PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
September 2019
629
were the old watchtower and the two minarets. Plane coordi-
nates were computed on the Bulgarian Gauss-Krüger Trans-
verse Mercator projection devised by Prof. Hristow where the
central meridians were 21º, 24º, and 27º East of Greenwich
(zones 7, 8, and 9), the scale factor at origin was 0.9999 on
the central meridian, and the False Easting was 500 km. The
False Northing was zero at 4,540,198.36 m, corresponding to
41º N on the International 1924 ellipsoid until 1942 when the
equator was adopted as the origin of the ordinate axis. For
this Grid, the Northing coordinates are labeled “X” and the
Easting coordinates are labeled “Y.” Furthermore, the False
Easting for zone 8, Y = 8,500 km, and for zone 9, Y = 9,500 km.
A new framework of precise leveling commenced at the
same time as the triangulation. Completed in 1929, the net-
work consisted of 18 closed loops with a total length of 6,445
km. Leveling was run along railroads and roads as well as
to all trig points within 4 km of the main route. The levels
were referenced to mean sea level at Varna where tide gauge
observations were performed continuously between 1928
and 1931 (note that a full metonic cycle is 18.67 years). Af-
ter 1938, leveling ties with Rômania, Greece, and Yugoslavia
yielded vertical datum discrepancies. With Yugoslavia, the
mean of three connections showed the Varna Datum to be
0.60 m higher than that of the Trieste Datum on the Adriatic
Sea. With Greece, the mean of five connections showed the
Varna Datum to be 0.15 m higher than that of the Kavalla
Datum on the Aegean Sea which in turn was 0.24 m high-
er than the Thessaloniki Datum, also on the Aegean. With
Rômania, the mean of three connections showed the Varna
Datum to be 0.35 m higher than that of the Constanţa Da-
tum, also on the Black Sea.
In 1947, after WW II, the State Geographic Institute was
closed and its functions were assumed by the Military Topo-
graphic Service of the Bulgarian Army. In 1951 the General
Board of Geodesy and Cartography (GUGKK) was established,
and in 1954 the National Survey was formed. The Geodetic
“System 1950” was adopted in 1950, and is the “System 1930”
Datum recomputed on the Krassovsky 1940 ellipsoid where
a
= 6,378,245 meters and
1
/
f
= 298.3. The result of the new
computations was that a shift of approximately 3.5˝ in position
occurred, but the original azimuth at Cherni-Vrukh was con-
strained. The grid system changed such that the False East-
ings were equal to 500 km at the central meridians, and the
scale factors at the central meridians were changed to unity.
The military version of the Gauss-Krüger Transverse Merca-
tor projection used 6° belts, while the civilian version remained
the same as used for the “System 1930,” which was 3° belts.
Note that the Gauss-Krüger Transverse Mercator projection
formulae used by the Soviets for Russia and all of the Warsaw
Pact nations was the one developed by Prof. Hristow!
In the 1960s it was decided to use the “System 50” only
for military applications and to introduce a set of plane co-
ordinates for civilian use. The new system consists of four
Lambert Conformal Conic projections on the Krassovsky
1940 ellipsoid; the four overlapping zones are K3, K7, K5,
and K9. Each zone is defined with a latitude of origin and
a scale factor at origin, and each zone has a different initial
azimuth of the central meridian in order to rotate (and obfus-
cate) the grid. The parameters remain secret to this day, and
this practice (including zone rotation) is typical of many of
the former Soviet Bloc countries. The plane coordinates are
used as “local” grids, and the life span of the secrets is dwin-
dling fast as civilians introduce high-precision GPS receivers
into the country. Outside of Bulgaria, it is easy to purchase
the formerly secret Russian Military Topographic maps at
1:50,000scale of Bulgaria. These materials are secret only
within the Bulgarian borders!
The Russian “System 42” Datum is referenced to the Krass-
ovsky 1940 ellipsoid. The origin is at Pulkovo Observatory:
Φ
o
= 59° 46´ 18.55˝ North, Λ
o
= 30° 19´ 42.09˝ East of Green-
wich, and the defining azimuth at the point of origin to Signal
A is α
o
= 317° 02´ 50.62˝. This was used in Bulgaria only for
military applications.
The Geodetic “System 1942-83” Datum is actually a mis-
nomer created by a novice in Bulgarian government ser-
vice. Although “System 42” does exist for Bulgaria, viz. the
1:50,000 scale Military Topo series, the “System 1942-83” was
introduced by the Army in 1983 as the “Unified Astro-Geodet-
ic Network of Central and Eastern Europe.” This is a spe-
cial-purpose military datum and is still classified a secret.
According to Dr. Valentin Kotzev, director of the Central
Laboratory of Geodesy, in June of 2001 the Bulgarian Gov-
ernment issued a decree for the adoption of a new geodetic
system. The “Bulgarian Geodetic System 2000” is based on
the fundamental parameters of the Geodetic Reference Sys-
tem of 1980. The European Terrestrial Reference Frame 1989
(ETRF-89) is introduced as the national coordinate system.
The ETRF-89 was extended in Bulgaria during campaigns
in 1992 and 1994. The new height system is defined as part
of the Unified European Leveling Network. The horizontal
coordinates are going to be computed on a single Lambert
Con-formal Conic projection defined with two standard par-
allels and a single central meridian yet to be defined. The
cadastral system of Bulgaria is prescribed on the new Grid. I
am told that the new system is controversial. I have person-
ally noticed that controversy is usually created when two or
more geodesists get together . . . .
Prof. Hristow (1902-1979) was the son of Kiril Hristov, one
of the greatest Bulgarian poets and writers of the 20th cen-
tury. Prof. Hristow was one of the greatest geodesists of the
20th century, and he received his Ph.D. from Leipzig, Ger-
many in 1925. Some of his accomplishments have just been
chronicled in this column, but he left a record of hundreds of
papers published worldwide that speaks for itself.
Thanks to John W. Hager, Dr. Valentin Kotzev, Dr. Mom-
chil Minchev, and the Defence Geographic and Imagery Intel-
ligence Agency of the United Kingdom.
