Jamestown Ferry Pier (USJT)

Extratidal Water Levels: 3-Day Observations

Click on any item in the chart legend above to hide and show that particular data trace.

Other Intervals

Last 30 days | Last 3 days | 36-hour Forecast

Legend Definitions & Datum
Sensor Observation (dashed = forecast)

The solid red line shows the observed water level (unverified data from the U.S. National Oceanic and Atmospheric Administration). The dashed red line shows our water-level forecast for the next 36 hours.

Predicted Astronomic (m30) Tide

The blue line shows our prediction of the astronomic tide. This incorporates two factors:

  1. The expected daily change in water level produced by gravitational interactions of the Earth, moon, and sun. These are the familiar tide charts, and can be predicted years in advance.
  2. A plus or minus "m30" value, which is a 30-day running mean of the actual water level observed at a tidal station. m30 represents a time-local departure from mean sea level due to seasonal variations in winds and currents, decadal fluctuations in sea level, and the long-term sea-level trend relative to land.
Residual (dashed = forecast)

The green line is the residual difference between the predicted astronomic tide and the observed water level. Values are positive when observed values exceed predicted values, and negative when observed values fall below predicted values. The solid green line shows the observed residual; the dashed green line shows our forecast for the residual over the next 36 hours.


Observed and predicted water-level values are displayed in feet and referenced to the elevation benchmark North American Vertical Datum of 1988 (NAVD88). This is the datum in which house and road elevations are typically given.

For a more detailed discussion, visit the Tidewatch Background page.


The Virginia Institute of Marine Science (VIMS) makes no warranty concerning any of the data displayed on this website ( Read our Disclaimer and Methods Disclosure statement.