Always trust your charts???

[john_morris_uk]

I disagree. It is stated that there may be features between contours (which I also interpret as along contours), which the survey may not have detected. Of course that point is more relevant to lead line soundings, than sonar, but even sonar is not uniform over every square mm of seabed.

Your confidence is misplaced on charts with older surveys. I agree that modern surveys will be be more accurate by a significant margin.

I can give examples, as I update my charts, however I can’t currently quote correction or chart. There is a lot of recent surveys being performed by private contracted, organisations, fish farms, wind farms. A few rocks and contours, WCoS, have been moved, I assume because the earlier surveys, by UKHO were not that accurate.

You’re drawing a false conclusion regarding the ‘private’ surveys. I spent a very happy period of my RN career with the Hydrographic Squadron of the Royal Navy. Lots of surveying is done by private contractors. Some of them even have small teams of RN surveyors on board. I know this because I used to go and visit them.

The point I was making is that In general terms, contours on UKHO charts are not the guesswork of a computer algorithm. But I agree that the sensible navigator always looks at the data source.

Charting corrections are being made all the time. So what?

Modern surveys are by multi beam and side scan sonar but the vast majority of the worlds oceans have not been surveyed in this way.

 

[NormanS]

Just replied to your similar question on the other thread.
In short, on a typical yacht MFD/chart plotter - NO I don’t believe you can currently see chart sources / levels of confidence (CATZOC).
The only way is raster charts on an app, and that isn’t always easy.

It is very relevant - as I am discussing with UKHO a large area of drying rocks entirely missing from UKHO charts in Scotland (and hence proudly showing safe 5m depth on Navionics, who use UKHO data, whereas a large area is drying) - this is shown as on old survey and low confidence level on the relevant paper chart, but no way of knowing this from my Navionics chart on Raymarine MFD.

As someone who sails extensively in the waters of the West of Scotland, where the charts many of the inshore areas are still based on 19th century lead line surveys, I am well aware of real and potential dangers. I have found a few, which I have reported to the Hydrographic Department, and charts have subsequently been corrected. Many areas were surveyed in the 1850s, and this was by men in wee boats, physically checking depths with lead lines, often in very difficult conditions. My own feelings are that, in general, the charts are amazingly accurate.
Some of the different conventions that have been used in the past have led to some modern mistakes. An example: At one time, in some old feet and fathom charts, drying heights were shown in Roman numerals. A certain rock was shown as drying 11 ( meaning 2'). When that particular chart was changed to metres, the "11" was mistakenly converted to 3.4m.
Another convention saw soundings in fathoms in as far as a 5 fathom contour, and then the soundings were in feet. That was really confusing.
I would be very interested to know the area of drying rocks that you are referring to.

 

[wonkywinch]

I've contacted C Map regarding the Solent updates since they still haven't amended to include Lentune Island which formed in March, was subject of a NTM and Navionics promptly amended.

Still waiting for a reply from C Map but quite concerned they weren't even aware of a charge large enough to be promulgated by a NTM six months ago.

 

[RunAgroundHard]

You’re drawing a false conclusion regarding the ‘private’ surveys. I spent a very happy period of my RN career with the Hydrographic Squadron of the Royal Navy. Lots of surveying is done by private contractors. Some of them even have small teams of RN surveyors on board. I know this because I used to go and visit them.

The point I was making is that In general terms, contours on UKHO charts are not the guesswork of a computer algorithm. But I agree that the sensible navigator always looks at the data source.

Charting corrections are being made all the time. So what?

Modern surveys are by multi beam and side scan sonar but the vast majority of the worlds oceans have not been surveyed in this way.

No false conclusions. The correction states the source of the data. It is a fact that earlier surveys were wrong in the instances where corrections have been made..

 

[john_morris_uk]

No false conclusions. The correction states the source of the data. It is a fact that earlier surveys were wrong in the instances where corrections have been made..

I thought you were drawing a conclusion from the fact that some surveys are done privately and therefore not thr responsibility of UKHO. I was explaining that lots of private surveys ARE giving data to UKHO and are actually commissioned by UKHO.

 

[AntarcticPilot]

I thought you were drawing a conclusion from the fact that some surveys are done privately and therefore not thr responsibility of UKHO. I was explaining that lots of private surveys ARE giving data to UKHO and are actually commissioned by UKHO.

Indeed, a major feature of charts (the coastline!) is usually sourced from other agencies (the OS in the UK, the unit I worked for (before I retired) in Antarctica).

One minor quibble - contours are always derived data; the actual measurements are spot depths (very closely spaced indeed for swath and side-scan bathymetry, but still spot depths). However, in most cases around the UK, the contours will be based on far more data than are shown on the face of the map; even in Antarctica, if there is bathymetry, there are often more soundings than are shown. I've had access to compilation data for some places for scientific use, and there are usually many times more soundings than are used on the chart, but of course, contours are based on all the data.

 

[dunedin]

I've contacted C Map regarding the Solent updates since they still haven't amended to include Lentune Island which formed in March, was subject of a NTM and Navionics promptly amended.

Still waiting for a reply from C Map but quite concerned they weren't even aware of a charge large enough to be promulgated by a NTM six months ago.

As C Map uses UKHO source data, it is quite possible UKHO have not updated their source data, or not issued it to the leisure chart publishers. Apparently this can take many months even after the source database is updated.

 

[bikedaft]

Just replied to your similar question on the other thread.
In short, on a typical yacht MFD/chart plotter - NO I don’t believe you can currently see chart sources / levels of confidence (CATZOC).
The only way is raster charts on an app, and that isn’t always easy.

It is very relevant - as I am discussing with UKHO a large area of drying rocks entirely missing from UKHO charts in Scotland (and hence proudly showing safe 5m depth on Navionics, who use UKHO data, whereas a large area is drying) - this is shown as on old survey and low confidence level on the relevant paper chart, but no way of knowing this from my Navionics chart on Raymarine MFD.

thanks for flagging it up to UKHO...

where is it, please?

 

[john_morris_uk]

Indeed, a major feature of charts (the coastline!) is usually sourced from other agencies (the OS in the UK, the unit I worked for (before I retired) in Antarctica).

One minor quibble - contours are always derived data; the actual measurements are spot depths (very closely spaced indeed for swath and side-scan bathymetry, but still spot depths). However, in most cases around the UK, the contours will be based on far more data than are shown on the face of the map; even in Antarctica, if there is bathymetry, there are often more soundings than are shown. I've had access to compilation data for some places for scientific use, and there are usually many times more soundings than are used on the chart, but of course, contours are based on all the data.

You’ve explained the very point I was trying to make so much better than my attempts.

 

[Stemar]

thanks for flagging it up to UKHO...

where is it, please?

Off Hurst Point

 

[Whaup367]

... I am discussing with UKHO a large area of drying rocks entirely missing from UKHO charts in Scotland ...

thanks for flagging it up to UKHO...

where is it, please?

Off Hurst Point

Is this linked to the "round UK" thread? A circular route "round UK" through the Pentland firth and Caledonian canal results in defining anything south east of Corpach as "off Scotland"?

 

[dunedin]

Off Hurst Point

The rocks I was referring to were somewhat further north - actually over 500nm furth North

 

[Draystone]

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.

Navionics Sonar Charts & The Missing Reef

Here's one person's experience of several serious errors in 2017. It would be nice to think Navionics are better now wouldn't it?

 

[AntarcticPilot]

Navionics Sonar Charts & The Missing Reef

Here's one person's experience of several serious errors in 2017. It would be nice to think Navionics are better now wouldn't it?

I have given detailed critiques of why the Sonar Charts should NOT be trusted previously. But the worst aspect is Navionics extrapolation of data between data points and the coastline.

 

[Draystone]

I have given detailed critiques of why the Sonar Charts should NOT be trusted previously. But the worst aspect is Navionics extrapolation of data between data points and the coastline.

Your explanations are always clear. Thanks.

 

[B27]

I have given detailed critiques of why the Sonar Charts should NOT be trusted previously. But the worst aspect is Navionics extrapolation of data between data points and the coastline.

The worst aspect is that Navionics defaults to this crap.
There are places near here where the SonarChart layer gives 2m of charted depth, but in reality, you can walk there at low tide.
I think someone up the Tamar is supplying them with a lot of duff data!

 

[Bristolfashion]

How many people think that their "bought it in 1972 and haven't updated it since" paper chart is more accurate?

 

[Fr J Hackett]

How many people think that their "bought it in 1972 and haven't updated it since" paper chart is more accurate?

As most of my charts tended to be in places that he hard rocks that didn't move much and had been adequately surveyed relatively recently by hydrographers and not yachtsmen with dodgy echo sounders I would have to say guilty M'Lord. However for my brief sojourn on the East Coast which was not so fixed I was very grateful for the annual surveying done particularly of places like the Deben entrance and the excellent Crossing the Thames Estuary to which I paid great attention.

 

[Roberto]

How many people think that their "bought it in 1972 and haven't updated it since" paper chart is more accurate?

One "bought in '72 chart and never updated" would/should suggest the user that the data is far from accurate, hence he/she would use it accordingly. If one does not, his/her mistake.
A modern electronic chart with 50cm spaced contours derived from pure interpolation would give the impression of having very accurate data and no way of determining the likely error (contrary from Catzoc in serious HO charts), The boat grounds, who induced the mistake?
One could well make charts with interpolated 10cm spaced contours and probably make a lot of money with users not knowing what a chart means.

 

[AntarcticPilot]

As most of my charts tended to be in places that he hard rocks that didn't move much and had been adequately surveyed relatively recently by hydrographers and not yachtsmen with dodgy echo sounders I would have to say guilty M'Lord. However for my brief sojourn on the East Coast which was not so fixed I was very grateful for the annual surveying done particularly of places like the Deben entrance and the excellent Crossing the Thames Estuary to which I paid great attention.

I hope not THAT guilty - a 1972 chart in the UK would be on the OS1936 datum, and potentially up to 100m away from WGS84 positions! I think it would also be a black and white chart, but I'm not sure when UKHO went to colour!