A key factor in the development of a digital cadastre is the type and density of control marks. They are essential for the re-establishment of boundaries and are a major cost factor both in their initial establishment and in their continual maintenance. There will always be a steady attrition rate for survey marks as streets are dug up for the installation and maintenance of services regardless of the type of mark and legislation for their protection. As a conservative estimate, each mark costs about $600 to establish and this includes the installation, measurement, calculation and initial entry into the mark data base. If there is a mark for every twenty parcels on average, then the cost of control in the traditional sense is about $30 per parcel or five times the current contract price being paid for parcel data entry, joining and adjustment in the Northern Territory.
While GPS is being used for mark coordination in some areas and this does reduce the cost of establishment by about $150 per parcel, but there is still the maintenance problem for all marks. In some ways we are designing the control system to match the technology of yesterday rather than look at what is available to-day and the way that the technology is moving.
If we look at sales of "survey accurate" GPS systems you will see that the price, utilisation and rate of development is similar to that of EDM systems in the 1970ís. If anything the rate of development of GPS is being pushed along at a faster rate and it is reasonable to expect that they will be in use in most survey practises within five years. At that time it will not matter if the SSM is in the same street, or the same block or even the same suburb. In other words, by the time we get a high density network fully established it will not be necessary
There is an alternative approach and that is to develop a network of "Active Trig Stations" for the urban areas. These would be static GPS stations each broadcasting data for differential GPS measurements. They would be similar in concept to the current wide area network which operates through 2JJJ but would provide centimetre precision instead of the "navigation level" accuracy offered at present.
The stations could be placed and operated by local councils, or by Optus or Telecom at their mobile phone repeater stations or by government or private operators. They could distribute their information by short wave radio, could simply use a "0055" number or simply a subscription based bulletin board or internet web page. In the 0055 case, the surveyor would ring the number, connect to his GPS and would be billed for the service through the phone system.
Such a system could provide significant cost savings as well as giving a high level of quality assurance. Each surveyor would need only one receiver instead of two or three. Each unit currently costs about $30,000 and while the unit price will undoubtedly drop in the near future, every unit in the field requires an operator or person to supervise it. However the most important benefit will be in quality assurance. Every survey will be based on the same marks with each point being fixed with respect three or more fixed points, each of which will be known to a very high degree of precision. Each base station would cost about $50,000 which is less than the cost of 100 marks and they could be self funding. In fact if each major surveying firm put in and ran a base station and they pooled their information, each firm would make a financial saving and SSMs and PMs would not be needed in many areas!
While the current marks are undoubtedly of great use and are playing an important role in the stabilisation of the cadastre, we should be aware that they are simply an interim measure and not a long term solution.