Surveyors have worked for decades to create a tremendously accurate reference
system across the United States
that helps define such things as property boundaries and flood plains
and is used in many other applications.
Today's geodetic datum are defined using data that has been collected since the
1800s
and augmented with data and models that use today's state-of-the-art technology.
Advances in science and technology like the global positioning system
or GPS, let us build models and tools with greater precision and accuracy
than was possible in the past.
The North American datum of 1983 called NAD 83
is the horizontal datum the United States. NAD 83
is based on a set of approximately 250,000 points. These points have
positions that were determined
both through the extensive triangulation network established over decades
and newer GPS observations.
All of these points were constrained to a geocentric origin,
specifically the center of an ellipsoid model,
which is a commonly used mathematical figure approximating the shape of the
earth.
With the help of more recent satellite-based observations,
we now know that NAD 83 is misaligned to the most modern definitions of
latitude and longitude
by over 2 meters. Our ability to detect this misalignment
comes from the current more accurate understanding up the true center of the
earth.
This offset causes many inconsistencies and mapping products referenced to NAD 83
versus the datum based on the true center of the earth.
Looking at the vertical reference frame the North American vertical datum
of 1988, or NAVD 88,
is based on an extensive network of leveling obervations from across the
country.
To make the network of observations internally consistent
one value was held constant: the local mean sea level value
at father point Rimouski, Quebec, Canada.
Every leveling observation includes some random observational error
as well as theoretical errors. While this averages out to less than half a
millimeter per mile,
these errors actually accumulate to a total error
of more than one meter between Miami Florida and Seattle Washington.
Additionally the NAD 83 positions and the NAVD 88 elevations
are inconsistent with the more accurate up-to-date models
that have evolved with advances in technology. These up-to-date models allow
us to create a more accurate reference frame then the surveying methods and tools
used in the past.
Because we still need to use NAD 83 positions
and the NAVD 88 elevations today we have to artificially alter the updated models
to fit with the existing reference systems.
Much of the NAD 83 and NAVD 88 reference systems
rely on marks in the ground that you must be able to physically access.
Unfortunately this physical infrastructure deteriorates over time.
Marks often get disturbed. Sometimes this is accidental
as when landowners inadvertently destroy a mark on their property.
Other times marks are destroyed when roads are rebuilt or widened.
Marks are intended to be very stable so we attach them to the earth asw
permanently as possible.
However physical events like front heave
or subsidence can move a mark if it is not attached to something more stable
like a deeper bedrock structure. Any motion of a mark requires that coordinates
be updated over time
an expensive and time-consuming task.
We do have strategies for replacing marks that are about to be disturbed or
destroyed.
but it takes time and effort. It would be prohibitively expensive to duplicate the
level of physical effort
and labor used to create our current datums.
To address the problems with the physical infrastructure of our current
datums,
and to integrate information from the most current technology,
NOAA's National Geodetic Survey plans to establish new datums
to replace NAD 83 and NAVD 88.