On 8 May 2010 LINZ reclassified the Order of all 5a control marks from Order 5 to Order 6.
The actual coordinates of these marks did not change, but the accuracy of these marks now is now 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 also have ellipsoidal heights. These will be the existing 16,000 5b marks.
What are Order 5a marks?
When the New Zealand Geodetic Datum 2000 (NZGD2000) was implemented, Order 5 marks were established to provide the ‘street-level’ control for cadastral surveying and other applications.
At the time the Order 5 marks were established there was also a pressing need to provide control marks to support the survey conversion process that preceded the rollout of Landonline. To enable this demand to be met in a cost-effective way, 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 were 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 were referred to as 5b marks. Note that the differentiation between 5a and 5b marks has not been shown on the geodetic database or Landonline.
Until this reclassification, New Zealand’s survey control system had 72,000 Order 5 marks, 56,000 5a marks and 16,000 5b marks.
Failure of 5a marks to comply with Order 5 accuracy standard
Although coordinates were initially assigned Order 5 accuracy, subsequent analysis showed that a majority of 5a marks did not comply with all aspects of the accuracy standard. This analysis was supported by anecdotal evidence from surveyors who 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.
Failure to meet the Order 5 accuracy standard was 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.
Following a re-analysis of the original Order 5a adjustments it was found that while many coordinates did not meet the Order 5 accuracy standard, in most cases they achieved the Order 6 standard. The re-classification means that the accuracy of the coordinates is better reflected by the new Order.
Impact on surveyors
This is one of the more substantial changes for the control system since NZGD2000 was implemented. However, the impact on surveyors using the control marks should be minimal. Order 6 marks can still 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 both the geodetic database and Landonline.
Plans to re-establish Order 5 control
As part of the geodetic annual programme, Order 5 control marks will be re-established in areas where the control marks have been 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 Annual Programme.