Precise level values for the Christchurch City Council Benchmark Network are now available. These new January 2014 Network values supersede all previously published benchmark data.
Benchmarks are levelled survey points assigned heights expressed in terms of a common datum (reference surface). In the case of Christchurch City Council, the datum used is known as Christchurch Drainage Datum (CDD).
Christchurch City Council Benchmarks enable the ready establishment of normal-orthometric heights in terms of CDD within the greater Christchurch area. For example; a benchmark would be used to physically establish design ground levels for subdivisions, invert levels for pipe works and specified floor levels for new buildings in terms of CDD.
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The level value for A82 is now of historic interest only. It had a recorded height of 20.333 feet (6.197m) above Lyttelton Vertical Datum 1937 (mean sea level 1937) prior to the 4 September 2010 Darfield (Canterbury) earthquake. Land Information New Zealand’s Hagley Fundamental (AF30) has been adopted as the new fundamental mark for Christchurch Drainage Datum. The offset of Lyttelton Vertical Datum 1937 to Christchurch Drainage Datum is 9.043m.
The earthquakes experienced in the Canterbury region between 4 September 2010 and 23 December 2011 put the integrity of the Council’s existing network of benchmarks in doubt. In response to each of the seismic events a Global Navigation Satellite System derived “emergency response” benchmark network was established as an interim measure to assist in the recovery effort with the intention that a higher accuracy “recovery response” precise leveled network would be established once seismic activity had subsided to a permissible level. The January 2014 Network is the embodiment of that vision.
The origin of CDD dates back to February 1876, the early days of the Christchurch Drainage Board where the need for a common level was evident very early in the quest to control Christchurch's drainage issues. An arbitrary figure of 50 feet (15.240m) above datum was chosen for the networks fundamental benchmark (A82), which was situated in the floor of ChristChurch Cathedral’s foyer. A value of 50 feet ensured that levels specified on engineering works in Christchurch City would be expressed as positive values, even when working in New Brighton and Sumner where infrastructure is often situated below sea level.
Christchurch Drainage Datum was initially called Christchurch Drainage Board Datum but upon amalgamation of the Drainage Board and it's activities into the Christchurch City Council the name was changed to Christchurch Drainage Datum (CDD).