On paper maps, the general information block of the chart shows the chart title, usually the name of the navigable water in the covered area, the type of projection and the unit of measurement (1:40,000, Soundings in Feet).
http://www.nauticalcharts.noaa.gov/mcd/learnnc_soundings.html
The numbers that appear all over the water portions of the chart are soundings.
They show how deep the water is at that point.
Large block letters at the top and bottom of the chart indicate the unit of measurement used for soundings.
So NOAA SOUNDINGS can be in FATHOMS (1 Fathom = 6 feet), FEET, or METERS (1 Meter = 3.28 feet).
Most are in feet but it depends on location and age : U.S. charts began switching from feet and fathoms to soundings in meters using the international standard for measuring depth.
Soundings in meters can be quickly converted to feet, by dividing by 3: the math isn't accurate, since a meter is slightly more than 3 ft. long, but it gives a quick, conservative estimate of the depth in feet.
SOUNDINGS IN FATHOMS indicates soundings are in fathoms or fathoms and fractions.
SOUNDINGS IN FATHOMS AND FEET indicates the soundings are in fathoms and feet.
For spot depths, the fathom will be the larger number while the fraction of fathoms or feet will be the smaller subscript number.
For example, you might see a spot depth on the chart of 42.
This indicates a depth of 4 fathoms and, if the soundings are in fathoms and feet, the small 2 indicates two feet, so in this case the depth is 4 fathoms plus 2 feet or 26 feet.
A similar convention is followed when the SOUNDINGS are in METERS or meters and tenths.
The larger number on the spot depths are in meters while the smaller number is tenths of a meter.
What about the level of water (tidal datum) ?All depths indicated on charts are reckoned from a selected level of water called the chart sounding datum.
http://www.nauticalcharts.noaa.gov/mcd/learnnc_soundingdatum.html
For NOAA charts the datum is usually Mean Lower Low Water (MLLW).
This is the average height of the lower low waters of each tidal day over a 19-year period.
Older charts use Mean Low Water and foreign charts can use other tidal datums.
The MLLW datum is lower than Mean Low Water (MLW). It is more conservative with the goal of being more safe.
You must understand the soundings and tidal datum for the chart you are using and take into consideration the height of tide and the tidal range to avoid grounding.
You also must be aware that there will be certain times of the year or weather conditions which can make the actual depth less than that depicted on the chart.
Note : If the tide table show a minus sign (like, "-1") that means that you need to subtract that number from the chart's water depth.
Similar depths are connected by dark blue Contour Lines (isolines or isobaths).
If there's no depth shown at a particular location, you can glance along the contour lines nearest the location until you see a number for the depth.
Typically, the area inside either the three fathom or one fathom contour line is tinted blue to provide quick identification of shoal water.
In the Marine GeoGarage, as we display the different charts in a seamless process, the general information for the chart may be masked by another chart.
So it could be impossible to get the info about the soundings unit.
In this case, waiting the future implementation of a user tool allowing to know the metadata associated with each chart, we advice the user to display with the 'coverage button' the reference of the map and to go to the NOAA website to read the legend and notes to confirm the unit of measurement used for the soundings :
Ex.: for chart 50 (soundings units in meters, datum MLLW) :
YouTube : Reading the nautical charts depth note
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