Depth indication close to shore

[bvk]

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 :-)

[Adelie]

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.

[Adelie]

Welcome.

I would suggest updating your profile with your general location and your boat make & model or “Looking” in the "Boat" category. This info shows up under your UserName in every post in the web view. Many questions are boat and/or location dependent and having these tidbits under your UserName saves answering those questions repeatedly. If you need help setting up your profile then click on this link: https://www.cruisersforum.com/forums...ml#post3308797

I would happily help more if the link above is not enough.

[boatman61]

Originally Posted by bvk

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 :-)

Depths printed on charts are usually the shallowest depth to be expected in the immediate area around the mark at MLWS.
Ergo if there is a 3 metre rise and fall of the tide on average, at High Water that area would be +/- 4.8 metres depth and at Low Water 1.8+ depending on the lunar cycle.

[bvk]

Originally Posted by Adelie

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.

In poorly surveyed areas there’s no guessing.

The area is very well surveyed. I am interested in your remark that you would expect 1m of depth between the 1.5 and 1.8 if it weren't for the tiny island. Is that based on a rule of thumb, experience, something else? In any case thanks you for your input,

[bvk]

[QUOTE=boatman61;3446260]Depths printed on charts are usually the shallowest depth to be expected in the immediate area around the mark at MLWS

Sorry, forgot to mention that there is no tidal effect for my example (lake). However if the indicated depths are the shallowest in 'the immediate area around' then I would like the chart to indicate its idea of 'immediate'. Is that 3m? 10? A percentage of the depth? Furthermore why indicate the shallowest point very close to shore (much shallower)?
Confusion still reigns.

[Adelie]

Originally Posted by bvk

The area is very well surveyed. I am interested in your remark that you would expect 1m of depth between the 1.5 and 1.8 if it weren't for the tiny island. Is that based on a rule of thumb, experience, something else? In any case thanks you for your input,

That’s me being conservative. If I had a 1.4m deep boat and needed to transit that line and there was no island i would do so at 1kt in calm seas or not at all.

[TrentePieds]

bvk:

I'm not sure just what it is you are confused about. I must be confused ;-)!

In Canadian waters you would use printed charts published by the Canadian Hydrographic Service. The charts are numbered as well at titled, of course, and Chart #1 is really a reference work in which you find explained the particular significance of every symbol you will ever find on a Canadian chart. FWIW, in Canadian waters you are legally required to carry paper charts regardless of how sophisticated your electronic navigation instruments may be.

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

Scull down a ways and see how it works!


Other nations have institutions equivalent to CHS and these institutions also publish charts. As you have not told us where you sail, I can't help you further just now, but I STRONGLY recommend that you keep Chart#1 (or you nation's equivalent of it) handy as an icon on your desktop so you have it handy for reference. If you'll be out of WIFI range you should probably print it out. 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.

TrentePieds

[StuM]

They are "spot depths". i.e. that's where actual sounding were taken when surveying the area.

Other than at the centre of that number, all you know is that anywhere within the blue area, the depth is somewhere between 0 and 3

[StuM]

Originally Posted by Adelie

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.

That is a dangerous and false assumption. If there was nothing marked between them then all we know is that everything on that line can be assumed to be above chart datum and at most less than 3m above.

[RaymondR]

Don't know about Canadian charts but Australian charts often have a small "reliability" diagram somewhere on them which shows the type of survey which was used to record the depth data from which the chart was compiled.

Being mindful that most charts were surveyed for the military and commercial shipping I am generally suspicious of what is illustrated for shallow water areas other than on port plans, particularly the location of those little asterisks which indicate rocks.

[StuM]

Originally Posted by TrentePieds

In Canadian waters you would use printed charts published by the Canadian Hydrographic Service. The charts are numbered as well at titled, of course, and Chart #1 is really a reference work in which you find explained the particular significance of every symbol you will ever find on a Canadian chart. FWIW, in Canadian waters you are legally required to carry paper charts regardless of how sophisticated your electronic navigation instruments may be.

Here is a link to CHS Chart#1:

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

Scull down a ways and see how it works!

See Page 41 - Item 10 "Sounding in true position"

[Alan Mighty]

Originally Posted by RaymondR

Don't know about Canadian charts but Australian charts often have a small "reliability" diagram somewhere on them which shows the type of survey which was used to record the depth data from which the chart was compiled.

Since about 2014 the International Hydrographic Organisation standard has been for marine charts to carry a legend block that gives Zone of Confidence (ZOC) of the position and depth data. Recent Aus charts (and no doubt the charts of other rich economies) meet that standard.

Whether you see that ZOC info block on your electronic chart display is another question.

The ZOC legend block refers to the IHO ZOC categories (A1, A2, B, C, D, U) that are explained in the usual documents (in the case of Aus, the Mariner's Handbook for Australian Waters and similar publications). I've attached a standard ZOC categories table.

In Aus and a few other areas, the position accuracy issue is a big one. The Aus continent moves about 70 mm each year. It's one of the faster movers among the plates. So position data on charts from, say, the 1990s are inaccurate by distances in the order of 1.5 - 1.8 metres (depending where in Aus).

Depth accuracy is, as they say, a different kettle of fish. Depths measured and confirmed by traditional techniques, such as hand-sounding by lead weight and verified by drag, measure a different depth from electro-acoustic means.

Hand sounding was assumed (and is still thought) to have reached the hard substrate. Dragging a wire cable was similarly believed to establish whether the hard substrate was uniform or at least without surprises.

The introduction of electro-acoustic sounding has changed (or is changing) the idea of the water column and depth. The issue is complicated because of a shift in the frequency of the preferred acoustic signal over recent years.

The accepted view at the moment (or at least pre-Covid) is to think of the water column and seabed in three layers:

* an upper layer of water with some suspended particles, including to the point of opaque turbidity, through which ships and acoustic signals pass;

* a layer of 'quick clay' also called the 'nautical layer' of precipitated silt, with a specific gravity just a little more than water (e.g. SG of 1.1 plus or minus), that a keel can drag through without (much) damage. A hand lead penetrates the quick clay. Depending on the frequency, sonar detects the layer and may call it the bottom.

* the hard bottom, which may be consolidated mud, sand, rock, etc. And the hard bottom is hard to your keel.

The quick clay/nautical layer may not be a fixed or uniform thing. In or near estuaries with high silt loads (the Plate is the case everyone knows: in the Plate, you first sink in the water, then you sink in the nautical layer; some commercial ports - e.g. Rotterdam - give a weekly report on the location and depth of the nautical layer, because it can move fast) Most any semi-enclosed marina is another example: in the marina I use as home, the nautical layer just grows. Any disturbance re-suspends the nautical layer for as short as a week or as long as several months. Changes in season, the pH of the water, and biological activity can precipitate the turbidity and even give the hard clay a temporary crust or measure of stability that can be enough to resist a hand lead.

# complicating all that is vegetation and other organic growth. In not a few areas, weed is robust and dense, to the point of being able to resist penetration by an anchor (not to mention a lead line). Depending on frequency, sonar can detect the vegetation. Sophisticated electronics and intelligent human interpretation can help.

[boatman61]

Never sailed in a lake so unfamiliar as to how they are/may be charted but, I would imagine it would be a fixed depth all around till the next number that says different.. where the demarcation lies however is for you to find..

[RaymondR]

Good Post AM.