606
September 2017
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
influence on the shape of the primary triangulation chains.
Consequently, since observation towers had to be built
anyway, the plan for chains of man-made towers to follow
strict graticule lines was a logical system to implement.
The longitude of the IGM circle in Palermo was determined
in 1902 which was Λ
o
= –58° 25´ 25.05˝West of Greenwich and
was later transferred to Belgrano in 1910, whose value was
A
o
= –64° 13´ 10.8˝ W. This is significant in the development
of the Argentine network because of the technology of
the time. Longitude transfers were a major technological
accomplishment over long distances, and were done with
telegraph wires. The successes of such longitude (time)
transfers document the modern development and settlement
of the Americas as well as the world. When we attempt to
model the systematic error in these old networks that form
the metric base of existing large-scale topographic maps, we
need to recognize that the major component of error is time
= longitude.
Early Argentine topographic mapping followed the
European model of the times, just like the or ganization
of their mapping agency. A common projection of the time
was the “Poliédrica,” (Polyhedric). As I have pointed out
in the past, this projection is mathematically equivalent
to the local space rectangular implemented in analytical
photogrammetry software. It is an ellipsoidal version of the
gnomonic projection, and it is easy to draft a graticule from
modest projection tables. The sheets were cut on the graticule
and were 2° of latitude by 3° of longitude. That longitudinal
spacing was convenient in later years when the IGM changed
its basic projection.
Starting in 1894, Astro stations were determined
throughout the Re- public. By 1919 there was at least one
point observed in every province of Argentina except for the
northern provinces of Chaco, Formosa, and Catamarca and
the southernmost provinces of Santa Cruz and Tierra del
Fuego. The majority of these points were located at junctions
or planned junctions of railroad tracks. Some of these Astro
stations were used as local datums until the national chains
of triangulation were able to incorporate the hinterlands into
the network. Mendoza 1894 was one example. Others include
Paraná 1908 where:Φ
o
= –31° 44´ 00.7˝ S, Λ
o
= –60° 31´58.5˝
W, which is the capital of Entre Ríos Province, and Santo
Tomé 1908 where: Φ
o
= –28° 32´ 34.380˝ S, Λ
o
= –56° 02´
09.225˝ W. Another old datum that is still occasionally used in
connection with oil exploration is the Chos Malal 1914 Datum
where: Φ
o
= –37° 22´ 30.3˝ S, Λ
o
= –70° 17´ 01.8˝ W. All of
these old datums were established when the Bessel 1841 was
the ellipsoid of reference for Argentina. However, the Chos
Malal Datum of 1914 is probably used on the International
1924 ellipsoid, which was later adopted for all mapping in the
country since 1926.
In 1926, the IGM adopted a new ellipsoid as well as a
new projection for the national topographic series. The
Gauss-Krüger Transverse Mercator was selected as the new
projection and Grid system. The scale factor at origin (m
o
=
1.0), the central meridians of the belts (C. M. =
l
o
= 72°, 69°,
66°, etc., West of Greenwich), the False Easting at C. M. = 500
km, and the False Northing = 10,002,288.299 m. Note that
the central meridians of the TM belts are the same interval
as the predecessor Poliédrica. The defining parameters of
the International ellipsoid (also called the Hayford 1909
and the Madrid 1924), are a = 6,378,388 meters, and
1
/
f
=
297. By 1926, the entire province of Buenos Aires (and more)
was completely triangulated, so the present origin of the
Argentine Datum of 1926 was included. The point of origin
is the Campo Inchauspe triangulation station in the town of
Pehuajó where: Φ
o
= –35° 58´ 16.56˝ S, Λ
o
= –62° 10´ 12.03˝
W. Different from most of the world, the proper name of the
Argentine classical da- tum is the same name as the town and
the origin point: “Campo Inchauspe Datum.”
According to Mr. Rubén C. Rodriguez, by 1954 ten loops
formed by chains of double triangles running along even-
numbered meridians and parallels were completed and the
first datum adjustment of Argentina was performed. All the
angle measurements, baselines at chain intersections, and
Laplace Azimuths determined at the same intersections and
at half distances on meridian chains were included in that
adjustment. By 1969 the network had grown to 19 loops, with
a few baselines determined with electronic distance meters.
There were 5,000 direction observations from 1000 vertices
(stations), and two Argentine geodesists adjusted the net-
work at the U.S. Army Map Service in Washington, D.C.
The mean error of the least squares adjustment by variation
of coordinates was 0.4˝. Simultaneously, Dr. Irene Fisher
directed the adjustment of the South American Datum of
1969 at Army Map Service (AMS) that included the Argentine
data set in all the observations of the entire continent.
As a side note, I was assigned to AMS at the time, and later
attended a classified symposium at Cameron Station, Virginia
where Dr. Fisher presented a paper on the SAD 69. I believe I
was the only junior officer attending, but there was a veritable
constellation of stars with all the generals and admirals there.
The flag rank officers sat in front of the audience, the senior
civilian geodesists sat in the middle of the audience, and I sat
in the rear with the rest of the peasants. Dr. Fisher walked
onto the stage, and the retired Austrian school teacher-turned
Senior Geodesist of the Army Map Service and Director of the
Department of Defense Gravity Control Library stopped at
the edge of the podium. She was about 4’ 10” tall (1.5 m),
Dr. Fisher paused, turned to the front row of 2, 3, and 4-star
generals and admirals and looked at one particular Navy
admiral. She frowned, addressed that giant of a man who was
seated, and started scolding the admiral as if he were a child.
She told the admiral (who was the commander of the U.S.
atomic submarine fleet), that “Your Captains are not taking
the proper observations for their gravity measurements. If
your Captains cannot provide the data in proper scientific
detail, do not waste my geodesists’ time trying to decypher
such garbage!” The Admiral cringed, said not a word, but
he nodded acknowledgement. The audience was in stunned
