sounder. The community link is via the app on your phone of course.

Yes, I'm aware - works very well for fresh water but tides are an issue as actual tide depends on barometric pressure and wind. On Tasmania's west coast we have 30cm tides - 'weather' makes a lot of difference (Bass Strait has 2m tides leading to stunning tidal races between the Indian/Southern Ocean and the Strait). In addition the accuracy of location of your depth sounder will impact the data.

They thus need to rely on frequent and many inputs to weed out the anomalies and then average. Works for well used locations - but well used locations are already well documented, in various forms. Where we have the big gaps are the less popular, call them more isolated, destinations like jlavery's St Kilda and our Tasmanian charts where we fall back on buckets of caution (Chiara’s slave) and the depth sounder(s) - we have two, one in each hull forward of the keels - and forward looking sonar (Interphase).

Jonathan

 

[Chiara’s slave]

Yes, I'm aware - works very well for fresh water but tides are an issue as actual tide depends on barometric pressure and wind. On Tasmania's west coast we have 30cm tides - 'weather' makes a lot of difference (Bass Strait has 2m tides leading to stunning tidal races between the Indian/Southern Ocean and the Strait). In addition the accuracy of location of your depth sounder will impact the data.

They thus need to rely on frequent and many inputs to weed out the anomalies and then average. Works for well used locations - but well used locations are already well documented, in various forms. Where we have the big gaps are the less popular, call them more isolated, destinations like jlavery's St Kilda and our Tasmanian charts where we fall back on buckets of caution (Chiara’s slave) and the depth sounder(s) - we have two, one in each hull forward of the keels - and forward looking sonar (Interphase).

Jonathan

Fresh water depth is surely even less predictable. Plus or minus 30cm is luxury in most rivers and lakes.

 

[john_morris_uk]

Does anyone not watch their echo sounder on coastal waters? Look at that bank that's appeared at Hurst. Even us, drawing 40cm, when creeping into an anchorage, we do 1kn and watch the numbers if there could be any doubt. Fairly sure Newtown creek changes regularly.

I might start a new thread on echo sounder usage!

 

[Chiara’s slave]

I might start a new thread on echo sounder usage!

Do it! My echo sounder has an automatic, speed variable alarm. The noise gets louder the shallower it gets and the faster we are going. She does calm down once the hook is down and holding

 

[john_morris_uk]

Do it! My echo sounder has an automatic, speed variable alarm. The noise gets louder the shallower it gets and the faster we are going. She does calm down once the hook is down and holding

I married one of those !

 

[Chiara’s slave]

I married one of those !

Very useful and reliable. The alarm makes tea, steers, passage plans and takes lines when we moor up. Regrettably, the bowline tying is a bit suspect, she blames it on being left handed.