828
November 2015
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
Land Office are pen-and-ink water-colored maps cast on
paper, not linen.
Hassler’s survey design was not restricted to only classical
triangulations as the objective was to produce navigation
charts in order to promote and encourage shipping trade
and commerce. The hydrographic soundings of water depths
needed vertical control referenced to local mean sea level, so
tide gauges and geodetic leveling were also critical elements
to the production of reliable charts that accurately charted
the hazards to navigation. After his untimely death, Hassler’s
successors continued the original Congressional charter of
the Survey of the Coast; later changing the name to the U.S.
Coast & Geodetic Survey (USC&GS) and eventually to its
present moniker, the National Geodetic Survey (NGS). While
the coasts were being surveyed and charted, triangulations
of the interior of the nation started being surveyed by the
USC&GS also. After the Civil War in the 1860s, several
catastrophic floods occurred in the lower Mississippi River
valley. The U.S. Congress called out the Army, and the Civil
Works Division of the Army Corps of Engineers was formed
with the primary mission of flood control and maintenance
of the channels of the navigable rivers and waterways of the
United States. The Corps of Engineers undertook the General
Survey of the Mississippi River in the 1870s, and formed a
continuous chain of quadrilaterals of second- and third-order
triangulation from St. Louis, Missouri to the Gulf of Mexico.
The Corps did not perform first-order surveys as that was
the mission of the USC&GS.
(Note: this was at a time in our
nation’s history when there was very little duplication of effort
by different arms of the federal government!)
An interesting
side note to the Corps’ triangulation activities was the
original simultaneous development of an invention used in
geodetic astronomy called the “Horobow-Talcott” level, a
double-vial attachment used with astronomical theodolites.
Horobow was a British Royal Engineer that published
his new invention in a British technical paper almost
simultaneously with Captain Talcott of the U.S. Army Corps
of Engineers. Apparently, Horobow got the idea into print a
bit earlier than Talcott, so he got “first billing.” The Corps
also conducted hydrographic surveys of the river depths of
the navigable rivers and waterways which necessitated the
establishment of tide gauges. One gauge, in particular was
at Biloxi, Mississippi which was used as the primary gauge
for the establishment of the “Mean Low Gulf – (MLG)”
hydrographic datum used as the reference to this day for
dredging operations at the Mouth of the Mississippi River
and upstream to Baton Rouge, Louisiana. MLG is nowadays
merely applied as an “index” to whatever is the current
vertical datum of the USA as determined by NGS which is
–0.41 feet below the current “zero” of the North American
Vertical Datum of 1988 (NAVD88).
Hassler’s first surveys were of the New England area of the
country, and those triangulations were eventually adjusted
into what was termed “The NewEnglandDatum.” Subsequent
adjustments of increasingly larger and larger areas of the
country were named “The North American Datum.” Finally,
triangulation arcs spanned the breadth of the continent,
and “The North American Datum of 1927” was published by
the USC&GS. This spectacular geodetic feat was the first
continental horizontal datum in the history of the world, and
practically every nation of the world adopted the methods,
techniques, standards & specifications written and published
by the USC&GS. Likewise, first-order geodetic levelings had
accumulated thousands of kilometers of observations, and
the first continental vertical datum was published as the
“Sea Level Datum of 1929.” Standards, specifications, and
instruments were adopted by the rest of the world, including
the famous “Fischer Level” instrument.
Topographic mapping duties were assumed by the U.S.
Geological Survey (USGS) in the 19
th
century, and the planetable
& alidade compilations were also cast on Hassler’s Polyconic
projection. This projection continued in the copperplate
engravings of the topographic maps and charts published for
decades, and were eventually supplanted in the late 1960s.
However, other projections were known in the federal mapping
agencies for decades prior to that change but were not embraced,
apparently out of typical federal agency inertia.
When the United States joined England in the first World
War, the first Americans to “hop the pond” and assist the
British were a small group of mathematicians and geodesists
of the USC&GS. Initially they assisted in the compilation
of projection tables of the “Nord de Guerre” zone extended
from France into Belgium. Apparently, notice of these tables
were taken by some American Artillery Officers during the
war because of the convenience of plane computations for
the indirect control of cannon fire. After the war, the North
CarolinaGeodetic Surveywrote to theUSC&GS, and requested
that a set of projection tables be compiled for their state. Soon
thereafter, the idea took off and the State Plane Coordinate
Systems were computed, established, and eventually legislated
for every state of the United States. In the 1930s, much of the
country was unemployed, and President Franklin Delano
Roosevelt created the Works Progress Administration (WPA).
A number of states had federally-funded and founded geodetic
surveys, including Louisiana. Triangulation traverse surveys
were conducted with transit and steel tape by the WPA, and
second-order closures were required to be better than one part
in ten thousand (1:10,000). As a consequence, one of the design
criteria of the State Plane Coordinate Systems computed
by the USC&GS was that the maximum systematic error
of the conformal projections could not exceed 1:10,000. This
rationale was enforced because the average civil engineer and
land surveyor had very humble computational power at their
disposal, and if they neglected to correct chained distances for
the scale factor at a point, they could still produce excellent-
quality (at the time) field survey closures using computations
on the local State Plane Coordinate System Zone of their state.
As technology continued to progress including the invention
of the electronic distance meter (EDM), local engineers,
surveyors and cartographers starting discovering that the
