Bembridge - why don't the move the channel?

[Mark-1]

If Navionics are correct it looks to me like it would make a lot of sense to change the bouyage of the 'official' channel at Bembridge south of the Fort which is a little bit deeper.

http://www.bembridgeharbour.co.uk/the-harbour/navionics

They must have a good reason not to, anyone know what it is?

Obviously Nothing to stop people going in from due east East and join around 2a anyway, but be nice to have it bouyed.

EDIT: The latest Navionics charts don't seem to agree with their web app - which is perhaps the answer.

 

[johnalison]

I don't know Bembridge, but it is quite common in other places for the soundings to be changed less often than the buoyage. If buoyage is based on local knowledge, it will normally indicate where the best water is. The place I know this best is around the Frisians, where it is quite common to follow the buoyed channel while ignoring the soundings shown on the chart or plotter.

 

[Angele]

Pretty sure they move the buoys whenever the channel moves. Pretty easy to do with a channel that dries. I would trust the buoys in the channel more than I would the Navionics chart. Especially if the latter has been "improved" with data from the public...

EDIT: The latest Navionics charts don't seem to agree with their web app - which is perhaps the answer.

Wouldn't be the first time....

 

[prv]

Pretty sure they move the buoys whenever the channel moves.

Possibly only to follow the existing entrance channel snaking about a bit, though? The appearance of a new, slightly deeper channel the other way round the fort might not prompt a complete rethink of the approach immediately.

Pete

 

[Angele]

Possibly only to follow the existing entrance channel snaking about a bit, though? The appearance of a new, slightly deeper channel the other way round the fort might not prompt a complete rethink of the approach immediately.

Pete

Dom spoke to the harbour guys at the weekend, and they told him the bar was 0.8m above datum. So far as I am aware, on spring tides, the bar is responsible for retaining water in the harbour when the tide is out. Surely if there really was a way in below datum, they would have noticed a big reduction in depths inside the harbour at LW springs.

I'm not aware they have.

I have my concerns over the way Navionics charts get updated (as you will recall from discussions on here about mysterious drying areas in the Solent and a deep water channel into Saint Vaast, that appeared and then promptly disappeared in rapid succession).

 

[Mark-1]

I have my concerns over the way Navionics charts get updated

Me too, but in this case Bambridge Harbour's own website is quoting the data so they must think it's reasonable. (Unless it's been updated since they linked to it.)

It's a low spring today - any forumites in Bembridge with a Camera?

 

[Angele]

Me too, but in this case Bambridge Harbour's own website is quoting the data so they must think it's reasonable. (Unless it's been updated since they linked to it.)

Fair point, but I'm guessing it may well have been (updated that is). I'm not sufficiently programming savvy to work that out from the script.

The buoyage list on their website was updated in May - so is fairly recent.

 

[prv]

Dom spoke to the harbour guys at the weekend, and they told him the bar was 0.8m above datum. So far as I am aware, on spring tides, the bar is responsible for retaining water in the harbour when the tide is out. Surely if there really was a way in below datum, they would have noticed a big reduction in depths inside the harbour at LW springs.

Must admit I haven't actually looked at the link at the start of the thread, just going on memories of an apparent second channel the other side of the fort shown on Navionics in previous years. What you say makes sense, and I agree Navionics is certainly not to be trusted implicitly.

Pete

 

[prv]

Me too, but in this case Bambridge Harbour's own website is quoting the data so they must think it's reasonable.

All they've done is included a frame containing http://webapp.navionics.com centred initially on Bembridge. It shows whatever Navionics is currently showing, nobody at Bembridge is curating it.

Pete

 

[BelleSerene]

If you go to the Navionics Web app ( http://webapp.navionics.com ) and select SonarChart in the bottom left-hand corner, you see their crowdsourced bathymetry computed from the depth sounders of users who are floating back and forth over an area all the time. This is far more recent (as well as far more detailed) depth information than is on their regular published charts.

Here's a screenshot of it. It shows that recently the drying bar is intact to the east of the Bembridge channel - a good reason not to move the buoyed channel to pass south of St Helen's fort.

The only thing that's less useful about this data is that it has a lot fewer data points in areas that dry at Chart Datum. This is presumably because most sailors chicken out of using 'green' areas on their charts when the tide allows it, as, um, my recent 'Solent Bramble bank' post!

View attachment 59999

 

[Angele]

If you go to the Navionics Web app ( http://webapp.navionics.com ) and select SonarChart in the bottom left-hand corner, you see their crowdsourced bathymetry computed from the depth sounders of users who are floating back and forth over an area all the time. This is far more recent (as well as far more detailed) depth information than is on their regular published charts.

Yes, I saw that. And I think that is what Mark meant by the EDIT in his OP.

The trouble is that I have, until now, had some faith in the Navionics webapp, provided the crowd sourced data (i.e. sonarChart) was turned OFF. Now I'm thinking I cannot even trust that....

 

[Topcat47]

There are no drying heights on the Navonics web screen shot. I've been in and out of the main channel in the past, but not this year. The Bar is above CD but not by a lot and close to HW I can pass easily (Snark draws a little under 6')

 

[dom]

I have my concerns over the way Navionics charts get updated (as you will recall from discussions on here about mysterious drying areas in the Solent and a deep water channel into Saint Vaast, that appeared and then promptly disappeared in rapid succession).

Fully agree, the crowd sourced data is interesting but comes with a health warning. In any event sand/shingle/mud banks are always prone to movement and the only data one can rely on is a proper bathymetric survey such as that periodically undertaken in Chichester Harbour:
http://www.conservancy.co.uk/assets/assets/Bar Survey June 2016.pdf

Even then a good storm can upset the apple cart. However, in the absence of such a survey I would personally stick to the buoys as someone is much more likely to complain if depths are not as shown, than with respect to some 'cunning passage' based upon a few leisure depth sounders, which may or may not be correctly calibrated

 

[prv]

The trouble is that I have, until now, had some faith in the Navionics webapp, provided the crowd sourced data (i.e. sonarChart) was turned OFF. Now I'm thinking I cannot even trust that....

I think we saw that well-illustrated by the St Vaast debacle, and some other examples posted on that thread (somewhere on the East Coast, and somewhere else in France copied from a French forum). I seem to remember a couple of rogue banks shown in the Solent in the past too (one near Portsmouth and one near Wootton).

No source is perfect - either too slow to reflect changes, or too eager to incorporate unreliable information - which just shows the benefit of having multiple sources (in my case, Navionics on deck and recently-corrected Imray paper on the table) and the importance of also trusting what your eyes and your depth-sounder are telling you directly. Recently in Greece we used Google Maps aerial photos to advantage too.

Pete

 

[Angele]

Fully agree, the crowd sourced data is interesting but comes with a health warning. In any event sand/shingle/mud banks are always prone to movement and the only data one can rely on is a proper bathymetric survey such as that periodically undertaken in Chichester Harbour:
http://www.conservancy.co.uk/assets/assets/Bar Survey June 2016.pdf

Even then a good storm can upset the apple cart. However, in the absence of such a survey I would personally stick to the buoys as someone is much more likely to complain if depths are not as shown, than with respect to some 'cunning passage' based upon a few leisure depth sounders, which may or may not be correctly calibrated

I'm fully behind you there, Dom. Preferably about 20m behind - just far enough to put the brakes on if you come to a juddering halt.

 

[dom]

I'm fully behind you there, Dom. Preferably about 20m behind - just far enough to put the brakes on if you come to a juddering halt.

Prob the wisest approach! Still, it looks like the Smart Alecs on the other side will come to an equally abrupt stop.

I don't know how to post pics but pic #6 (click to expand and notice port Buoy #2 to orientate oneself) from this year's St Helens Fort walk prima facie^* suggests six of one, half a dozen of the other:
http://iwradio.co.uk/news/crowds-br...envira_id=1670201#envira-gallery-wrap-1670201

*: I am excluding a religious explanation

 

[Mark-1]

Recently in Greece we used Google Maps aerial photos to advantage too.

I've been staggered how good Google maps are for finding the deep bit of muddy creeks. Far better than charts which often make no attempt to define smaller creeks.

https://drive.google.com/open?id=1wkYQ_rEpY7tL5XUnPv0veuBmhtw&usp=sharing

A slightly different point, but above there's a track of me (amongst other things) following the echo sounder round the bottom of Thorney Island. It was interesting how close it contours the mud as it appears on the google maps photo, whenever that was taken. Also that the track out of the harbour matches the location of the obvious drying bit on the way out. It was also staggering how the 20 year old CMAP charts that were torrented years ago also show the bottom to be similar at both points in spite of the bouy having completely moved. I image it's all moved a lot and it's just fluke that's moved it back at the moment.

Yes, I saw that. And I think that is what Mark meant by the EDIT in his OP.

I should have been clearer - I was comparing latest update on the Navionics app on my phone with the Navionics Web App. Not Sonar charts on either.

 

[Mark-1]

I don't know how to post pics but pic #6 (click to expand and notice port Buoy #2 to orientate oneself) from this year's St Helens Fort walk prima facie^* suggests six of one, half a dozen of the other:

That's conclusive and the event could have been designed to answer my question. Thanks, your google-fu is strong.

 

[Angele]

That's conclusive and the event could have been designed to answer my question. Thanks, your google-fu is strong.

Nah! I reckon those coming from Bembridge are all pygmies, whilst those from St Helens are all giants.

 

[Mark-1]

Nah! I reckon those coming from Bembridge are all pygmies, whilst those from St Helens are all giants.

http://heartburnentertainmentworldwide.com/international/isle-of-wight/isle-of-wight-stilt-walkers/