Chart Datums
The depths and heights in the chart need a plane of reference: the Chart The depths and heights in the chart need a plane of reference: the Chart Dat
Datum um (se(see e intinteraeractctive ive figfigure ure belbelow)ow). . DepDepthths s are are ususuaually lly desdescricribed bed witwithh respect to low water reference planes (yielding lower charted depths, which respect to low water reference planes (yielding lower charted depths, which are safer) and heights are shown with respect to high
are safer) and heights are shown with respect to high water reference planeswater reference planes (again, yielding lower vertical clearances on the chart, which are safer). As (again, yielding lower vertical clearances on the chart, which are safer). As such, the chance that the
such, the chance that the observed depthobserved depth or vertical clearance beneath aor vertical clearance beneath a bridge is smaller than the
bridge is smaller than the charted depthcharted depth oror heightheight is rather small.is rather small.
In
Some Chart Datums and their
Some Chart Datums and their abbreviations:abbreviations:
1.
1. MHWSMHWS : Mean High Water Spring: Mean High Water Spring
2.
2. HWHW : High Water: High Water
3.
3. MHWNMHWN : Mean High Water Neap: Mean High Water Neap
4.
4. MLML : Mean Level: Mean Level
5.
5. MLWNMLWN : Mean Low Water Neap: Mean Low Water Neap
6.
6. MLWSMLWS : Mean Low Water Spring: Mean Low Water Spring
7.
7. LATLAT : Low Astronomical Tide: Low Astronomical Tide
Overview Overview
1.
1. TideTide: The vertical rise and fall of the surface of a body of water caused: 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 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 lesser extent the sun, upon different parts of the earth when the positions of the moon and sun change with respect to the earth.
the moon and sun change with respect to the earth.
2.
2. Spring TideSpring Tide: The tidal effect of the sun and the moon acting in: 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 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.
line (full moon or new moon). The range of tide is larger than average.
3.
4. RangeRange: The vertical difference between the high and low tide water: The vertical difference between the high and low tide water levels during one tidal cycle.
levels during one tidal cycle.
5.
5. Tidal DayTidal Day: 24 hours and 50 minutes. The moon orbits the earth every: 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 month, and the earth rotates (in the same direction as the moon's orbit) on its axis once every 24 hours.
its axis once every 24 hours.
6.
6. Tidal CycleTidal Cycle: One high tide plus a : One high tide plus a successive low tide.successive low tide.
7.
7. Semi-diurnal TideSemi-diurnal Tide: The most common tidal pattern, featuring two: The most common tidal pattern, featuring two hi
highghs s anand d twtwo o lolows ws eaeach ch daday, y, wiwith th mimininimamal l vavariariatition on in in ththe e heheigight ht of of successive high or low waters.
successive high or low waters.
8.
8. Diurnal TideDiurnal Tide: Only a single high and a single low during each tidal: 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 day; successive high and low waters do not vary by a great deal. Such tides occur, for example, in the Gulf
occur, for example, in the Gulf of Mexico, Java Sea and of Mexico, Java Sea and in the Tonkin Gulf.in the Tonkin Gulf.
9.
9. Mixed TideMixed Tide: Characterized by wide variation in heights of successive: Characterized by wide variation in heights of successive high and low waters, and by longer tide cycles than those of the semidiurnal 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 cycle. Such tides occur, for example, in the U.S. Pacific coast and many Pacific islands.
Pacific islands.
10.
10. Chart DatumChart Datum oror Tidal reference planesTidal reference planes: These fictitious planes are: These fictitious planes are used as the sounding datum for the tidal heights.
used as the sounding datum for the tidal heights.
11.
11. Drying HeightDrying Height: Clearance in meters (or feet in old charts) above the: Clearance in meters (or feet in old charts) above the chart datum.
chart datum.
12.
12. Charted DepthCharted Depth: Clearance in meters (or feet in old charts) below the: Clearance in meters (or feet in old charts) below the chart datum.
chart datum.
13.
13. Observed DepthObserved Depth: Height of tide + charted depth: the actual depth in: Height of tide + charted depth: the actual depth in meters.
meters.
14.
14. Height of lightHeight of light: The height of light above : The height of light above the bottom of its structure.the bottom of its structure.
15.
15. ElevationElevation: The height of the light above the chart datum.: The height of the light above the chart datum.
16.
16. Rule of TwelveRule of Twelve: Assuming a tidal curve to be a perfect sinusoid with: 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 a period of 12 hours. The height changes over the full range in the six hours bet
betweeween n HW HW anand d LW LW witwith h ththe e folfollowlowing ing frafractictions ons durduring ing eaceach h resrespecpectivtivee hour:
hour: 1/12 21/12 2/12 3/12 /12 3/12 3/12 2/12 3/12 2/12 1/12.1/12.
17.
17. Rule of SevenRule of Seven: The change from spring range to neap range can be: The change from spring range to neap range can be as
assusumemed d lilinenearar, , eaeach ch daday y ththe e rarangnge e chchanangeges s wiwith th 1/1/7t7th h of of didiffffereerencncee between the spring and neap ranges. Hence, the daily change in range = between the spring and neap ranges. Hence, the daily change in range = (spring range -
(spring range - neap range)/7.neap range)/7.