Editing Tide (section)

== Navigation ==
[[File:Tide legal use.gif|thumb|left|US civil and maritime uses of tidal data|alt=Chart illustrating that tidal heights enter in calculations of legally significant data such as ''boundary lines'' between the high seas and territorial waters. Chart shows an exemplar coastline, identifying bottom features such as longshore bar and berms, tidal heights such as mean higher high water, and distances from shore such as the 12 mile limit.]]

Tidal flows are important for navigation, and significant errors in position occur if they are not accommodated. Tidal heights are also important; for example many rivers and harbours have a shallow "bar" at the entrance which prevents boats with significant [[Draft (hull)|draft]] from entering at low tide.

Until the advent of automated navigation, competence in calculating tidal effects was important to naval officers. The certificate of examination for lieutenants in the [[Royal Navy]] once declared that the prospective officer was able to "shift his tides".<ref>{{cite book |title=The Mariner's Mirror |url=https://books.google.com/books?id=lagPAAAAIAAJ&q=%22shift+his+tides%22 |author=Society for Nautical Research |date=1958 |access-date=2009-04-28 |via=[[Google Books]] |archive-date=2023-09-16 |archive-url=https://web.archive.org/web/20230916153531/https://books.google.com/books?id=lagPAAAAIAAJ&q=%22shift+his+tides%22 |url-status=live }}</ref>

Tidal flow timings and velocities appear in ''tide charts'' or a [[tidal stream atlas]]. Tide charts come in sets. Each chart covers a single hour between one high water and another (they ignore the leftover 24 minutes) and show the average tidal flow for that hour. An arrow on the tidal chart indicates the direction and the average flow speed (usually in [[Knot (unit)|knots]]) for spring and neap tides. If a tide chart is not available, most nautical charts have "[[tidal diamond]]s" which relate specific points on the chart to a table giving tidal flow direction and speed.

The standard procedure to counteract tidal effects on navigation is to (1) calculate a "[[dead reckoning]]" position (or DR) from travel distance and direction, (2) mark the chart (with a vertical cross like a plus sign) and (3) draw a line from the DR in the tide's direction. The distance the tide moves the boat along this line is computed by the tidal speed, and this gives an "estimated position" or EP (traditionally marked with a dot in a triangle).

[[File:Tidal Indicator Delaware River ca1897.jpg|thumb|Tidal Indicator, Delaware River, Delaware c. 1897. At the time shown in the figure, the tide is {{frac|1|1|4}} feet above mean low water and is still falling, as indicated by pointing of the arrow. Indicator is powered by system of pulleys, cables and a float. (Report Of The Superintendent Of The Coast & Geodetic Survey Showing The Progress Of The Work During The Fiscal Year Ending With June 1897 (p. 483))]]

[[Nautical chart]]s display the water's "charted depth" at specific locations with "[[Echo sounding|sounding]]s" and the use of bathymetric [[contour line]]s to depict the submerged surface's shape. These depths are relative to a "[[chart datum]]", which is typically the water level at the lowest possible astronomical tide (although other datums are commonly used, especially historically, and tides may be lower or higher for meteorological reasons) and are therefore the minimum possible water depth during the tidal cycle. "Drying heights" may also be shown on the chart, which are the heights of the exposed [[seabed]] at the lowest astronomical tide.

Tide tables list each day's high and low water heights and times. To calculate the actual water depth, add the charted depth to the published tide height. Depth for other times can be derived from tidal curves published for major ports. The [[rule of twelfths]] can suffice if an accurate curve is not available. This approximation presumes that the increase in depth in the six hours between low and high water is: first hour — 1/12, second — 2/12, third — 3/12, fourth — 3/12, fifth — 2/12, sixth — 1/12.