In their book on the Ordnance Survey,  Owen and Pilbeam state:
"The steel observing tower, called a Bilby tower, had been developed in America for surveying its vast flatlands and prairies. Each tower was in fact two indep endent structures. The instrument was supported on the inner tower and the observer and his assistant stood on the outer one. They needed careful erecting, including the laying of concrete footings for the legs. It was found to be easier for a trig pillar to be built under the Bilby tower, once it was in position. At full heights the towers rose to 103 feet and took about 10 hours to erect"
- Ordnance Survey- Map Makers to Britain since 1791', Tim Owen and Elaine Pilbeam, p.123
- The History of the Retriangulation of Great Britain, Section 2.07 'Steel Observing Towers'