Larger/Smaller numbers on chart

[BugzyCan]

Looking at a chart from OpenCPN, it sometimes has the high/low depth listed small/large, and sometimes large/small. Can someone tell me why this is? Oh, maybe this is meant to represent a decimal, not high/low?

[StuM]

You second guess is correct. They are not high/low or low/high. They are single numbers and are the depths below chart datum in decimal units.


The one on the left is 2.8 ( i.e. a bit less than 3) the one on the right is 26.8 (i.e. a bit less than 27)



Depending on settings , those numbers may be feet, metres or fathoms. Based on the shading, I'd guess it is metres.

[StuM]

Note the values with an underscore beneath the whole number are heights above chart datum. i.e. likely to be exposed at low tide.

[BugzyCan]

Originally Posted by StuM

You second guess is correct. They are not high/low or low/high. They are single numbers and are the depths below chart datum in decimal units.

The one on the left is 2.8 ( i.e. a bit less than 3) the one on the right is 26.8 (i.e. a bit less than 27)

Depending on settings , those numbers may be feet, metres or fathoms. Based on the shading, I'd guess it is metres.

Meters is correct. Do the numbers change depending on the current tide level, or is that determined some other way? I know I can turn tides on/off, and it displays as a bar.

[sv_pelagia]

Originally Posted by BugzyCan

Meters is correct. Do the numbers change depending on the current tide level, or is that determined some other way? I know I can turn tides on/off, and it displays as a bar.

OPENCPN:
NO, charts give the depth at datum (at 0 tide). Turning tides on/off does not change this (that simply toggles tide stations on/off).

How do you determine depth at current level? Add the tide level to the charted depth.

[StuM]

Originally Posted by BugzyCan

Meters is correct. Do the numbers change depending on the current tide level, or is that determined some other way? I know I can turn tides on/off, and it displays as a bar.

No, the numbers never change they are depths below "chart datum".


Chart datum depends on what country produced the chart.


For Canadian charts, it can vary
Canadian Hydrographic Service

"CHART DATUM is the plane of reference for soundings and is stated in the title of each chart. Supplementary information may be given in a table, note or graph. Chart datum is the low water plane to which are referenced the depths of water over features permanently covered by the sea and the elevations of those features which are periodically covered and uncovered. Some low waters can be expected to fall below chart datum. This only occurs under certain astronomical conditions, but, where the range of tide is small, meteorological disturbances may cause even average tides to fall below chart datum.
In tidal waters, the CHS uses the level of Lower Low Water, Large Tide or Lowest Normal Tide as its reference plane for chart datum. In non-tidal waters, a low water level is adopted as datum."

[StuM]

Added: More info
https://www.tides.gc.ca/eng/info/verticaldatums


For navigational safety, depths on a chart are shown from a low-water surface or a low-water datum called chart datum. Chart datum is selected so that the water level will seldom fall below it and only rarely will there be less depth available than what is portrayed on the chart. The following three criteria place somewhat more restriction on its choice: chart datum should be:

1. so low that the water level will but seldom fall below it,
2. not so low as to cause the charted depths to be unrealistically shallow, and
3. it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities.

On most Canadian coastal charts the surface of lower low water, large tide, or LLWLT , has been adopted as chart datum, but the term " lowest normal tide, " or " LNT, " has been retained on the charts since it encompasses a variety of other choices for chart datum on some older charts.

[BugzyCan]

Excellent, thank you for that information. That helps a lot.

[cal40john]

Chart #1 is the reference to find most of what you need -

Canadian Hydrographic Service

[SV__Grace]

I always wondered about that, thanks for asking.

For users of Navionics-

When you are seeing a depth number [sounding] with multiple digits where it looks like you have one larger and one smaller number, treat it as a decimal unless you have your unit set to Fathoms.

Meters or Feet:
Larger (regular font) number = full depth unit (Meters or Feet)
Smaller (subscripted) number = 1/10 depth unit
Where your depth unit is set up on the chart plotter or mobile app as feet or meters.



In the example above, the depth units set to feet, you have depth soundings of 32.8 feet (approximately 32 feet 10 inches).
Fathoms and feet: depths are specified in fathoms (units of six feet of depth) and feet. So, large digit 2 and small digit 4 would be 2 fathoms 4 ft (or 16ft total).

[grantmc]

Here is what I feel is an excellent reference text. Not only is know how to read a chart a wonderful skill, it could save lives.

Nigel Calder's How to Read A Nautical Chart

In addition here is a link to my Dropbox folder in which you will find downloadable pdfs of both the USA and New Zealand International chart symbols and abbreviations.