Invention of the Tellurometer – a giant leap in measurement

Document Sample
Invention of the Tellurometer – a giant leap in measurement Powered By Docstoc
					          Measurement and Instrumentation

Invention of the Tellurometer
– a giant leap in measurement
by J R Smith, History Group, FIG

If one traces the measurement of distance back through the mists of time for the determination of long distances with a high degree of accuracy,
one comes to Eratosthenes around 230 BC. The librarian in Alexandria, he took an interest in all manner of subjects including knowledge of the
size of the Earth.

By that time the Earth was considered to be            Initially the baselines in triangulation were         in the late 1950s when the last such line was
spherical rather than flat but there had been no       measured by a variety of means — chain,               measured in Kenya. At the time the Directorate
reasonable estimate of its size.                       wooden rods, glass rods, metal rods, rods             of Colonial Surveys (later to become DOS, or
                                                       set in boxes of ice, pairs of rods of different       Directorate of Overseas Surveys), was observing
Previous attempts had been made but with little
                                                       material, steel tapes and other forms. Such           a triangulation scheme down the west side of
or no scientific basis. So to obtain a value for the
                                                       rods were generally about 10 feet long. Then          Kenya from Isiolo in the north, southwards to
circumference of the Earth Eratosthenes used
                                                       in 1885 the Swedish Professor, Edvard Jäderin,        150 km south of Nairobi where it turned south-
the astronomical knowledge of the time together
                                                       introduced the idea of using tapes suspended          east to Mombasa and then north up the coast
with a method, the principles of which, remained
the same until the advent of satellite technology      in catenary, rather like telephone wires. After       to Malindi. The idea was that the loop north
(see Fig. 1). He said that if you have two far         various developments on this idea, 1896               along the coast and then back westwards to
separated positions that lie on the same               saw the introduction in France of a material          Isiolo should be completed by traverse.
line of longitude, and you measure the                 called invar that had a very low coefficient of
                                                                                                             The baselines were measured in catenary as
distance between them on the ground                    expansion. By using this material for the wires
                                                                                                             13 250 m at Malindi and 21 095 m at Isiolo. That
and also determine their separation in                 that were suspended in catenary these two
                                                                                                             at Isiolo took six weeks to measure out and back
terms of the angle subtended by the two                items, catenary and invar, reduced many of
                                                                                                             and took numerous personnel. That at Malindi
positions at the centre of the Earth, then you         the major problems of baseline measure but still
                                                                                                             was a little quicker. Then someone came
can      determine         the      circumference.     kept to the principle used by Eratosthenes.
                                                                                                             along with a box of electronic tricks, called the
One of the many interpretations of his
                                                       After extensive use between the two World Wars        Tellurometer, and measured the Isiolo base
results gave a value that equated to
                                                       baseline by catenary came to an abrupt end            in about 20 minutes to a very comparable
46 250 km.

The introduction of triangulation by Snellius in
1615 laid the foundation of a more accurate
way to determine the length between the
points as in the principle of Eratosthenes but
the angular separation still relied on good
astronomical       observations.       Triangulation
depends on the accurate measurement of one
or more baselines and then the measurement
of all the angles of the triangles in the chain.

Later in the late 18th century the definition
of the new unit of a metre was such that the
circumference was 40 000 km. This had been
determined from the results of two chains of
triangulation, one in France and the other in
Peru. In addition, by this time theories suggested
that the Earth was not spherical but spheroidal
and thus did not have a single radius. After
controversy between Newton in England and
the Cassini family of astronomers in France in the
early 18th century, it was agreed that Newton
was correct in deciding on an oblate (flattened
at the poles) rather than prolate (flattened at the
Equator) shape, hence not so easy to determine
as the sphere. This meant that the length of
1˚ of latitude varied in length with the latitude
(see Fig. 2).                                              Fig. 1: Determination of the size of the Earth.      Fig. 2: How the length of 1˚ varied with latitude.

26                                                                                                                                      October 2009 - EngineerIT
accuracy and similarly that at Malindi. The
Isiolo base had suddenly become the last such
line to be measured in catenary and a new
era had dawned for the surveyor. The traverse
from Malindi to Isiolo had been estimated to
take between 2 and 2½ years to complete by
traditional methods but with the Tellurometer
it took just 28 days to cover the 402 miles in
26 traverse legs.

In this context one must not overlook the fact
that the Geodimeter was invented some
10 years before the Tellurometer but had                             Fig. 3: Dr. Wadley using the             Fig. 4: Dr. Wadley demonstrating use of the whirling
disadvantages in respect to weight, range                              prototype Tellurometer.                   hygrometer for the determination of humidity.
and bad weather capabilities. However
the Geodimeter used light waves whereas
the Tellurometer used radio waves and this                                                                   appeared over the years until 1985 and the
                                                       The theory of the Tellurometer was simple, using
difference accounted for the better all round                                                                MRA 7 with a range of 50 km and accuracy of
                                                       the formula
ability of the latter instrument.                                                                            ±15 mm. That instrument is still in production
                                                       D = (v.t)/2                                           and widely operated in deep-mine safety
So what was this new instrument and where
                                                       where:                                                applications where it is used to continuously
had it come from. The spur to invent such an
                                                                                                             monitor the position of the moving cages. In
instrument came from Col. Baumann, who                 D = the distance required,                            addition models were developed for use both
was Director of Trig. Surveys in South Africa
                                                       v = the velocity of light in a vacuum                 from aeroplanes/helicopters and ships.
from 1945 to 1955. However the technology
was a result of developments during World                                                                    In May 2009 a meeting was held in Cape
                                                       t = the travel time for the signal to go from
War II in Radar, then Shoran and Hiran to get                                                                Town to commemorate the 50th anniversary
                                                       t = A to B and back again
the technology required for use at survey                                                                    of the Tellurometer and this was repeated in
accuracies. Baumann had in 1954 stipulated             Unfortunately it was not quite so simple as that      Johannesburg on 15 October 2009. Those of
                                                       as there were a whole host of new sources of          us who were taught on chains and tapes had
that he would like “an accuracy of better than
                                                       error to take into account. For example, the          to adapt to the new technology and much of
1 in 105 at over thirty miles, a resolution of a few
                                                       important figure of v was at the time taken as        what we learnt at college became obsolete
inches, line of sight and easily man-portable.”
                                                       299 792 km/s but how accurate was that in             overnight. Dr. Wadley has much to be thanked
This was no mean requirement considering that          survey terms?                                         for, albeit posthumously, as he revolutionised
other countries had spent years and millions of                                                              how surveying was carried out.
                                                       Then there was the problem of how accurately
dollars trying to achieve just this. However Wadley    could the time interval be determined? In fact it     Contact Jim Smith, Survey Review,
and colleagues at the Telecommunications               was possible to record time in nanoseconds (ns)
Research Laboratory (T.L.R), a part of the newly       where 1 ns equates to around 0,3 m.
established Council for Scientific and Industrial      What of the velocity figure? While testing his
Research (CSIR), had a prototype operating             prototype equipment Wadley concluded
within a matter of months, and in January              that the value of 299 792 for the velocity
1957 was demonstrating it to an international          needed to be modified by an increase
audience in Cape Town. The first production            of 1,87 ppm to 299 792,6 km/s. So both
models appeared during the same year (see              Bergstrand with the Geodimeter and Wadley
Figs. 3 and 4).                                        with the Tellurometer were instrumental in
                                                       modifying the accepted value for the velocity
So just over 50 years ago surveyors were forced        (see Fig. 3).
into a sudden change away from triangulation
to trilateration and traverse. The added               With the first model of Tellurometer, the M/RA1,
complication was that mechanical equipment             readings were taken off as a series of dial pattern
was replaced by electronic “black boxes” and           settings and this proved to be very tedious to
                                                       achieve. Then followed a page of calculations
the traditional use of a screwdriver to cure many
                                                       to achieve the result. An accuracy of ±2 inches
of the surveyors’ problems was replaced by a
                                                       over 35 miles was quoted and similar for the
technology that was not screwdriver friendly.
                                                       MRA 2 that first appeared in 1959 (see Fig. 4).
Many of today’s readers will never have used
                                                       In 1964 a prototype of the MA100, the
catenary taping and nearly as many will not            Modlite, appeared and this was altogether
have used the early models of Tellurometer.            different in that it was designed for short
Now little more than 50 years later satellite          ranges and used near infra-red light rather
technology has to a great extent replaced              than radio waves. The MA100 itself was
the Tellurometer although it is still being            masterminded by Dick Hölscher and Paul
manufactured in Cape Town but the company              Cabion and came on to the market in 1968.
is now called Tellumat.                                Further models of both radio and light versions

EngineerIT - October 2009                                                                                                                                       27

Shared By: