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A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [1] Common chart datums are lowest astronomical tide (LAT) [1] and mean lower low water (MLLW).
Depth contour lines show the shape of underwater relief. Coloured areas of the sea emphasise shallow water and dangerous underwater obstructions. Depths are measured from the chart datum, which is related to the local sea level. The chart datum varies according to the standard used by each national Hydrographic Office.
UKC = Charted Depth − Draft-/+ Height of Tide. Ship masters and deck officers can obtain the depth of water from Electronic navigational charts. [5] More dynamic or advanced calculations include safety margins for manoeuvring effects and squat. [7] Computer systems and software can be used to manage and calculate UKC for ships and ports.
A fathom line or fathom curve, a usually sinuous line on a nautical chart, joins all points having the same depth of water, thereby indicating the contour of the ocean floor. [24] Some extensive flat areas of the sea bottom with constant depth are known by their fathom number, like the Broad Fourteens or the Long Forties, both in the North Sea.
Charts often showed a great deal of detail of features on land as well as sea. Depths were shown by individual soundings while hills and mountains were shown by hatch marks. Printing was in black and white, but some charts were hand-coloured, either to emphasise water depth or terrain, or to indicate specific features such as lighthouses.
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(The Center Square) – Street racers could soon face a rude awakening under legislation considered by state lawmakers on Thursday to require speed-limiters for certain offenses. Rep. Mari Leavitt ...
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