PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
May 2015
357
A
couple summers ago I decided to see
what I could find by keying in the
coordinates of various classical datum
origins throughout the world. To my surprise,
there are quite a number of datum origins that
are readily identifiable through archived imagery.
I used Google Earth™ software, and the following
observations are offered as a sample for those
interested in looking these up as well as others.
Please note that the maximum difference between
various classical datums and the WGS84 datum is
usually an East-West displacement due to the past
difficulties in maintaining accurate time based on
international standards such as Washington, D.C.,
Paris, Greenwich, Berlin, etc., from long ago. The
fact that Google™ uses a spherical approximation to
the WGS84 ellipsoid also produces some positional
errors, but primarily in the North-South component
.
Córrego Alegre
is the origin for the primary mapping
datum of Brazil, Φ
o
= 19º 50´ 15.14˝ S, Λ
o
= 48º 57´ 42.75˝ W,
referenced to the International 1924 ellipsoid. Since this is a
middle 20
th
century classical datum established with relatively
recent technology, the chronometers used were probably
calibrated with radio time signals. As a result, there’s very little
East-West systematic error between the classical coordinates
and what is geo-referenced with Google Earth ™. Also see:
(Sampaio, A.C.F., Sampaio, A.d. A. M., Datum Córrego Alegre:
o estado da arte de sua existência ou não,
Brazilian Journal
of Cartography
, ISSN 1808-0936, 10 pages).
Carthage
is the origin for the Tunesian 1925 datum,
Φ
o
= 36º 51´ 06.5˝ N, Λ
o
= 10º 19´ 20.64˝ E, referenced to the
Clarke 1880 ellipsoid. The East-West displacement is minor,
perhaps 30 meters, but the North-South displacement is
several hundred meters South. Although for decades I knew
“Carthage” was the name of the datum origin, it was not until
I used Google Earth™ that I realized that it was the name
of a cathedral. Furthermore, there is a difference in ellipsoid
parameters; the local datum is well known to have some
whopping systematic errors from old colonial surveys. Army
Map Service contracted with the French Institut Géographique
National in the 1950s to model some of these errors, and the
non-linearity of the region is legend.
DATUM ORIGINS WITH
Mirador Tower
is the origin for the Belvedere Datum of
Vietnam, Φ
o
= 21º 01´ 58.5˝ N, Λ
o
= 105º 50´ 05.95˝ E, referenced
to the Clarke 1880 ellipsoid. Established by the French back
in colonial days, the latitude component (in spite of different
ellipsoid parameters) is nearly in perfect agreement with
Google Earth™ geo-referencing to a spherical WGS84 “datum,”
but the East-West displacement puts the Hanoi Flag Tower
(Mirador Tower) about a kilometer to the East of the map tack.
The
Aya Sofia Mosque
is the origin for the Balikesir 1934
datum of Turkey, Φ
o
= 41º 00´ 30.0709˝ N, Λ
o
= 28º 58´ 52.7237˝
E, referenced to the International 1924 ellipsoid. Latitude
appears about 10 meters off (different ellipsoid parameters),
and longitude about 30 meters too far East from the finial of
the dome. Although the old grid coordinate system of Turkey
is well known to use the Aya Sofia Mosque as an origin point,
it is curious that the datum name corresponds to a province
quite a distance from Istanbul.
Johnston Memorial Cairn
is the origin for the Australian
Geodetic datum of 1966, Φ
o
= 25º 56´ 54.5515˝ S, Λ
o
= 133º 12´
30.0771˝ E, referenced to the Australian National ellipsoid.
There is close correspondence in longitude that is not
surprising for 1966, but there’s a large disparity in the North-
South component (too far south) likely due to the difference
in the ellipsoid parameters and meridian distance from the
equator. This also marks the spot in the geographic center of
Australia.
Bogotá Observatory 1935
is the origin of the Bogotá 1941
datum, Φ
o
= 04º 35´ 56.57˝ N, Λ
o
= 74º 04´ 51.30˝ W, referenced
to the International 1924 ellipsoid. There is a large circular
bare spot in the grass about 50 meters to the southeast from
the old coordinates located by the Google Earth™ map tack;
perhaps that is where the old observatory used to be located
on the top of the hilltop park.
Kandawala
is the origin of the Ceylon 1929 datum, Φ
o
= 07º 14´ 06.838˝ N, Λ
o
= 79º 52´ 36.670˝ E, referenced to
the Everest 1830 ellipsoid. The Trigonometrical station is a
concrete tower located at the intersection of a North-South
road just to the East of the Google Earth™map tack, and there
is very little latitude displacement in spite of the considerably
different ellipsoid parameters. There is a ground level image
of the tower!
Kandilli Observatory
is the origin of the Kandilli datum
of Istanbul, Φ
o
= 41º 03´ 48.899˝ N, Λ
o
= 29º 03´ 55.2˝ E,
referenced to the Bessel 1841 ellipsoid. The observatory is
