476
July 2019
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
The baselines measured for the Algerian triangulation, with
dates of execution, are Blida (1854, 1912), Bône (Annaba)
(1866, 1885), Oran (1885, 1910), Laghouat (1914), Ouargla
(1920), Mercheria (1932), Biskra (1932), and Navarin (1949).
The original mapping was cast on the ellipsoidal Bonne projec-
tion – the ubiquitous projection
du jour
for the Europeans of
the time. The North African (ellipsoidal) Bonne Grid Latitude
of Origin (
j
o
) = 35° 06´ N (39
G
00 N), the Central Meridian (
λ
o
)
= 2° 20´ 13.95˝ East of Greenwich, and, some time before WWII,
the False Easting and False Northing were changed from zero
to 100 km for each. Interestingly, this old Bonne Grid still influ-
ences current mapping in that grid limits of the Lambert Conic
Grids are still defined by Bonne Grid values. The sheet bound-
aries of the new Lambert Grids are commonly computed by a
reversion of the late Prof. Karl Rinner’s Bonne power series
formulae published in
Zeitschrift für Vermessungswesen
during
the 1930s. That reversion allows cartographers to compute the
intersection of a constant Bonne Grid value with a chosen arc
of the parallel or of the meridian. Those intersections then were
used to define the limits with the graticule of the Lambert Con-
ic Grids computed by John W. Hager of the Defense Mapping
Agency (
ex
Army Map Service) in 1974.
Based on original triangulations of the French Army, a
local (temporary) Astro station was established in the port
city of Oran by Capitaine Faure during 1905-1906. Station
Tafaraoui coordinates are Φ
o
= 39
G
3778.26˝ N and Λ
o
= 3
G
1532.06˝ East of Paris. The reference azimuth to Tessala is
α
o
= 62° 09´ 57.73˝ and the ellipsoid of reference is the Clarke
1880 (IGN). The observations were later adjusted and used
in the 1930 hydrographic survey of that portion of the coast
of Algeria and the port of Oran. The Lambert Conic Grid was
used by the French Navy for the hydrographic survey.
The reader will notice that I have left off the word “confor-
mal” when describing the Lambert Conic Grids of Algeria. That
is because the original systems that succeeded the ellipsoidal
Bonne Grid in 1906 were not fully conformal. There are two
original zones: for Nord Algerie, the Latitude of Origin (
f
o
) =
36° North (40
G
), the Central Meridian (
l
o
) = 2° 42´ (3
G
) East of
Greenwich, and the Scale Factor at Origin (m
o
) = 0.999625544.
For Zone Algerie Sud, the Latitude of Origin (
f
o
) = 33° 18´ North
(37
G
), the Central Meridian (
l
o
) = 2° 42´ (3
G
) East of Greenwich
also, and the Scale Factor at Origin (m
o
) = 0.999625769. The
False Origin is 500 kilometers for Eastings and 300 kilometers
for Northings
for both zones
, and the same convention as used
in the adjacent Kingdom of Morocco (
PE&RS
, June 1999). The
complete replacement of the Bonne Grid for original topograph-
ic mapping in Algeria did not happen until 1942.
During the 19
th
century, projection table computations were
performed by hand, and all formulae were commonly truncat-
ed past the cubic term to ignore infinite series terms consid-
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