PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
November 2015
829
first-order triangulation stations had some significant errors
in their published coordinates. The National Academy
of Science published a paper documenting the apparent
deficiencies of the NAD27, and recommended that Congress
appropriate necessary monies to re-compute the old classical
datum along with additional scale control through a proposed
Transcontinental Traverse that would span the nation from
east to west in several routes using first-order theodolites
and EDM instruments. This was in the early 1970s, and the
U.S. had entered the Space Age, including the invention and
implementation of artificial geodetic satellites. The USC&GS
had the BC-4 cameras (Army surplus) for intercontinental
triangulation, the Army had the ANNA satellite and later the
SECOR system for geodetic triangulation, but these systems
took weeks or months of observations to acquire sufficient
observations to compute a single latitude, longitude, and
ellipsoid height!
(This was a significant improvement over
the classical method that took years to decades.)
During
the middle 1970s, the Navy TRANSIT satellite system and
geoceiver were declassified for public use. The USC&GS
acquired some of these electronic instruments including the
famous Magnavox 1502 geoceiver that enabled observations
referenced to the World Geodetic System Datum of 1972.
The observations obtained at collocated Transcontinental
Traverse stations showed that there were still significant
residual systematic errors present! Nevertheless, the re-
computed North American Datum of 1983 included all
of the original classical triangulation observations, the
Transcontinental Traverse observations, and the occasional
TRANSIT observations. The majority of the resultant errors
of the NAD83 were now at least at the sub-meter level.
For a number of years into the middle 1980s, individual
states entered into cooperative agreements with the NGS to
observe a representative sampling of stations to establish
the High Accuracy Reference Network (HARN) and similar
names and acronyms with TRANSIT observations. The U.S.
Air Force commenced the NAVSTAR system of artificial
satellites that promised “real-time” positioning and dynamic
locating called the Global Positioning System (GPS). It was
found that a number of academics, some in geodesy but most
in geophysics could obtain geodetic-quality positioning using
a differential approach to positioning. The U.S. Coast Guard
started installing Permanent GPS receivers with broadcast
capabilities to commercial shipping activities, and the
concept of Continuously Operating Reference System (CORS)
sites was born. Ultra-precise positioning was now available
to anyone with access to a dual-frequency receiver!
Problems had been developing with the vertical datum
in the U.S. also, and observations were performed through
several transcontinental levelings in what was termed “Basic
Net A.” The re-adjustment of the national vertical datum
was accomplished and published in the 1990s as the North
American Vertical Datum of 1988, but Congress had taken
notice of the enormous cost of the new observations of “Basic
Net A.” Congress directed the NGS to come up with a solution
to the cost of what had become approximately $1,500 per mile
of first-order differential leveling; a cost that would continue
to skyrocket as the price of labor continued to rise with the
cost of living in the United States.
In 1992, the NGS published the plan for the National
Height Modernization Program that would employ the first
newmethodology to measuring elevations in over 5,000 years:
GPS Leveling
. The Soviet Union was kaput. The possibility
of WWIII breaking out any day with nuclear warfare had
faded, and peace was breaking out everywhere (for a while,
anyway). The mathematical model of the Earth’s gravity
field, the GEOID, was declassified by the U.S. Department
of Defense as it was no longer needed to be Top Secret for
the targeting and guidance of Inter Continental Ballistic
Missiles. Congress approved NGS’s plan and the installation
of numerous GPS CORS sites commenced both by NGS and by
private/public entities. The implementation of the GEOID is
well-employed if not well understood in exquisite detail, and
ellipsoid heights are now converted to equivalent orthometric
elevations referenced to the NAVD88 now performed on a
daily basis nation-wide. The reliability of the transformation
from “h” to “H” is not of as precise a level of accuracy as one
would want; it is now in the decimeter range for much of the
continental U.S. However, the cure for the GEOID malady
is in the works. GRAV-D is a current program of the NGS in
that airborne gravity surveys are being flown currently for
the entire United States with the plan to develop a three-
dimensional datum, including a vastly-improved and accurate
GEOID for a new datum of the United States in 2022. It is
hoped that the new datum will employ a centimeter-level (on
the average) accurate GEOID for the entire continent.
For some areas of continuous crustal motion, it is expected
that a quasi-geoid will be a necessary enhancement for the
NAD22 as is being observed for Louisiana by LSU with
terrestrial gravity observations (relative and absolute)
and 100+ CORS sites throughout the state and the Gulf
Coast operated and maintained by the LSU Center for
GeoInformatics.
This concludes the Grids and Datums coverage of the
countries of the world that I have written about since 1997.
This last column was written off the top of my head with
zero look-up or references and just personal memory of what
has happened (geodetically) in the U.S. since the late 1960s.
Next month will be an epilog and summary of this series of
columns. With respect to this history of geodetic operations
in the United States, much is due to the copious publications
of the USC&GS and the NGS, including in particular by the
late Joseph Dracup, a wonderful man that was very kind to
me. Contrary to what some of my students believe, Ferdinand
Hassler was not a buddy of mine – he was before my time.
The contents of this column reflect the views of the author, who is
responsible for the facts and accuracy of the data presented herein.
The contents do not necessarily reflect the official views or policies of
the American Society for Photogrammetry and Remote Sensing and/
or the Louisiana State University Center for GeoInformatics (C
4
G).
