The water levels fluctuate vertically throughout the day in response to tidal movement. Tides are influenced by the gravitational pull of the moon and the sun, along with other factors such as coastline configuration, water depth, seafloor topography, and local weather patterns. As the moon orbits around the Earth its gravitational pull creates a bulge of water. The height of the tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth. A bulge simultaneously occurs on the other side of the Earth away from the moon. These bulges represent high tide, while the areas between the bulges experience low tide. The world's oceans subsequently rise and fall in response to the position of the moon and sun.
The period between a high tide and a succeeding low tide is an outgoing or ebb tide. The period between a low tide and a succeeding high tide is an incoming or flood tide. Slack tides occur near low or high tide events when the water is relatively still.
Tides also vary by geographical location. Tides are higher when the Earth is closest to the moon and strongest overall when the moon and sun are aligned, increasing the total gravitational pull. In the northern hemisphere, tides are lowest during the winter when the Earth is furthest from the sun. The tidal range (difference between high and low tide) also varies throughout the year and is largest during the spring.
Tide data are collected at fixed monitoring stations along the coast. Stations are identified using Latitude/Longitude coordinates and gauges record water levels throughout the area. Tide gaging stations are maintained by the National Oceanic and Atmospheric Administration's National Weather Service.
Tide gauges are programmed to take measurements at 6-minute intervals with each measurement consisting of a set of 181 one-second interval water level samples centered on each tenth of an hour. The 181 samples are averaged, a three standard deviation outlier rejection test applied, and the mean and standard deviation recalculated and reported along with the number outliers. The reported measurements have 0.01 foot resolution and are stored in system memory. Timing of the system is controlled by an oscillator located in the GOES satellite transmitter which is accurate to 2 seconds per month. Every three hours the data are transmitted via GOES to the satellite downlink. In addition, telephone connections can be used to retrieve data and to interact with the system. Laptop computers are used by field personnel to check and maintain the system. The backup water level sensor is a strain gauge pressure transducer that records data on a separate data collection platform which is optically coupled to the satellite radio. Yearly second order class I geodetic levels are run from the primary sensor reference point to the local bench mark network.
For detailed information on methods for data processing read Computational Techniques for Tidal Datums Handbook, U.S. Dept. of Commerce, NOAA, National Ocean Service, Center for Operational Oceanographic Products & Services.