Extratropical Water Level Guidance
What is a datum?
In the context of this site, datum refers to a vertical tidal datum.
The four datums used by this site are defined as follows. For more on
tidal datums, please see
NOAA's National Ocean Service (NOS) tidal datum definition page.
Highest Astronomical Tide|
| "The [height] of the highest
predicted astronomical tide expected to occur ... over the National Tidal
Datum Epoch (NTDE). The present NTDE is 1983 through 2001." HAT is
an estimate of the highest tide predictable strictly from the
effects of gravity.
Mean Higher High Water|
| "The average of the higher high
water height of each tidal day observed over the NTDE."
Mean Sea Level|
| "The arithmetic mean of the hourly water
heights observed over the NTDE."
Mean Lower Low Water|
| "The average of the lower low water
height of each tidal day observed over the NTDE."
Each datum has its uses:
provides an estimate of where the "grass line" is. Crossing HAT
is an indication that flooding will occur as people tend to build to the grass
line, thus it has direct human impact.
is an estimate of how high water gets each day; however it is
exceeded by the tidal cycle alone for approximately half the month. This site
uses it as a warning that waters are likely to be high, so please pay attention.
is the average water surface. Deviations from MSL provide a
precise description of unexpected amounts of water, but it is
difficult to directly tie it to human impacts due to the variability of
the tide range centered on MSL.
is an estimate of how low water gets each day and is the
standard datum used by NOS tide stations. It is useful for mariners
concerned with running aground.
What do the Datum buttons do?
The buttons allow the graphs or text to be displayed in other datums.
The reasons for this specific choice of datums were:
- HAT the datum which best approximates the grass line and
hence flooding. (Note: MHHW was omitted as HAT serves
this purpose better.)
- MSL the datum most familiar to the general public.
- MLLW to be consistent with NOS and earlier versions of this site.
NOTE: "Surge Guidance" and "Anomaly" values are changes in water
level, so are "datumless". This means that while they appear to move on the
graph when toggling datums, they actually continue to be centered on 0.