Reclassification of order 5 (5a) control marks
LINZ will be changing all the 5a marks from order 5 to order 6. Actual coordinates will not change, but the accuracy of these marks will now be more appropriately reflected by their new order.
This will mean that all of the order 5 marks in the geodetic database will comply with the order 5 accuracy standards and will have ellipsoidal heights. These will be the existing 16,000 5b marks.
The reclassification is scheduled to take place in May, subject to successful testing. A firm date will be communicated closer to the time.
Impact on customers
This is one of the more substantial changes for the control system since NZGD2000 was implemented. However, the impact on customers should be minimal. Order 6 marks can be used in a cadastral survey in the same way as order 5 marks. Coordinates are not changing, so jobs in progress when the change occurs should not be impacted. Marks will retain their geodetic codes, and will remain in the geodetic database and Landonline.
New Zealand’s survey control system
New Zealand’s survey control system has 72,000 order 5 marks. These marks provide the ‘street-level’ control for cadastral surveying and other applications.
When the New Zealand Geodetic Datum 2000 (NZGD2000) was established, order 5 coordinates were generated using two distinct processes. Firstly, existing traverse data from control surveys was captured and adjusted in terms of NZGD2000. These are commonly referred to as 5a marks. Secondly, where there were areas lacking existing control traverses, existing cadastral marks were surveyed with GPS to assign them order 5 coordinates. These are commonly referred to as 5b marks. Each year, additional 5b marks are surveyed as part of the geodetic annual programme.
Currently, there are 56,000 5a marks and 16,000 5b marks in the survey control system.
Failure of 5a marks to comply with order 5 accuracy standard
Although coordinates were initially assigned order 5 accuracy, subsequent analysis has shown that a majority of 5a marks do not comply with all aspects of the accuracy standard. This analysis is supported by anecdotal evidence from surveyors who have had difficulty achieving acceptable closes using 5a marks. At the time the order 5 coordinates were assigned, full statistical tests were not carried out to confirm that accuracy standards were met. Order 5 was effectively treated as a default order, assigned to almost all coordinates captured from control traverses.
Failures of the accuracy standard are primarily the result of two factors: low observation accuracies and poor network geometry. Although fully complying with specifications in place at the time they were made, many control traverse observations are simply not accurate enough to comply with the current accuracy standard. This is particularly true of control traverses carried out prior to the use of EDM technology. Poor network geometry often relates to the fact that prior to the use of GPS, all lines had to be intervisible. This means that sometimes no direct vector has been observed between two adjacent marks. Order 5 networks may also feature long rural control traverses, along which errors quickly propagate.
Plans to re-establish order 5 control
As part of the geodetic annual programme, order 5 control will be re-established in areas where the geodetic marks will be changed to order 6. This process will take a number of years and will need to be prioritised against other requirements, such as establishing control in rural areas.
Work on this has already commenced, with order 5 control being re-established in a number of areas as part of the 2009/10 Annual Programme.
Any comments regarding this change can be forwarded to LINZ by emailing customersupport@linz.govt.nz with 'Geodetic 5a reclassification' in the subject line.
