Interest Pages

Len Beadell - Astronomy in Surveying

 

LEN BEADELL's Cooke, Troughton and Simms Tavistock Theodolite

written by Connie Sue Beadell, March 2019

 

    There has been a lot written about Len and his fellow surveyors; the hardships that they endured, the type of country that they were mapping and why they were there. Not so much is written about the gear that they used, in particular the non-digital theodolites used back in those days almost obsolete in this current digital age. In Len's case, as you may know he learnt his craft from surveyor Scout Master Mr JTC Richmond ("Skip"), the instrument used by him a Vernier 4 screw base Cooke Troughton & Simms theodolite (Skip eventually gave Len this instrument which we still have in our family collection, see right). Skip worked with the Sydney Water Board for 43 years, on weekends taking Scouts on trips to the Blue Mountains to find, clear & replace old trigonometric stations. He was a mentor to many young boys including Len, some of whom we have met over the years with plenty of stories to tell.
    Skip died in July 1976 but he had a profound effect on one young lad who was to make surveying his life's work.
Skip's Vernier theodolite
Skip's 4 screw vernier theodolite
Paint mark on Len's Tavistock theodolite
Paint mark Len put on foresight
    At some point early in his career Len was issued with a brand new Cooke Troughton & Simms Tavistock theodolite brought especially for him from England (so named for the conference at Tavistock, Devonshire, in 1926 during which time modifications were made to existing theodolite design). It was an optical micrometer transit instrument. After his desert career was over, this instrument, on which Len performed nearly all of his desert work, found its way back to WRE in Salisbury, South Australia, where it was left in a corner gathering dust. Just prior to Len's retirement in 1988 it was unearthed and returned to him, Len very excited to have his old instrument back. He took possession of it, WRE apparently telling Len that on paper it didn't actually exist anymore anyway! We also still have this instrument in our family collection, old paint marks still in evidence on the foresight put there by Len to make it easier to see during his observations. Safely in its box, secured with spring-loaded metal packings, Len wrapped the whole thing in thick sponge rubber and flannelette to protect it from years of bush-bashing, corrugations and other rough outback conditions. As Len once said, he relied on it for his life many times (before the advent of hand-held GPS's) apart from being a vital piece of equipment for his desert surveying program. As a point of interest, inside the booklet below, Len had made a note that in May 1949 the instrument was purchased for £340.
Len's Tavistock theodolite & manual
Len's trusty Cooke Troughton & Simms Tavistock theodolite & manual
some of Len's astro fix calculations
A snippet of Len's astro-fix calculations

 

    Here are a few technical specs of the Tavistock theodolite listed in the above booklet -:

Len's tavistock with paint mark on foresight
Len's theodolite, closer

 

    The basic use for a theodolite -:

    For the determination of horizontal and vertical angles, that is basically it! Simple! Actually it is a complex field but very interesting for those keen on surveying, mapping, astronomy and maths. In Len's case for astro-fixes, with the theodolite fixed in one position, readings from stars on opposite sides of the sky were taken for a mean result (for latitude stars north & south, longitude east & west). This is a feature of transit theodolites and helped to overcome various errors that could affect the result (including instrumental imperfections and refraction (the bending of light as it passes through the atmosphere making stars appear higher than they really are)). Len took readings from stars at about 35-40 degrees above the horizon to reduce refraction as well as being a more convenient angle for the theodolite. He would also try and select ones transiting at a convenient hour of the evening. He used the circum-meridian method where stars were read very close to their culmination and with the time, a correction was made to calculate the star's altitude when crossing his meridian. More opposite pairs were read & calculated depending on the accuracy of the result required, for example he read 20 pairs for some of his Woomera observations and even allowed for the time it took for time signals to reach him from Honolulu via his wireless (required for longitude, the correction somewhere about 0.08 seconds). Sun Observations were observed too when Len was only interested in a latitude reading. These were relative to the sun crossing his meridian (the north-south line through your position on earth) and a mean reading was worked out from observations on the top & bottom of the sun (bigger object than a star). Considerations in general included corrections for celestial refraction, parallax (for sun observations due to our position on the earth looking up, the stars too far away to be an issue), timing issues (a problem for early explorers and sea navigators relying on chronometers and the like before radio time signals were available), accurate placement of the theodolite over the surface mark (or corrected for) to name a few. By the way, for ground distance measurements issues with measuring tapes were considered as well, like standardised lengths, temperature contraction and expansion of the tape, amount of tension and sag, slope of country and at times correction of alignment if a straight line was not possible.

    Len's early career in the Army Survey Corp. involved operational mapping in New Guinea, near Port Moresby and later Wewak. His CSIR work in 1946 involved astronomical observations for a mapping program from aerial photographs of country in Arnhem Land, NT. His Woomera and subsequent surveys included the setting up of trig stations, contour surveys of the terrain, laying out sites for buildings, airstrips, rocket & bomb infrastructure etc (e.g. Woomera, Emu, Maralinga & the Giles Weather Station), surveys for the Rocket Range CentreLine, identifying & fixing features off aerial photos, participating in triangulation surveys and traverses (including for the Worldwide Geodetic surveys) as well as fixing positions of geographic & topographic features and building a road network. None of it was done on his own and tribute must also be paid to the many other dedicated men who also worked hard under at times extreme conditions. None of it would have been possible without the trusty theodolite.

 

    A very basic run down of an astronomical observation (astro-fix)

Diagram for latitude
Diagram for latitude.
The zenith is the point from the centre of the earth through your position.

    Latitude - Our position relative to the equator, north or south. It is equal to the angle read from the theodolite (horizon to star - 1 on the diagram) plus the angle from the nautical almanac (angle from the star to the equator - 2) taken away from 90O (the angle between our horizon and zenith).

    Longitude - Our meridian relative to the Prime Meridian through Greenwich, England (which is 0O of longitude). This is more complicated and requires the use of time signals. Len would find a star and read its angle of altitude & bearing and record with the exact time the reading was taken. You can then work out when it will cross your meridian (with spherical triangle calculations) and the nautical almanac will allow you to work out the time it will transit the Prime Meridian. The difference in these 2 times converted to degrees, minutes & seconds is equivalent to your longitude.

A piece of (survey) cake.......

 

Connie Sue Beadell
Text & Images Copyright : March 2019

 

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