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.
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