Base measurement using

Macca

base measuring equipment

 

This involved measuring the base line by catenary taping using the specialised equipment.

The equipment included

Three standard invar tapes of 24m length, graduated at 1mm intervals for only 0.1m  at each end.

Two field invar tapes of 24m length, graduated at 1mm intervals for only 0.1m  at each end.

One 6m invar tape graduated throughout.

Two Straining tripods c/w straining wires and weights. The pulley is adjustable both laterally and vertically.

Measuring heads including magnifiers & centering devices.

A theodolite for alignment.

Various tripods

Level & staff

Tape supporting poles

Canvas screens.

 

To the best of my recollection the bases were in the order of three hundred metres.

The field work proceeded in the following manner.

A number of tripods with tribrachs & measuring heads were aligned and set out from one end of the base using a theodolite and a steel tape. The setting out had to be done so that each bay was within the range of the invar tape.

The first and final bay measured in a session involved taking standardisation measurements using the three standard tapes and the two field tapes using the same procedure.

All tapes to be used in a measurement were removed from the drum which was approximately two feet in diameter and attached to two wooden poles via springs so as to avoid straining the tapes. The straining tripods were set up and aligned with the bay to be measured. Each tape in turn was removed from the carrying poles attached to the straining wires, positioned across the measuring heads and the weights gently lowered to apply the tension. The final alignment using the adjustable movement of the pulleys completed. Six pairs of readings are now taken at each end on the graduated section with the tape being moved between each pair of simultaneous readings. The readings were taken using a magnifier to 0.1mm. The difference between the forward & back readings for the six sets to agree to 0.3mm. The mean difference applied to the nominal tape length of 24m gives the measured length. Temperatures being taken for each set of measurements. All the remaining bays are measured with only the field tapes. To avoid personal equation errors the two observers change ends every ten bays. The final bay will normally be measured using the six metre tape only. The heights of the measuring heads being determined by levelling. This gave one peculiarity, as most levels gave an inverted image the short staff used has the figures upside down, this was fine until we received a new automatic level which gave an upright image.

The whole measurement is then repeated in the opposite direction, in the event of the base not being completed in the day marks are transferred to a peg in the ground to carry on the next day. The specification was for the forward & back measurement to agree to 1 part in a million, one part in five million was achieved on more than one occasion. In the event of there being any wind  the canvas screens were held up to shelter the bay being measured. The observers included S/Sgt Bob Louden and Sgts. Pete Riffle & Dis Dyal.

 

I cannot find any photographs of the period but thanks to the ORDNANCE SURVEY I can show  photographs of the Macca Base equipment in use during the measurement of the CAITHNESS BASE.

The Macca base equipment in use during the measurement of the Caithness Base

The Macca base equipment in use during the measurement of the Caithness Base, click to enlarge

Crown copyright Ordnance Survey

Published with thanks to and the permission of Ordnance Survey

 

Straining and measuring tripod & head

Straining and measuring tripod & head, click to enlarge

Crown copyright Ordnance Survey

Published with thanks to and the permission of Ordnance Survey

 

 

The other end of a bay

The other end of a bay, click to enlarge

Crown copyright Ordnance Survey

Published with thanks to and the permission of Ordnance Survey

 

Aligning theodolite

Aligning theodolite click to enlarge

 

Crown copyright Ordnance Survey

Published with thanks to and the permission of Ordnance Survey

 

T.B. Edwards

Crown copyright Ordnance Survey

Published with thanks to and the permission of Ordnance Survey

 

 

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