As the earth’s climate is changing, rising sea levels are affecting low-lying coastal
cities in the form of more frequent tidal inundation and coastal flooding.
In many places, both the frequency and severity of extreme flood events have been rising,
and scientists expect this trend to continue or worsen in the future.
Even nuisance flooding events, minor flooding which causes public inconvenience, have dramatically
increased in frequency over the last 50 years.
To better plan appropriate responses, local decision makers need coastal intelligence
in the form of accurate data to gauge the likely impacts of higher water levels.
Geospatial infrastructure provides these data in the form of very precise and accurate positional
information from both land and water level observations.
On land, Continuously Operating GPS Reference Stations, or CORS use permanently-mounted
GPS antennas to receive signals broadcast from global positioning satellites orbiting
the earth.
Using CORS in coordination with their own GPS equipment, surveyors are able to obtain
high-accuracy coordinates and elevations at any point of interest at the 1-2 cm level
of precision.
These coordinates are expressed with respect to national reference frames.
These are defined and maintained by NOAA’s National Geodetic Survey as part of the National
Spatial Reference System (NSRS).
But in coastal communities, land elevations only tell half the story.
To predict coastal flooding, these elevations must be referenced accurately to local water
level observations.
Tide stations, another key component of geospatial infrastructure, provide this detailed tracking
of local water levels over time.
The long term averages of water levels at different stages of the tide are used to define
local tidal datums.
These include mean lower low water, mean sea level, mean higher high water and others.
Due to hydrodynamic effects, these datum values are only accurate for a limited area along
the coast, in the vicinity of the tide station.
Tidal datums, computed for each tide station, are referenced to a local network of permanently
fixed points called tidal bench marks.
These bench marks are used to measure the stability of the tide station relative to
the land over time.
By using GPS on these bench marks, and by referencing to nearby CORS, surveyors can
relate the tidal datums from tide stations to the NSRS.
By connecting tidal datums to the NSRS, they can be accurately related to nearby land elevations.
With this geospatial information, community planners can assess recurrent high water risks
to roads, schools, hospitals and other critical infrastructure.
They can also predict how sea level rise will change the risk of future flooding, and identify
areas of vulnerability.
Models can fill in data gaps where critical elevation and water level information is not
currently available, however this information is less accurate.
Communities may decide to strategically invest in new geospatial infrastructure as a way
to increase the precision and accuracy of their local data.
Precise and accurate measurements from geospatial infrastructure improve coastal resilience
and allow communities… - to have confidence that levees will be built
high enough, - to understand how long evacuation routes
will remain dry, - and to update building codes to protect
homes and businesses from coastal flooding.
Using the geospatial information already available through NOAA, combined with strategic local
investments, geospatially-enabled communities can more confidently define the relationship
between elevations and water levels, and better plan for an uncertain future.