How we Measure Up and Minimise Errors

At Fox and Associates we get involved in some huge surveys, monitoring buildings and land for movement patterns.  Most of our monitoring work before the sequence of earthquakes in Canterbury that began in 2010 was for construction projects where we were required to check land and buildings near a construction site for changes.  Movement sometimes occurs after pile driving, extensive excavation or drainage.

At present, we are called on mainly to monitor changes resulting from natural seismic activity.  There is little tolerance for error and our challenges include:

• Locating all monitored positions to within five or ten millimetres
• Working around blocked lines of sight between positions on large sites
• Finding reliable reference points when the entire city has moved 

 
We deal with the first challenge by determining, even before we get out into the field, how precise our client needs the measurements to be: with a tolerance of 1mm; 5mm; 10mm or more?  We can then choose the best measuring instrument and processes to match the job, the site and the client’s expectations.  We constantly strive to eliminate errors in our fieldwork but, in fact, every measurement naturally has some margin of error.

One of our most-used instruments is the Total Station (or electronic theodolite), for measuring distances and angles.  This precise tool measures distance with an accuracy of between 2mm and 3mm per kilometre.  For data to be helpful, of course we need to measure FROM somewhere TO somewhere else.  When we set up the Total Station on a tripod over a survey mark, the FROM point, it is possible to mistakenly add a 3mm–5mm error.  The same thing may happen at the target, the TO point.  The resulting measurement might be out by 10mm or more, and multiple end-on-end measurements will compound this error.  However, choosing the right instrument, setting up correctly and measuring more than once will minimise these errors.

The site itself presents challenges if we cannot see all of the points we are monitoring from one position.  With one set-up station, we know there can be only one set-up error, which will be the same for each measured position.  This is the ideal, which is rare, and we commonly have to use more than one set-up station, which results in more than one set-up error.

We get around this problem by creating a network of measured observations with our Total Station between survey marks around the site.  We add to this network using data from other instruments for position including the GPS and the barcode level for height.  Most of the measurements are taken more than once.

We calculate the best-fit co-ordinates from all these measurements by using least squares network analysis.  Put simply, this process compares all measurements from the various sources, finds the differences or ‘errors’, and minimises them to deliver a more accurate result.

There has been significant movement over most of Christchurch and, after the February 2011 quake, many of the survey control monitoring marks had moved.  Finding a solid reference point outside the earthquake zone proved a challenge.  We now use a relatively unaffected LINZ GPS station about 160 kilometres away at Lake Taylor that continuously tracks and stores position information.  Using data from several hours of continuous GPS measurement we are able to include this mark in a number of our networks.  It has held fixed co-ordinates and helps us see large-scale movements. 

Dealing with these challenges and completing many monitoring projects in an ever-changing environment has added to our considerable pool of experience.  We can confidently process and present monitoring data in a way that clearly shows relative direction and magnitude of movement across monitoring stations with a high degree of accuracy.  We look forward to new challenges as Christchurch rebuilds and reinvents itself.