Surveying today relies heavily on the global positioning system,
GPS, or similar systems from other countries,
collectively known as global navigation satellite systems.
Today's satellite-based technology can help people determine
accurate positions and elevations more efficiently than in the past.
The benefits of using GPS to determine accurate positions and elevations
will be even greater when NGS to find new datums.
Let's take a look at the planned new datums and some of their expected benefits.
Horizontal positions are currently defined by the North American Datum
of 1983 or NAD 83. This will be replaced by a new geometric datum
which provides latitude, longitude, height and time information.
Vertical positions or elevations are currently defined by the North American
vertical datum
of 1988 or NAVD 88. This will be replaced with the new vertical
or geopotential datum. The new datums will help lessen the impact of aging
physical infrastructure
like marks, that currently help define NAD 83
and NAVD 88.
The new geometric datum that will replace NAD 83
will rely completely on reference stations that continuously receive GPS
information.
NGS maintains the CORS network
which stands for continuously operating reference stations.
Users can reference scores to help accurately determine any geometric
position,
which includes latitude, longitude and some elevation information.
CORS provides elevation information computed using an ellipsoid model of the
earth.
An ellipsoid can be used as a rough approximation of the shape of the earth,
however, we know that the earth is not shaped like a perfect ellipsoid.
As a result we need another model.
One that reflects the varied topography of the earth including features such as
mountains and valleys.
This new model called a geoid, is based on the variations in gravity
across the earth's surface.
The geoid model is the basis for the new vertical or geopotential datum.
This new datum will combine the ellipsoid height
and the geoid height at the same point to calculate its elevation.
To help improve accuracy of the new vertical datum,
NGS is completing a major project called Gravity for the Redefinition of the
American Vertical Datum,
or GRAV-D.
NGS is flying over the entire US and collecting a unique,
continuous gravity data set that will help us build a better geoid model,
which will improve the accuracy of elevations measured using GPS technology.
When the new datums are adopted, positioning information for existing
structures and marks will change.
The scale of the change will vary depending on your location,
ranging from 0.5 to 1.5 meters in a horizontal direction,
and 0 to 1.3 meters in elevation.
These changes, based on our newest positioning technology and scientific
knowledge,
will provide more accurate positioning information. They will have an impacts in
areas ranging from transportation,
construction, flood plain mapping, agriculture and more.
Even though the change is years away, the time to prepare is now.
While tools will be available to facilitate the transformation of
specific position information,
it can take years to change products services or databases.
You may want to begin examining current databases and records that may be
impacted
and plan for the effort required to update.
In addition, many state and other governmental entities
passed laws requiring that public works or maps
reference NAD 83 or NAVD 88.
Those laws or contracts will need to be changed to reflect the new datum.
Starting to plan now will help you be well positioned for the change.
Resources are available and continue to be developed by NOAA's
National Geodetic Survey.