| The Web site is updated every 5 minutes from the data collection site
at the RNLI Station, Hayling Island
The link to tidal predictions (in the Latest Report page) is from the United
Kingdom Hydrographic Office Position coordinates (latitude and longitude) are
referred to WGS84 chart datum.
Tidal heights are reduced to Chart Datum, which is approximately the level
of Least Astronomical Tide.
The following table shows briefly how the reported data is derived.
| Measurement |
Sensor |
Sampling Frequency |
Averaging |
Reporting Interval |
| Wind Speed |
Anemometer |
Every second |
5 minutes |
Every 5 minutes |
| Wind Gust |
Anemometer |
Every second |
Taking max of 3-second running average |
Every 5 minutes |
| Wind Direction |
Wind Vane |
Every second |
Vector addition over
5 minutes |
Every 5 minutes |
| Air Temperature |
Thermistor |
Every second |
5 minutes |
Every 5 minutes |
| Sea Temperature |
Thermistor |
Every second |
5 minutes |
Every 5 minutes |
| Barometric Pressure |
Barometric
Pressure Transducer |
Every
minute |
5 minutes |
Every 5 minutes |
| Tidal Height |
Pressure Transducer |
Every second |
5 minutes |
Every 5 minutes |
Wave Height(average) *Note 1 |
Pressure Transducer |
Every second |
15 minutes |
Every 5 minutes |
Wave Height(maximum) *Note2 |
Pressure Transducer |
Every second |
15 minutes |
Every 5 minutes |
Wave Period *Note 3 |
Pressure Transducer |
Every second |
15 minutes |
Every 15 minutes |
*Note 1 - Wave height is calculated by
subtracting the minimum water depth from the maximum water depth. An average is
then taken over 15 minutes.
*Note 2 - Maximum wave height is calculated
by subtracting the minimum water depth from the maximum water depth. The maximum
is then recorded.
*Note 3 - Wave period is calculated by
taking a Fast Fourier Transform on 512 consecutive samples of the water depth.
A spatial maximum is then performed on the resulting histogram. This gives the
bin with the greatest frequency. The frequency is calculated from the bin number.
Wave periodicity is the reciprocal of this. The number of samples has been limited
by the available battery power at the Bar Beacon. |