Tidal Heights

Tides are caused by the gravitational pull of the moon and sun on the oceans. In general, there are two high and two low tides each day. (However, this can vary in different locations.) Coastal areas that are susceptible to tides have both fluctuations in water depth and changing tidal currents. In most areas, tides ebb (flow out) and flood (flow in) twice a day. ( Go Back )

A tidal chart shows the depth of the water over a period of time. Peak current flows are on the mid-slope of the chart, as the slowest current flows at the peaks (high tide) and troughs (low tide). To the right, is a tidal chart adapted from the NOAA website "Tides Online". Notice the difference between the predicted tide (blue) and the actual tide in red. This difference can be caused by changes in barometric pressure, winds blowing toward shore, and regional rainfall filling local rivers and straits.

NOAA Tides Online (You can view tide tables and graphs from various locations around the USA): http://tidesonline.nos.noaa.gov/monitor.html

If a tidal chart is not available, one may use the rule of twelfths to estimate the rate of rise of the water. First calculate the height of the tide as the difference in water height at the tide's peak and trough. Divide this number by 12. If one assumes the tide will rise over a 6 hour period, it can be estimated to rise 1/12 the first hour, 2/12 the second , 3/12 the third, 3/12 the forth, 2/12 the fifth, and 1/12 the sixth hour.

Tidal heights may be added to the soundings (water depth) found on navigation charts to estimate the water depth at any point in time. Several caveats should be heeded.

	Make sure you are using the same units in the tide tables and graphs as on the nautical charts.
	Make sure Zero on the tide tables and graphs corresponds to the sounding levels listed on the chart. The soundings on different charts can be measured from a different reference water level. The most common method in the US is to use the Mean Lower Low Water Level. This is the average level of the lowest daily tide level over 19 years. Read More About Methods of Reporting Chart Sounding
	Since a mean level is used, the water level can be lower that that listed on the chart. Tide tables and graphs can have negative water levels. This usually happens during a "spring tide" where the sun and moon are at 180 degrees or 0 degrees to each other. The least tide variation occurs when the sun and moon are at 90 degrees to each other. This is called a neap tide.
	Tide tables and graphs only provide estimates of water height. Always calculate in a safety factor.

NAOO Tides Online (You can view tide tables and graphs from various locations around the USA): http://tidesonline.nos.noaa.gov/monitor.html

Free Tide Prediction PDA Software for Plam OS : http://www.toolworks.com/bilofsky/tidetool.htm