PE&RS June 2018 Full - page 337

PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING June 2018 337
SECTORINSIGHT:.gov
Education and Professional D evelopment in the G eospatial Information Science and Technology Community
By Dr. Dru Smith, NSRS Modernization Manager, NOAA’s National Geodetic Survey
The 2022 Modernization of the National Spatial Reference System
Should I be in a different lane of traffic? Is my house
in a flood plain? Have I climbed every 14k in Colo-
rado? Can the mast of my cargo ship fit below that
bridge? How quickly is that Levee subsiding? How
do I use lidar over time to monitor my city/county/state?
These questions and dozens of seemingly unrelated others all
require accurate knowledge of the position of things and, with
increasing frequency, knowledge about the changes to those
positions. By position, I mean not only the latitude, longitude
and height but also more subtle knowledge like which refer-
ence frame is being used, which type of height you’re talking
about and what definition of “zero height” is being used.
If all of these subtle details keep you up at night, you prob-
ably work for the National Geodetic Survey (NGS). Located
within the Department of Commerce’s (DOC) National Oce-
anic and Atmospheric Administration (NOAA), NGS traces
its roots back through the U.S. Coast and Geodetic Survey
(USC&GS) to the Coast Survey and ultimately to the Survey
of the Coast, established in 1807, making it the oldest scien-
tific agency in the federal government. It is the mission of
NGS to “define, maintain and provide access to the National
Spatial Reference System” (NSRS). The NSRS is the system
of latitude, longitude, height, gravity (and other positional
quantities) that is required to be used by all non-military fed-
eral geospatial agencies. It is the NSRS which enables U.S.
Army Corps of Engineers (USACE) levee surveys, U.S. Geo-
logic Survey (USGS) topographic maps and Federal Emer-
gency Management Agency (FEMA) floodplain maps to over-
lay one another with consistency. Aside from just the federal
agencies, many state governments use the NSRS too. That’s
pretty important for an agency with under 200 of NOAA’s
12,000 employees.
Technology has driven latitude, longitude and height to be
ubiquitous. Smartphones, car navigation systems and oth-
er technology put your location at your fingertips, but rarely
is the metadata and/or the accuracy behind those locations
available.
This hardly matters when the question is “what road am I
on?”, but if the question is “what lane am I in?” then things
get complicated. Similarly, when hiking, it might be inter-
esting to know how high up you are to a few tens of feet or so,
but if the issue is determining height differences between two
neighborhoods subsiding at different rates in New Orleans,
then accuracy and metadata are key pieces of information.
And this brings me back to NGS and the NSRS.
Historically, a self-consistent system for determining lati-
tude and longitude was called a “horizontal datum”, while a
similar system for determining orthometric heights (known
colloquially, but inaccurately, as “heights above mean sea
level”) has been called a “vertical datum”. The term “hori-
zontal datum” has, however, been replaced in geodetic usage
by “reference frame” as space techniques and 3-dimensional
positioning became the way to get latitude and longitude.
NGS has established, either alone or in conjunction with
neighboring countries, a series of country-wide horizontal and
vertical datums over the years for the last century. However,
the last time these were updated was in the 1980s (the North
American datum of 1983, or NAD 83 and the North American
Vertical Datum of 1988, or NAVD 88), just before the advent
of the Global Positioning System (GPS). Why is that import-
ant? Because it means that decisions like “where should the
center of Earth be, when computing latitude and longitude?”
and “how can we make a height system be self-consistent over
a continent” were all made and implemented before satellites
began orbiting Earth’s center of mass or measuring its grav-
ity field with unprecedented accuracy. Without belaboring
the point, the 30+ year old datums of the NSRS contain sys-
tematic errors that exceed meters, while the accuracy needs
and availability of accuracy to the public has been growing
steadily towards centimeters (and smaller!). For this reason,
NGS embarked upon a plan called NSRS Modernization in
2007, with the intent of replacing NAD 83 and NAVD 88 con-
currently in the year 2022. What makes this problem par-
ticularly difficult is the dynamic nature of the Earth. To be
specific:
• tectonic plates rotate, changing latitudes and longitudes
by a few centimeters every year
Photogrammetric Engineering & Remote Sensing
Vol. 84, No.6, June 2018, pp. 337–338.
0099-1112/18/337–338
© 2018 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.84.6.337