The big cleanup in the aftermath of the Canterbury earthquake may be underway -- but the headaches have only just started for some people for whom the Earth really did move.
Canterbury surveyors, who normally rely on a network of control marks such as "trig" points to calibrate aerial photographs and other mapping, have been told some of them within 100km of Darfield may need major work to be sure of top-flight accuracy.
A basic geodetic network is overseen by a government agency, Land Information NZ (Linz), partly because it is an essential tool in setting and identifying property boundaries, siting buildings, and setting out utility services such as power and gas.
But the 7.1 magnitude earthquake in mid-Canterbury last Saturday has knocked some of those reference points out of kilter.
And a survey-accurate digital cadastre (SDC) -- measurements which cover 70 percent of the nation's land parcels and 50 percent of the boundary points -- is supposed to be accurate to at least 20cm.
But initial estimates of displacements indicate that many of the SDC coordinates in the Christchurch area will no longer comply with standards of absolute accuracy.
Surveyors have been told by Linz that all field work in surveys started before the quake but completed later will have to be accurate for the survey completion date.
"You will have to assess the reliability of your original field work, and undertake any re-surveying as necessary," Linz said on its website.
"Boundary definition, including recalculations, will need to take into account any movement or distortion due to the earthquake".
Linz is now working with GNS Science to make an initial re-survey of the key reference marks, which should give an initial estimate of the magnitude and extent to which the land has been deformed. GNS will use the information to understand the mechanisms of the quake, and Linz will use it to work out how to correct its survey control system around Darfield.
The fieldwork is expected to be completed by Monday , with preliminary results available later that week.
Later, Linz expects to have to carry out a major survey of its control points to pinpoint their new positions.
"It is likely to be at least six months before such surveys take place, as it is necessary to wait for most of the ongoing post-seismic movements to subside," Linz said.
This would be easiest to do by making comparisons with accurate survey data on marks from shortly before the earthquake, but most of the best control marks in Canterbury have not been surveyed by state science companies or other government agencies since the late 1990s.
Linz is requesting data from any agencies or companies that have recorded high-quality GPS information within 100km of Darfield in the past 2 years.
