PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
July 2020
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French Navy, and these were based on local astro stations
that served as origins of grids computed on the Hatt Azimuth-
al Equidistant projection. The Tabola River survey had its or-
igin at Cabrion Base North End (1936) where Φ
o
= 9° 56´ 08.1
N and Λ
o
= 13° 54´ 42.4˝ West of Greenwich.
The defining azimuth to Cabrion Base South End was α
o
= 123° 34´ 00˝, and the baseline length was 1017.537 m. I
was wondering why the French performed a survey in such
a tiny locale that did not even appear on the standard CIA
map of the country. I examined my
Carte Générale of Guinée
and noticed that there is a road to there through the town of
Koba that winds north up into the hills. Apparently, some-
thing valuable was being trucked out of those hills to the port
of Taboriya. Hager found one at “Binari 1949 (code BIN) at
the I.G.N. Astro, latitude = 10° 30´ 26.2˝ N, longitude = 14°
38´ 45.0˝ W (1) or ... 41.03˝ (2) or ... 40.0˝ (3), Clarke 1880.
Position (1) is from
Annales Hydrographiques
, 4e série, Tome
Sixième, Année 1955, p. 247. Position (2) is from
Annales Hy-
drographiques
, 4e série, Tome Dixième, Années 1959-1960, p.
65, Paris 1961. Position (3) is a footnote to (2) and refers to the
1954 survey by M. Sauzey.”
There have been some other rather curious coordinate sys-
tems devised for Guinea during the 20
th
century. Prior to and
during WWII, there were a number of military Grids that
were collectively termed the “British Grids.” These were all
documented and computed into projection tables by the U.S.
Army. One published by the U.S. Army Corps of Engineers
Lake Survey in 1943 was the Guinea Zone based on the Lam-
bert Conical Orthomorphic Projection Tables. The de-fining
parameters are Latitude of Origin, (
j
o
) = 7º N, Central Merid-
ian (
l
o
) = 0º (Greenwich), Scale Factor at the Parallel of Ori-
gin (m
o
) = 0.99932, False Northing, FN = 500 km, and False
Easting, FE = 1,800 km. The wording for the projection is
characteristically British, as is the method of presenting the
defining parameters, and the projection is definitely the fully
conformal version rather than the French Army version of the
time. Furthermore, the parameters given for the Clarke 1880
ellipsoid were the British version where
a
= 6,378,249.145 m
and
1
/
f
= 293.465. This grid continued in use by the U.S. Army
Map Service for a couple of de-cades after WWII.
Immediately after WWII , the French Institut Géographique
National devised a number of grids for l’A.O.F. useage as of
12 December 1945. The region of French Guinea was to be
covered by two fuseau, or zones: “Fuseau Sénégal” with
l
o
= 13º 30´ West of Greenwich, and “Fuseau Cote d’Ivoire” with
l
o
= 6º 30´ West of Greenwich. The scale factor at origin (m
o
)
= 0.999 and the ellip-soid of reference was to be the Interna-
tional (Hayford 1909) where
a
= 6,378,388 m and
1
/
f
= 297.
Because there was not a great deal of existing mapping in
French West Africa at the time, most datums were established
by astro shots and few classical chains of quadrilaterals had
been surveyed. The introduction of a new ellipsoid was there-
fore not of major geodetic importance to existing cartographic
work. French Navy Hydrographic surveys of the late 1940s in
Guinea were cast on the Fuseau Sénégal Grid. When I was in
college, I once read a science fiction novel about a disgruntled
cartographer on a lonely expedition to a new planet. He chose
risqué names for his gazetteer, and that fact went undiscov-
ered for many years. While perusing the report of the French
Navy hydrographic survey of the mouth of the Saloum River
(
Mission Hydrographique de la Côte Ouest d’Afrique, 11 Mai
1950 – 18 Mai 1952
), guess what I found? Yep, an Ameri-can
vulgarism and an American gangster’s name for triangulation
stations!
The U.S. Army Map Service concocted the Universal Trans-
verse Mercator (UTM) Grid System for worldwide use in
1948. France had been trying to gain an international con-
sensus for some sort of similar system, and quickly adopted
the UTM for most of its colonies. As of 30 September 1950, all
new surveying and mapping of French Guinea was done on
the UTM Grid. That situation remains to this day. The only
information available on a datum shift from the local datum
to WGS84 for the entire country of Guinea is the entry in NI-
MA’s TR8358.2 for “Dabola Datum” where ∆a = –112,145, ∆f
× 10
4
= –0.54750714, ∆X = –83m ±15m, ∆Y = +37m±15m, and
∆Z = +124m ±15m. This four-point solution was published by
NIMA in 1991. Because there is only a 1:200,000 scale map
published of Dabola, and there are no 1:50,000-scale topo-
graphic maps nearby, I am unable to find a plausible reason
for the choice of this transformation name or location other
than it is more or less in the center of the country. I have
found no other evidence of such a datum in existence.
U
pdate
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