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Old 17-07-2021, 21:50   #16
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Re: Depth indication close to shore

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Originally Posted by boatman61 View Post
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..
Lake or coast, if it's in the 0-3m band, 1 metre away from where a 1.5m depth was measured, it could well be 0.2m or 2.8m (especially if the bottom is rock or coral). And the lat/lon of that sounding won't be accurate to anywhere close to 1m anyway.

And if it was a sounding taken years ago on a sandy bottom, it's likely to have changed by now anyway.

Just consider spot heights (soundings) as an indication of likely depths in their general vicinity.
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Old 17-07-2021, 23:23   #17
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Re: Depth indication close to shore

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Originally Posted by TrentePieds View Post
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.


TrentePieds
There appears to be one case now where you don't have to carry paper charts in Canada.

https://www.charts.gc.ca/help-aide/faq-eng.html

Canadian Hydrographic Service digital charts meet the requirements of the chart carriage regulations under certain circumstances:
CHS Electronic Navigational Charts (ENCs) meet the requirements provided they are used with an Electronic Chart Display and Information System (ECDIS).
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Old 17-07-2021, 23:42   #18
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Re: Depth indication close to shore

Id like to reiterate what one poster said

A spot depth is exactly that. It’s only good for that point and depending on the age of the survey there could be a significant error in the spot depth position
You can’t make ANY assumptions about the depth around it. There could be an rock beside it that the survey missed

Where a line was dragged to “clear “a depth this is normally indicated
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Old 18-07-2021, 03:47   #19
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Re: Depth indication close to shore

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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
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?
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Old 18-07-2021, 03:57   #20
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Re: Depth indication close to shore

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Id like to reiterate what one poster said

A spot depth is exactly that. It’s only good for that point and depending on the age of the survey there could be a significant error in the spot depth position
You can’t make ANY assumptions about the depth around it. There could be an rock beside it that the survey missed

Where a line was dragged to “clear “a depth this is normally indicated

How do you discern between spot soundings and 'minimum depth' soundings? Given the contour lines on the chart a great many more measurements must have been made in the area than shown on the map, and with a device that could measure to at least as low as 1.2m. So if the mapping procedure in specific areas if different then it would be quite useful to know it but I've never seen it in a map though. Then again I am not navigating in Canada or I might know better.
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Old 18-07-2021, 04:20   #21
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Re: Depth indication close to shore

I can say I would have someone up the bow looking in that screenshot in the blue area if you have a 1.5 draft, assuming the water is clear enough.
A bold helmsman would trust the chart completely.
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Old 18-07-2021, 04:20   #22
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Depth indication close to shore

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How do you discern between spot soundings and 'minimum depth' soundings? Given the contour lines on the chart a great many more measurements must have been made in the area than shown on the map, and with a device that could measure to at least as low as 1.2m. So if the mapping procedure in specific areas if different then it would be quite useful to know it but I've never seen it in a map though. Then again I am not navigating in Canada or I might know better.


What do you mean by “ minimum depth. “ standard practice is all depths are “ reduced “. To LAT ( lowest astronomical tide ) or MLLW ( mean lower low water )

Depths are not “ minimum “ measurements per se

Contours were largely drawn by joining spot depths.

As in everything, do not take what you see on a chart as infallible

So for example the area coloured inside say a 5m contour , does not mean everything is at 5 metres minimum it means essentially that it’s 5m maximum and you could have any depth from 0-5m in there.

The spot sounding are there to give you an “ indication “ of what the ground profile is within the contour area. But again it’s a function of the survey requirements

Modern surveys in high use areas will be perform with very accurate high tech systems and will generally be highly reliable

Older surveys in out of the way areas especially non commercial waters were often very sparse and subject to a degree of error

You have to live with the ambiguity I’m afraid.
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Old 18-07-2021, 04:32   #23
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Re: Depth indication close to shore

If you really feel that the soundings aren’t useful, ENCs allow you to turn them off. I find them useful for multiple purposes, not least of which is to keep me off the bottom. Since there are normally no discontinuous contours on the bottom outside of stray rocks and bommies that are hopefully charted, the depth adjacent to the soundings can’t really be much different.
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Old 18-07-2021, 05:45   #24
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Re: Depth indication close to shore

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Originally Posted by goboatingnow View Post
What do you mean by “ minimum depth. “ standard practice is all depths are “ reduced “. To LAT ( lowest astronomical tide ) or MLLW ( mean lower low water )

Depths are not “ minimum “ measurements per se
Modern survey techiques with things like side scan sonar and towed arrays means that now we can get complete bottom coverage.
In that situation, some charts now do show "minimum depth soundings". The bottom wiill be gridded (frequently 25x25m or 50x50m) and the minimum depth within that grid will be plotted in its exact location.


Of course that can only be done iin areas which have had reasonably recent comprehensive surveys. .


I've neer actually seen an indication on a chart of whether soundings are gridded "minimum depths" or not.
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Old 18-07-2021, 08:10   #25
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Re: Depth indication close to shore

Here’s a good introduction to hydrographic surveys

http://www.fao.org/3/i1883e/i1883e05.pdf
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Old 18-07-2021, 09:42   #26
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Re: Depth indication close to shore

TrentePieds,
Thanks! It is nice to know our Canadian neighbors have some good sense.
FWIW, in Canadian waters you are legally required to carry paper charts regardless of how sophisticated your electronic navigation instruments may be.
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Old 18-07-2021, 10:36   #27
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Re: Depth indication close to shore

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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.

Not always. Be sure to check your chart's Datum, which will specify the reference point. In the US, it's MLLW, which stands for Mean Lower Low Water (look it up if you don't understand). Other places use different references....the MLWS that Boatie refers to, sometimes the lowest astronomical tide, sometimes an average, but always specfied and quite important.
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Old 18-07-2021, 11:46   #28
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Re: Depth indication close to shore

This from an old guy who spent his life at sea, river, lake, etc.

A chart i have from Charlotte harbor, Florida, for example, has a note with something to the effect that soundings are from about 100 years ago.

Storms and tides change things from year to year, at least. I have seen coral reefs ripped apart as deep as 100’. What does a hurricane do to shallow water???

My rule of thumb: when in water shallower than twice my draft., slow down, proceed with caution. Feel my way with depth sounder and shot line of necessary.

Sand shifts, reefs grow.

Just my 2cents.
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Old 18-07-2021, 11:57   #29
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Re: Depth indication close to shore

For US coastal chart (NOAA) whether ENC or RNC (aka paper charts), the depths are at chart datum MLLW; the minimum depth a low low water. Heights are Mean High Water MHW, or the official shoreline at MHW. The numbers related to charted depths are sourced from survey data. The white space between charted depths can have mixture of source data and age, but does not include a more dense depth interval, based upon the depth of that area and the chart scale sounding interval. Remember, the chart is a scaled product, and can only display what the scale allows cartographically.

The blue tint area can vary with chart scale; some charts aka 80k might have the blue tint at 18ft (5.48m), while a 20k scale product may have the blue tint at 12ft (3.66m). Cartographically, the blue tint represent an area that could be considered dangerous, thus the color presentation.

So the next question, where are you sailing and who is the charting authority? Each Hydrographic Office has standards for data and presentation. Most have adopted the international standard of S57, soon to transition to S100 and 102 (IHO).

Now that said, the source of a lot of chart plotters is the charting authority, where the private sector firms use the public domain data (NOAA) to make their chart proprietary, such that one pays to use the product.

Lastly, every charted depth has an depth uncertainty attributed; how certain that the source authority verifies within a depth range. Charted depths are usually at the minimum 0.5m or less of variability accuracy for that specific charted depth; one needs to consider other environmental conditions (local water levels).

Bottom line, be cautious on the side of being prudent and navigate safely.
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Old 18-07-2021, 12:01   #30
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Re: Depth indication close to shore

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Originally Posted by oldbuzzard View Post
This from an old guy who spent his life at sea, river, lake, etc.

A chart i have from Charlotte harbor, Florida, for example, has a note with something to the effect that soundings are from about 100 years ago.

Storms and tides change things from year to year, at least. I have seen coral reefs ripped apart as deep as 100’. What does a hurricane do to shallow water???

My rule of thumb: when in water shallower than twice my draft., slow down, proceed with caution. Feel my way with depth sounder and shot line of necessary.

Sand shifts, reefs grow.

Just my 2cents.
A good 2 cents and common sense.
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