After studying this section you should understand the differences between horizontal and vertical lines or surfaces and why these are important in levelling. You should know what a levelling datum is and be aware that a national levelling datum has been set up by the Ordnance Survey and is known as Ordnance Datum Newlyn (ODN). You should be aware that bench marks are points of known height specified on a chosen datum and that these can either be Temporary or Transferred Bench Marks (TBMs) or Ordnance Survey Bench Marks (OSBMs). In addition, you should appreciate the difficulties of using an OSBM for any levelling and why these are being replaced by GNSS-based heights.
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- Chapter 2