Shelley's Diaries for Geospatial
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A trig station on Sugarloaf in the Port Hills is a reference point used by surveyors. Image: LEARNZ.
Diary 2 - Wednesday 29 August 2012
Field Trip Name: Geospatial
Field Trip Place: Christchurch
Diary: 2 of 4
Weather: Spectacular!
Where's Shelley: Christchurch
Kia ora everyone, Shelley here
It was a stunning day to be out exploring today. We braved a frosty start out at Charing Cross near Darfield where we met Nic Donnelly and Steve Bensberg from LINZ (Land Information New Zealand). These guys are geospatial experts and they had promised to show us how location based information is helping with the rebuild of Christchurch.
Charing Cross was the perfect place to start the day as this is where the epicentre of the Darfield earthquake was located. Way back on 4 September 2010, in the early hours of the morning this 7.1 magnitude earthquake struck. The forty seconds of violent shaking that followed sparked a series of earthquakes over the following months that have changed Christchurch forever.
There were no signs of earthquakes at Charing Cross but Nic explained that geologists can locate the epicentre of an earthquake by using seismometers with exact GPS co-ordinates. A computer programme can then use the distance between seismometers and the time it takes for the seismic waves to reach each device to calculate where the epicentre is. This geospatial data helps people to predict the likelihood of future earthquakes in certain areas.
We found a quiet place to talk to St Peter’s School during our first audioconference. Nic enjoyed answering their questions and managed to explain a lot of uses for geospatial data.
Alvin wanted to see some signs of the earthquakes so we headed to Highfield Road. This country road was once straight and flat but has changed shape. This is because the Greendale fault runs under the road and this area moved four metres sideways and over one metre upwards. This land movement is still quite obvious even now, long after the road has been fixed. Hedges, fences, irrigation channels and even the railway line nearby also show this movement. This movement can easily be measured using a tape measure but more accurate measurements can also be made by surveyors.
Nic is a surveyor and his job is to find the exact location of things. He showed us the reference points that surveyors use to measure off. These points are located throughout the country. Nic knew the GPS co-ordinates for one point that was close to Highfield Road so we headed out there. Using GPS and a tape measure Nic thought he had found where the reference point was. These points are all buried, but luckily they are made out of metal, so to make sure we had the right place Nic used a metal detector. Sure enough after a little bit of digging we found the mark. You can watch the video to find out more about how these marks are used to accurately locate things down to the nearest millimetre.
Steve wanted to show us how some things in Christchurch had changed location due to the earthquakes, so we headed back into town. Steve took us to Avondale, one of the eastern suburbs which has been badly damaged by the quakes. We found a bridge that was obviously not in the same place as it used to be. The land on either side had dropped while the bridge itself stayed at the same level as it was supported by deep piles. The change in levels was obvious but to measure changes in land levels over all of Christchurch would take a really long time. This is when some clever technology called LIDAR can be used. This works like radar except it uses light waves rather than radio waves to bounce off the land surface to measure height. By flying a plane over the city and using LIDAR the different levels of the city were mapped. Changes in land levels have helped identify which areas are now at risk of flooding. You can watch the video to find out more.
After lunch we spoke to Kaiapoi Borough School during our second audioconference. They knew a lot about the Canterbury earthquakes and were interested to find out more about how geospatial data is helping in the rebuild of Christchurch. You can listen to the recording of today’s audioconferences.
Geospatial data has also been used to monitor rock fall hazard in the Port Hills. Thousands of rocks tumbled down hillsides during the February and June earthquakes, damaging homes, roads and tracks. To find out more about rock falls, GNS Science has mapped where rocks fell and measured the distance that rocks travelled. This information has allowed maps to be made to show areas at risk of future rock falls. It was amazing to hear that over the last year and a half thousands of rocks have been accurately located and mapped.
We ended the day at Sugarloaf, up on the Port Hills. You can see where we were in the panoramas. We could see for miles from our position near a trig station. These stations are set up as reference marks and can be used instead of GPS in areas where GPS doesn’t work. GPS stands for Global Positioning System and relies on signals from three or more satellites to work. If you can’t see the sky then you won’t be able to get a GPS signal, so this is when surveyors measure using trig stations instead. You can watch the video to find out more about how GPS works.
The GPS in Nic’s car showed that we had covered a large distance today so we headed back into town for a rest, ready for another big day tomorrow.
See you in the morning,
Shelley the LEARNZ Field Trip Teacher (and Alvin)
