ACCURACY IN LEVELLING.

Waikato Times, Volume XXVI, Issue 2152, 24 April 1886, Page 1 (Supplement)

ACCURACY IN LEVELLING.

Remarkable accuracy is now attained by engineers in cutting tunnels through mountains, working from doth end*. Thus at the Musconetcong Tunnel, on theLehit?h Valley Railway, the alignment tested to 0.04 foot, or has than £ inch. In this case levels were ran over a mountain 5,000 feet long and 450 feet above the line ; also into the tunnel at •ach end about 2,500 feet to the point wfeere the headings meet. These tested to o*ols foot, or less than one-fifth of an inch. The chaining carried by steel tape measurements over the same distance tested, on the headings coming together, 0.52 foot, or 6.4 inches. This accuracy is to a large extent a developement of recent year*. In the annals of the state of Pennsylvania (Pennsylvania state improvements— main line) it is mentioned as a 'remarkable specimen of accurate levelling than in a circle of 12 miles that was levelled the error was only 1.2 foot. This was in 1824. About fifty years afterwards, or in 1878, United States •engineers engaged in running a line of levels between the Chesapeake and Delaware bays, for the pnrpose of determining the feasibility of a ship canal, tested some new levelling instruments furnished by the Government. A line of test levels was run over 10 miles, and the difference of level on the closing bench «oark was only 0.005 foot.— lron.