600
October 2020
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
vey covered much of western and central
Livonia in 1904-05, and eastern Livonia
and Lettgallen were covered with the 1906
and 1912 nets. Lower-order nets to fourth
order supplemented the primary chains.
These Czarist Russian surveys were com-
puted on the Pulkovo 1904 Datum or the
Yuryev II Datum, both of which are refer-
enced to the Bessel 1841 ellipsoid where a
= 6,377,397.155 m and
1
/
f
= 299.1528128.
After Latvia achieved Republic status,
the new Latvian Survey Office began to
unify these various nets in stages, before
commencing with their own survey activi-
ties in 1924. The results of this program of
unification are published in
Latvijas Valsts
Trigonometriskais
. The Pulkovo 1904 Da-
tum points were recomputed into Yuryev
II Datum based on 14 available first-order
Scharmhorst points, and this was then
put into the Senks Soldner Grid. The 1905
Russian net had already been computed
on the Yuryev II Datum, and was accept-
ed into the Old Gaizinkalns Soldner Grid
where
j
o
= 56º 52´ 15.184˝ N and
l
o
= 25º
57´ 34.720˝ East of Greenwich. No false
origin was used. Similarly, the 1906 and
1912 nets were already on the Yuryev II
Datum, and those form the Old Vitoleieki
Soldner Grid where
j
o
= 56º 40´ 08.64˝ N ,
l
o
= 27º 15´ 11.79˝ East of Greenwich, and
no false origin was used. Tenner’s survey
had been calculated on the Walbeck 1819
ellipsoid and was now recomputed on the
Bessel 1841 ellipsoid and controlled by
the Yemel’yanov-Nikifirov survey of the
Puci-Sarmes side. This had the effect of
introducing a slight swing to the orienta-
tion of the Tenner triangulation. The Lat-
vian Kulkigan and Puci systems cover this
net. For most of eastern Latvia, the earlier
Russian surveys could be used to form the
basis of the Latvian triangulation as the
station centers could be found. In Courland
and Semgallen the original triangulation
dated back to 1820 and few of the old sta-
tions could be recovered. This necessitated
a complete new survey, which was started
in 1924 from the Liepaja-Paplaka base and
extended in stages to the Puci-Sarmes side
and thence to Riga and Jelgava. The origin
of this system is the Yuryev II Datum val-
ue of Riga St Peter’s Church “tower ball”
which is the same station as the Tenner
first-order point. The Yuryev II Datum azi-
Because of the ground-
breaking work the
University of Latvia’s
Institute of Geodesy
and
Geoinformatics
published on their re-
search to develop a
one-centimeter geoid
model and the inven-
tion of a relatively
inexpensive
Digital
Zenith Camera (DZC)
by Dr. Ansis Zarins,
I traveled to Riga a
couple summers ago.
LSU’s Center for
GeoInformatics had
acquired an absolute
gravity meter along
with a couple relative
gravity meters and
various related instru-
ments and vehicles
with the same objective for the State of Louisiana. Since their DZC only
works at night with clear skies for star shots, days were open for the staff
to take me around Riga and environs. A major point of interest for me was
the origin of the Yuryev II Datum and the General Latvian Triangulation
Net Datum Origin is Riga St. Peter’s Church (top of the Riga Church spire).
The Latvian Geodesists then took me inside of the church, and a plaque
in the floor of the vestibule, directly under the plumb line of the spire dis-
played the actual Datum Origin:
By the way, their DZC worked flawlessly, and LSU purchased one for
€100K, including training. Our DZC is currently kept busy here in Loui-
siana observing the deflection of the vertical at all of our GPS CORS sites.
Deflections of the Vertical in Latvia
