The following information relates to rule 3.1 and the accuracy requirements for non-boundary marks on a cadastral survey.
Applying rule 3.1 ensures there is an accuracy framework for the network of non-boundary survey marks that enables boundary positions to be accurately determined in relation to one another and correctly reinstated in the future.
In summary, the standard for survey accuracy [r 3.1] specifies two tiers of accuracies and a cap that applies between non-boundary marks. These are:
- a more stringent level of accuracy to be met between 95 % of new and old marks (a 95 % confidence level) [r 3.1, Table 1(a)],
- a less stringent level of accuracy to be met between all marks including cadastral survey network marks, but exempts other adopted non-boundary marks (100 %) [r 3.1, Table 1(b)], and
- a 0.5 m cap [r 3.1, Table 1(c)].
The following information explains these requirements in more detail
More stringent standard for accuracy of non-boundary marks
This first tier is more stringent and requires a 95 % likelihood that the relationship between the marks specified in the rules meets the specified accuracies. This 95 % standard applies to all new work but not to adopted work. The standard uses the root sum squared (RSS) method to calculate the accuracy value [r 3.1, Table 1(a)].
This second tier is less stringent with tolerances approximately 50 % greater than the most stringent standard.
The standard requires the relationship between all (100 %) of the points specified in the Rules, including all adopted cadastral survey network marks to meet the specified standards. Other adopted non-boundary marks as part of an adopted 'traverse' used as evidence in locating an existing boundary point are exempt (refer to r 3.1(b)). A cadastral survey network mark is a mark with a Landonline order 6 or better (LINZR65302: Ruling on cadastral survey network marks).
The standard uses a simple sum method to calculate the accuracy value [r 3.1, Table 1(b)].
This means that up to 5 % of the relationships between new work are permitted to be outside the more stringent accuracy standard providing they are within the less stringent standard.
0.5 metre cap for accuracy of non-boundary marks
The accuracy standard has a cap of 0.5 m, which has an impact on widely spaced marks [r 3.1, Table 1(c)].
Application of accuracy standards for accuracy of non-boundary marks
The accuracy standards are distance dependent. Note that the cap comes into effect at 3130 m.
Confidence levels can be estimated through a least squares analysis and are influenced by the number and location of the vectors connecting the points (the strength of the geometry), and the accuracy of those vectors. Good survey practice, strong geometry, accurate measurement, and the inclusion of additional vectors may provide assurance the required confidence levels are met.
Note that where circuits include new and adopted vectors, the circuit closure may not be a definitive indicator of the survey meeting the accuracy levels. This is because of the different accuracy specifications in rules 3.1(a) and 3.1(b).
With the exception of those adopted non-boundary marks as part of an adopted 'traverse' used as evidence in locating an existing boundary point (r 3.1(b)), the accuracy specifications apply to all new and old non-boundary marks including:
- permanent reference marks (PRMs),
- witness marks, and
- old or adopted cadastral survey network marks
- other non-boundary marks in the survey (traverse marks).
The standard applies between specified non-boundary marks:
- joined by measured vectors,
- joined by adopted vectors (for exceptions refer to less stringent standard for accuracy of non-boundary marks above),
- joined by calculated vectors, and
- not directly joined by vectors.
All adopted vectors that define the spatial relationship between cadastral survey network marks or between boundary points must comply with the less stringent accuracy standard [r 3.1, Table 1(b)]. There is no provision for allowing the adoption of vectors that fail the new accuracy standards, but meet the survey accuracy tolerances in place at the time of the original survey.
Where adopted 'traverse' vectors are used as evidence in locating an existing boundary point, the points are not required to meet the accuracy standards in rule 3.1 [r 3.1, Table 1(b)]. An example could be an old centreline traverse.
These vectors must be copied correctly from the source data into the CSD [r 8.4] and be included in the Diagram of Survey noting that an adoption may incorporate a bearing adjustment [r 8.4] .
The accuracy requirements for non-boundary marks [r 3.1], in combination with the accuracy requirements for witnessing [r 3.6], ensures there is a survey accurate relationship between the boundary point and non-boundary marks that are in close proximity.
This enables a boundary point to be confidently reinstated from any one of these non-boundary marks in the future.
Figure 3: Ensuring correct spatial accuracy between points
There is no specified class of accuracy for the survey accuracy standard. The same accuracies apply irrespective of the class of accuracy applicable to boundary witnessing [r 3.6] and boundaries [r 3.3].