338
June 2018
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
SECTOR
INSIGHT:
.
gov
• glacial isostatic adjustment is both bulging the crust of
the North American continent by a few centimeters per
year near Hudson bay while also changing the gravity
field in the region
• the crust in New Orleans is subsiding at a few milli-
meters to centimeters per year, but the changes are so
localized that one neighborhood might have a different
subsidence rate than another one nearby.
All of these changes are unimportant when accuracy needs
are at the “few meters” level. But that time has passed. So,
while NGS can provide tools to use GPS to position some-
thing to a centimeter, such a position is useless unless it is
also
time-tagged
. This is a huge change from any historic
part of the NSRS in the past. NAD 83 and NAVD 88 were
basically “static” systems, with hundreds of thousands of
points, each with a single published coordinate. Attempts
to show the true movement of a point, or its location through
time, were only partially implemented. With the NSRS Mod-
ernization in 2022, time dependency will be fully embraced
as the cost of centimeter-accuracy and built into the system
from the ground up.
What will the system look like? To begin, NGS will define
four “terrestrial reference frames”, one for each continent
upon which large U.S. civilian populations reside. They are:
• North American Terrestrial Reference Frame of 2022
(NATRF2022)
• Pacific Terrestrial Reference Frame of 2022 (PATRF2022)
• Caribbean Terrestrial Reference Frame of 2022
(CATRF2022)
• Mariana Terrestrial Reference Frame of 2022
(MATRF2022)
Each reference frame will rotate with its respective plate, so
that the largest time-dependent part of latitude and longi-
tude can be mitigated. Residual velocities will be modeled
in an Intra-Frame Velocity Model (IFVM) for users who need
to compare surveys and maps across the years. For heights,
NGS will replace NAVD 88 (and all other vertical datums in
the NSRS) with the North American-Pacific Geopotential Da-
tum of 2022 (NAPGD2022). This will be a self-consistent sys-
tem of heights, geoid, gravity and deflection of the vertical.
What will be the biggest changes? First, NGS will advocate
that GPS-based Continuously Operating Reference Stations
(CORS), rather than passive geodetic control marks, be the
primary access to the NSRS. This is simply because, of all
components in the NSRS, only the CORS are checked daily
for their position and velocity. Secondly, orthometric heights
will come primarily from GPS surveys. Third, and most im-
portantly, NGS will cease advocating the concept of “a single
coordinate for a single point” as was done historically. Rath-
er, every time a point is re-surveyed, a new coordinate may be
computed, and over time, the historic movement of that point
will become clear. Thus it is hoped that subsidence values
will become obvious, and help coastal managers understand
and predict
future
subsidence.
There is a tremendous amount of work to do, not only from
the scientific development side but from the education side
as well. That’s why NGS has been promoting this change for
11 years already. But heads up, it’s just 4 years away now!
Readers interested in greater detail than provided here are
directed to NGS New Datums page:
See you in 2022!
Author
Dr. Dru Smith was Chief Geodesist of NOAA’s National Geo-
detic Survey from 2005 until 2015, and now serves as the
NSRS Modernization Manager, responsible for overseeing
the replacement of NAD 83 and NAVD 88. He first entered
NGS in 1995 after receiving his Ph.D. in geodetic science
from The Ohio State University. He has published over 40
papers on research topics ranging from geodetic surveying to
ionosphere determination to geoid modeling.
He is a member of the American Geophysical Union, the
International Association of Geodesy and is a Fellow of the
American Congress on Surveying and Mapping (now the Na-
tional Society of Professional Surveyors) and has previously
served on the Board of Directors for the American Association
for Geodetic Surveying. He has received the Bronze, Silver
and Gold medals from the Department of Commerce for out-
standing federal service.
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