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Old 18-07-2021, 12:40   #31
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Re: Depth indication close to shore

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That would at least explain things, but leaves the question as to why these numbers are shown at all. If the the depth directly to north, east, south and west of the spot sounding can be between 0-3m, the indication can hardly be useful to anyone, for any purpose?
Occasional isolated soundings were common on older charts but I notice that on many of the newer ones they just leave areas blank and outline them as "unsurveyed".
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Old 18-07-2021, 17:50   #32
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Re: Depth indication close to shore

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That would at least explain things, but leaves the question as to why these numbers are shown at all. If the the depth directly to north, east, south and west of the spot sounding can be between 0-3m, the indication can hardly be useful to anyone, for any purpose?

The numbers are useful in that you can believe someone found that depth at that approximate location at some time in the past. As far as I'm concerned, this is a very useful indication indeed. Certainly a world better than knowing only that the depth is probably somewhere between 0 and 3 meters! As always in navigation, use you own judgement, caution and knowledge of what's typically to be found on the bottoms in the local area (always mud, usually rocky, whatever).

Contour lines, especially in areas subject to change like rivers and ocean near river mouths, can be even less accurate than spot depths. They might have been just drawn or generated by a computer program or by hand, using whatever soundings were available at the time.
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Old 18-07-2021, 19:01   #33
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Re: Depth indication close to shore

I would always assume some inherent inaccuracy in chart depths in shallow water so while taking the charts as a guide I would recommend the following: I have first calibrated my B & G depth sounder to read zero when keel touches a sand bank. Then in shallow water if one has to go there go very slowly and constantly monitor depth so that you can reverse out of trouble without doing any damage.
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Old 18-07-2021, 20:06   #34
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Re: Depth indication close to shore

I take the view that if it sticks out of the water at chart datum they would show it on the chart and consequently it's probably safe to go there if there is tide equal to the ships draft otherwise stay out.
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Old 19-07-2021, 01:16   #35
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Depth indication close to shore

If it’s “ sticks out of the water “ you can see it

Pushing you luck in shallow water is exactly that. The only real info is local knowledge.
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Old 23-07-2021, 08:02   #36
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Re: Depth indication close to shore

Can I suggest that you read a book, take a course or sail with someone who is experienced who can explain it. I am not being dogmatic but this is a serious safety issue.
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Old 23-07-2021, 08:12   #37
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Re: Depth indication close to shore

OP, if you do not have the qualifications to read your map correct (whatever map you have), then don't use it.
Simple as that.

If you MUST sail between flats and rocks, in unknown waters, do it in your dinghy, before you crash your boat.

People are trying to help you.
Please provide your boat type and draft, your map source, location, etc.
If you don't, people may not help you next time.
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Old 23-07-2021, 10:09   #38
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Re: Depth indication close to shore

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Originally Posted by Adelie View Post
For a well surveyed area the 1.5 & 1.8m depths are consistent for the area marked. If the was nothing marked between them I would believe that there was at least 1m of depth along the line between them. A drying rock or very small island is shown. I would go NE of the island to transit between those 2 marked depths.

In poorly surveyed areas there’s no guessing.

I think you may have misstated that. In an area marked 1-3 meters, anything shallower than 3 meters is specifically marked and you can assume anything unmarked is at least three meters, and marked depths only have as much depth as is called out. If you draw 1.4 meters, you can pot around wherever you like in there as long as you don't go over anywhere marked 1.4 meters or less.
The chart probably says when the survey was taken, and they usually indicate depth at mean low tide, which means that unless you get there on the @four days a year with unusually low tides, at just the right time, you will find at least as much water as marked.
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Old 23-07-2021, 11:23   #39
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Re: Depth indication close to shore

It is important to be aware of local conventions regarding charting. In the UK and Europe for instance Chart Datum (CD) is Lowest Astronomical Tide (LAT). This is lower than the US convention of average low water. The actual height of tide is modified by storm surges and high and low pressure systems. A high pressure system will actually lower the height of tide by one centimetre (about two fifths of an inch) per millibar. We in the UK have been experiencing a persistent high pressure system at 1026mb, the standard is 1013, so low water is 13cm (about 6”) than charted.
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Old 23-07-2021, 15:33   #40
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Re: Depth indication close to shore

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Hope somebody can help me with this one. I have been navigating for some time so it is a little embarrasing to ask but I would like some help on reading the depth in a chart...

In deeper water, I assume that the number in the chart is the minimum depth encountered in the area. But what do the numbers close to shore mean?

Look at the example below, depth in meters but that is irrelevant to the issue.
The darker blue is the 0-3m depth area. Now, what does the 1.8m mean? It is not the minimum depth since that is, quite obviously, 0m. E.g. it is clearly also not safe to navigate straight from 1.8 to 1.5 with a 1.5 draught. So I have no idea what I can use these soundings for. If you have more information please help :-)
Regardless of how well this area has been surveyed, it is clear from inspecting the chart that there will be many seabed obstructions lurking to take your keel off. If you must navigate shoreward of the 3m line, do so with extreme caution, safe speed, daylight only with a lookout posted forward, and be ready to reverse your propulsion and back away from danger. Picking high water will help too as the frequency of dangers encountered will reduce, but there will still be some.
Did you read all the notes and cautions under the chart title? This is mandatory! You may find answers to your questions there... And bonne chance!
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Old 23-07-2021, 15:58   #41
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Re: Depth indication close to shore

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Originally Posted by Herb Shallcross View Post
I think you may have misstated that. In an area marked 1-3 meters, anything shallower than 3 meters is specifically marked and you can assume anything unmarked is at least three meters, and marked depths only have as much depth as is called out. ........
I think not. The only thing you can assume is there will be (or should be) one metre, not three metres; plus tide height of course.
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Old 23-07-2021, 17:06   #42
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Re: Depth indication close to shore

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Originally Posted by Herb Shallcross View Post
In an area marked 1-3 meters, anything shallower than 3 meters is specifically marked and you can assume anything unmarked is at least three meters
Absolutely, 100% incorrect!

Anything unmarked is LESS THAN 3m and may be as low as 1m.
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Old 23-07-2021, 19:07   #43
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Re: Depth indication close to shore

In the area to the west of the deeper channel illustrated in bvks post, since it has both obstructions above datum and isolated rocks which do not uncover and could be in a tide race between the land areas I would avoid going into it mainly because of it's overall character and probably even so if it was deeper.
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Old 24-07-2021, 00:43   #44
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Re: Depth indication close to shore

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Absolutely, 100% incorrect!

Anything unmarked is LESS THAN 3m and may be as low as 1m.
Fair dinkum! I admire your patience, StuM.

I gave up wanting to correct anything wrong on the Net a while back. I'll indulge in a little ...

The revolution in nav charts took place starting in 2014. I've mentioned it once before, upthread at post #13: the revolution of nav charts using the Zone of Confidence system.

The ZOC is a partial answer to the questions raised in this thread and one other recent thread on sailing in shoal waters.

If you have a ZOC-compliant paper chart, published since 2014, you'll find a tiny diagram (usually at the bottom margin) telling you what parts of that chart have what ZOC.

If you're using ENCs, your e-chart display system should allow you to see a layer showing triangles and stars superimposed on the chart. You only look at that layer when you're doing your passage planning, because it obscures the detail. I've not seen an e-Chart display aimed at the recreational market that shows that ZOC layer (but I've not been shopping so I don't know the marketplace well). Professional pilots carry their own laptops with software that shows the ZOC and of course the latest eCharts. And apart from those in the Suez Canal (it seems) they've all done their passage planning multiple times and considering every weather condition.

ZOC A1 (6 stars in a triangle) guarantees you that on the date of survey, the depths reduced to LAT (or similar lower low mean tide in some jurisdictions) were accurate to plus or minus 0.6 - 0.8 metres (2 feet to about 2.6 feet). In some A1 areas, usually in a commercial port, you can find areas of special survey, with depth accuracy supposedly of 0.25 m (which must be about 9 inches).

In an A1 area, I'm happy to operate with a Under Keel Clearance of as little at 0.8 metres, taking into account wind and wave (wind can starve an area of water depth; wave causes pitching), tide, and of course the barometer (high pressure can lower the depth). In most ports boasting A1 ZOC, the VTS will read out the tide variance to the published tide prediction about once an hour.

To repeat, in ZOC A1, when I'm employed on a bridge (and I haven't had a paying job for 2 years and it looks like I may never have again) I will conn a vessel in water to a depth of draught + 0.8 meters.

ZOC A2 (5 stars in a triangle) guarantees you that on the day of survey the depth accuracy was plus or minus 1.2 - 1.4 metres. Something like 4 - 5 ft.

But it's A2, not A1. So I would only be comfortable with a UKC of 2 metres (with my job and reputation on the line when at the conn in a hull I do not own). And near any coast, I'd think about silting (?recent flood rain?) and sunken debris (again, from rivers and streams).

ZOC B (4 stars) covers much of the water in which I sail recreationally. Surveys can date back to the 1990s. Horizontal position accuracy is around 50 metres or better. Depth accuracy is about plus or minus 1 metre (note that it is not 1.0 metre, just 1 metre, so that's the signal to tell you that no one really really knows) - for spot depths and for contours. No one guarantees an undetected unknown feature (close to shore I've seen a metal pipe structure ditched from perhaps a commercial fisher, the structure was just exposed at a low tide) in a ZOC B.

I like a UKC of about 5 metres in ZOC B. If it's a designated channel or my own home waters, of course no problem.

ZOC C (3 stars) covers even more of the water in which I sail recreationally. I figure that the best charts that are not using the ZOC system are about ZOC C. The surveys are a mix of recent and older. The older surveys will often include passage sounding (a line of closely spaced spot depths on the track into a port or around a headland). Spot depths are just spot depths - they NOT repeat NOT maximum depths nor are they minimum depths. They are just spot depths.

If you see spot depths in a regular spaced pattern, you can have a tiny bit more confidence. If they are irregularly spaced, who knows? In ZOC C, the accuracy of water depth for both contours and spot depths is 6 metres, repeat 6 METRES or about 19 ft. To make you even more cautious, the horizontal uncertainty can be anything up to 500 metres. That's very many boat lengths. Channels and passage soundings are usually okay. Undetected obstructions could be anywhere (including with 500 metres of a passage sounding)!

Your judgment is important. In waters unfamiliar to you, you think about what sort of vessel usually uses these waters, what draught they have, and what track they would follow. That's because the chart may have been drawn with those vessels in mind.

The closer to shore and the closer you are to known rocks and reefs, the more cautious you are. Siltation is one thing. In the tropics, coral growth can be rapid (but not in turbid water).

ZOC D (2 lonely stars) marks surveys that could ancient (Captain Cook, Matthew Flinders). The sort of chart quality I was used to up to the 1980s. And still regarded as acceptable outside waters for commercial shipping in a lot of the world. Horizontal uncertainty greater than 500 metres. No quoted depth accuracy. Spot depths are usually not spaced evenly, but they might be there for a reason. Or not.

Passage soundings might pass within spitting distance of an obstruction (and how would you know with that level of horizonal accuracy? Only because the passage sounding likely is now marked by beacons or buoys or a transit or range on shore). Most anything could be 1 metre below the surface and be exposed only at dead low water.
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Old 24-07-2021, 01:43   #45
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Re: Depth indication close to shore

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Originally Posted by TrentePieds View Post
bvk:
Because the world is going "paperless", Chart #1 is no longer being printed, and you are not likely to find residual stock at chandlers. To make up for that, CHS published Chart #1 in PDF format and recommends - given the significance of colours in chart making - that you print it out on a colour printer - not just a B&W job.

Here is a link to CHS Chart#1:

https://www.charts.gc.ca/publication...index-eng.html

If you don't have a colour printer, an office products store like Staples can do it for you, but I think that that would cost more than buying a colour printer :-)!

All the best.

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Interesting that the Canadian Gov't is telling you to print it, but on on the 2nd page of the pdf, it states that digital or hard copy reproduction is PROHIBITED without written permission from the Hydrographer General of Canada . . . .
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