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Temporary CORS Networks for Tsunami Relief: Aceh
Researchers have recently established a temporary reference station network to test the potential of using high-precision GPS surveying techniques to re-establish land boundaries when survey monuments and other marks have been destroyed.
They reported on their experience in the wake of the December 2004 tsunami in Aceh at last year's conference of the International GNSS Society.
F.H. Adiyanto and Craig Roberts from the School of Surveying and Spatial Information Systems at the University of New South Wales in Sydney, told the conference that huge numbers of Aceh's cadastral monuments, which relate directly to cadastral boundaries, were destroyed.
The earthquake had two major impacts for established land boundaries. The first was horizontal and vertical ground surface displacements. 'The magnitude of these displacements was between 0.1 and 2.7 metres horizontally and 0.04 and 0.32 metres vertically.
'The second major impact concerned cadastral and geodetic control points, which simply disappeared.
'Boundary reconstruction needed to be done quickly and accurately. Until this was done, it was impossibly to begin the work of rebuilding. Traditional methods of reinstating boundaries could have been used, but they would have taken many years to complete. The decision was made to use GPS, which would deliver huge productivity improvements.'
To carry out cadastral mapping to the required accuracy, differential techniques or, even better, Continuously Operating Reference Stations (CORS) networks, are required.
Single baseline RTK suffers from distance-dependant errors that limit the length of a baseline to no more than 10 km, which is far too short. A network is the only practical option.
Normally, CORS networks are permanent fixtures. Base stations, which are surveyed in and connected to permanent power, monitor GPS and transmit error correction messages continuously. The question is whether it would be advantageous to do this on a temporary basis.
Network RTK can certainly deliver the required results but it suffers from its own restrictions. It has in-built distance limitations, depends on a robust communications network and requires a reliable power supply.
Furthermore, the base stations and network software are expensive items. So a temporary CORS networks was considered as an alternative. Using the network in a leapfrog fashion could solve the cost issue. The same essential equipment could be re-used in several interconnected sub-networks to cover the 100 km x 6 km tsunami affected area. It would be easy to set up base stations on higher order monuments. After completion, the network could be re–packed and moved to another region. The outlay for the receiver hardware and software would be a one-off cost and the monuments could be re-observed at a later date as required.
Even a temporary network needs to be connected for 72 continuous hours for sufficient data for the computation of co-ordinates for permanent reference points. These reference stations must be connected to the International Terrestrial Reference Frame (ITRF).
Internet-based GPS services are the easiest way to do this. For instance, Geoscience Australia runs the Auspos online GPS processing system. It was developed to provide Geodetic Datum Australia 94 (GDA94) co-ordinates for Australian users, but also provides ITRF-based co-ordinates for worldwide users. This service can be used for global datum connection when deploying temporary CORS networks.
One caution: the ITRF coordinate system is defined at a particular epoch. To be used in Aceh, it had to be transformed into the Indonesian co-ordinate system used for cadastral purposes (TM3, a derivative of UTM).
The problem with this is the communications network. Communications seem to be very vulnerable to natural disasters, although it is possible to re-establish mobile telephone networks relatively quickly.
In the case of the Aceh exercise, the data was post-processed, but it would certainly have been possible to do it in real time.
The researchers concluded by saying that the use of a CORS network could have unexpected advantages.
'The use of GPS presents an opportunity for the future,’ they said. 'Ít could provide a foundation for developing a spatial data infrastructure in the region.
'If such an infrastructure were in place and the Indonesian legal cadastral system permitted, land parcels could simply be re-established using co-ordinates, without the need to find existing survey marks, monuments or evidence from surviving family members,’ they said.
However, they admitted such a proposal would be controversial.
'There are a range of issues related to the use of RTK GPS for parcel boundary measurement with respect to cadastral regulation. On the one hand, Government Regulation No. 24/1997 allows the use of GPS in cadastral surveying. On the other hand, there is no specific direction on using GPS and some surveyors are reluctant to adopt it.
'It is in direct contravention of the principle of ‘monuments over measurements’, but given the unique circumstances in Aceh and the unfortunate likelihood of such an event re-occurring, such a step would be prudent,’ they said.
