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.
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.