baseline

Base lines used in triangulating the centre line of the Bridge

from Museums of History NSW - State Archives Collection

• 1 older comment, and then…
• BobMeade said:

  See Government Surveyor Mr. E. A. Amphlett measuring Base Line "B" in this photograph: and Governor of NSW Sir Dudley De Chair standing at Point E in front of Government House in this photograph: An explanation of the process of finding the centre line of the bridge by triangulation and using the centre line to position the main bearings for the bridge is here: sydney-harbour-bridge.bos.nsw.edu.au/engineering-studies/... At the right hand side of the page brought up by that last link is a picture of another version of the drawing shown above.

• beachcomber australia said:

  Does anyone understand that? Please explain.

• Mich said:

  I don't know why a line from point C to point D would not have sufficed.

• Mich said:

  To answer your question, they already know where C and D are but they don't know the distance between them. They don't have a steel tape measure 2000 metres long, and even if they did, they can't measure it across the harbour. But they do have a steel tape measure 500 metres long, and a big lawn at Government house. So they mark out 2 points in the garden at Government house, and they know exactly the distance between them with their steel tape measure. They they measure the angles to C and D. After they do this, they can use trigonometry to measure the exact distance between C and D so they know how long to make the bridge. The description that they needed to do this to lay out the centreline from C to D is not an accurate description in my opinion. They had C and D laid out. The purpose of this rigmarole was to determine the exact distance from C to D.

• beachcomber australia said:

  Mich mich_sydney_2007 Thank you. I can see how that works in 2D, but unless the variance in heights of A - H is known 3D would be inaccurate.