Re-heighting Canterbury

In the wake of the Canterbury earthquake, work is underway to re-establish the survey control system in the affected region. One of the components most urgently in need of re-establishment is the National Height Network.

The National Height Network consists of a series of precise levelling benchmark runs throughout New Zealand. These marks have very accurate heights on them, which other agencies such as local authorities use to calculate heights for their infrastructure. Accurate heighting is critical for systems such as sewers, where there is a need to predict the direction and speed of fluid flow.

The plan is to re-level approximately 500km of first order precise levelling benchmark runs in Canterbury, as shown in the plot below. These runs have been selected based on analysis of vertical movements after the earthquake, calculated from GNSS data.

To reflect advances in technology in the decades since precise levelling was previously undertaken, approximately half of the benchmarks will also be surveyed using GNSS. For any future earthquake, this will make it far more efficient to determine the impact on the precise heighting infrastructure. It also enables an accurate horizontal position to be determined, making it easier to locate and protect the benchmarks in the future.

The fieldwork and data reductions will be carried out by survey consultants working under contract to LINZ. As with all high-value geodetic work, service providers are selected via an open tendering process using the Government Electronic Tenders Service (GETS). Work will commence early next year and is expected to be completed by May 2011.

To ensure that results are made available as soon as possible to support the recovery effort, provisional data will be supplied to LINZ on a regular basis. Once this has been checked, provisional heights will be computed and supplied to local authorities.