The chart datum is the level of water that charted depths displayed on nautical charts are measured from. The chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.
Depths are usually described with respect to low water reference planes yielding lower charted depths, which are safer and heights are shown with respect to high water reference planes (again, yielding lower vertical clearances on the chart, which are safer). As such, the chance that the observed depth or vertical clearance beneath a bridge is smaller than the charted depth or height is rather small.
Here are some chart datums and their abbreviations:
MHWS : Mean High Water Spring
HW : High Water
MHWN : Mean High Water Neap
ML : Mean Level
MLWN : Mean Low Water Neap
MLWS : Mean Low Water Spring
LAT : Low Astronomical Tide
TERMS
Tide: The vertical rise and fall of the surface of a body of water caused primarily by the differences in gravitational attraction of the moon, and to a lesser extent the sun, upon different parts of the earth when the positions of the moon and sun change with respect to the earth.
Spring Tide: The tidal effect of the sun and the moon acting in concert twice a month, when the sun, earth and moon are all in a straight line (full moon or new moon). The range of tide is larger than average.
Neap Tide: This opposite effect occurs when the moon is at right angles to the earth-sun line (first or last quarter). The range of tide is smaller than average.
Range: The vertical difference between the high and low tide water levels during one tidal cycle.
Tidal Day: 24 hours and 50 minutes. The moon orbits the earth every month, and the earth rotates (in the same direction as the moon's orbit) on its axis once every 24 hours.
Tidal Cycle: One high tide plus a successive low tide.
Semi-diurnal Tide: The most common tidal pattern, featuring two highs and two lows each day, with minimal variation in the height of successive high or low waters.
Diurnal Tide: Only a single high and a single low during each tidal day; successive high and low waters do not vary by a great deal. Such tides occur, for example, in the Gulf of Mexico, Java Sea and in the Tonkin Gulf.
Mixed Tide: Characterized by wide variation in heights of successive high and low waters, and by longer tide cycles than those of the semidiurnal cycle. Such tides occur, for example, in the U.S. Pacific coast and many Pacific islands.
Chart Datum or Tidal reference planes: These fictitious planes are used as the sounding datum for the tidal heights.
Drying Height: Clearance in meters (or feet in old charts) above the chart datum.
Charted Depth: Clearance in meters (or feet in old charts) below the chart datum.
Observed Depth: Height of tide + charted depth: the actual depth in meters.
Height of light: The height of light above the bottom of its structure.
Elevation: The height of the light above the chart datum.
Rule of Twelve: Assuming a tidal curve to be a perfect sinusoid with a period of 12 hours. The height changes over the full range in the six hours between HW and LW with the following fractions during each respective hour: 1/12 2/12 3/12 3/12 2/12 1/12.
Rule of Seven: The change from spring range to neap range can be assumed linear, each day the range changes with 1/7th of difference between the spring and neap ranges. Hence, the daily change in range = (spring range - neap range)/7.
