The following information relates to the use of a vertical datum on a cadastral survey that includes survey marks or boundary points with reduced levels.
The official vertical datums for cadastral surveys are defined in LINZR65301 - Ruling on official vertical datums (PDF bytes) (PDF)
From 1 January 2019 NZVD2009 is replaced as an official vertical datum by NZVD2016. For more information see the questions and answers below.
The following questions and answers have been prepared to assist surveyors during the transition to NZVD2016 as an official vertical datum.
How do I determine whether my survey needs to be in terms of an official vertical datum?
Rule 4.3(a) requires a cadastral survey to be in terms of an official vertical datum where a 3rd order or better vertical control mark is within 200m of a class A or 500m of a class B boundary point defined by reduced level.
How do I determine whether my survey is within 200 metres of a vertical control mark?
Within the Landonline spatial view select All Layers/ vertical/ Vertical-Order 1-2 and Vertical-Order 3 to display the location of official vertical control marks in relation to the property you are surveying.
To determine compliance with Rule 4.3(a) you will need to extract the relevant vertical control marks from Landonline and import into your survey software to calculate a join. Alternatively, coordinates for relevant marks can also be obtained from the LINZ Geodetic Database and a join computed between that and a boundary point defined by reduced level.
What if my survey is within 200 metres of an NZVD2016 vertical control mark, but my client or the Council requires levels in terms of a local vertical datum?
You can continue to use the local vertical datum as long as it is an official datum in terms of LINZ ruling LINZR65301.
What if I want to use an alternative or assumed datum where I am required to use an official vertical datum?
You must use an official vertical datum. In very rare circumstances the Surveyor-General may issue a dispensation.
What happens if I commenced my survey prior to 1 January 2019?
As a transitional measure, an alternative or assumed datum that is permitted under rule 4.3(b) may continue to be used where a survey commenced before 1 January 2019.
In order for this dispensation to apply, the survey report must confirm that the survey commenced before 1 January 2019.
This transitional measure is valid for the period 1 January 2019 to 30th June 2019.
What happens if resource consent was issued prior to 1 January and it requires levels in terms of an assumed datum, but I have not yet started the cadastral survey?
You will need to apply to the Surveyor-General for a dispensation.
Does Rule 7.3.4(b) apply where the only vertical control marks within the distance specified by rule 7.3.2(a) are not in terms of the official vertical datum I am using?
No, Rule 7.3.4(b) only applies where the vertical control marks are in the same terms as the datum you have chosen.
Has there been any changes to official vertical control marks and their orders?
Official vertical control marks remain unchanged and as defined in LINZ ruling LINZR65303.
A vertical control mark is specified as an NZGD2000 mark that has been assigned a height in terms of an official vertical datum with a Landonline order 3V or better.
Where reduced levels must be in terms of the official vertical datum, connection to a control mark is not expressly required if levels can be obtained from other existing heighted marks in terms of that datum.
For a stratum boundary, if one or more vertical control marks are within 150 m (class A) or 500 m (class B) and one of these marks satisfies the criteria for a witness mark, then it is to be used as a heighted witness mark [r 7.3.4(b)] .
In the case of a heighted boundary, reduced levels must be in terms of an official vertical datum if a vertical control mark (3V or better in terms of an official vertical datum) is within:
- 200 m of any class A boundary point which is defined by the use of a reduced level, or
- 500 m of any class B boundary point that is defined by the use of a reduced level.
The distances specified are 'as the crow flies'.
An alternative vertical datum or an assumed datum may be used when rule 4.3(a) does not apply [r 4.3(b)].
Where an existing development is in terms of an alternative or assumed datum, dispensation will be required to allow reduced levels in subsequent stages to be in the same datum.
In some cases, local authorities have heighted survey marks, but these heights are not recorded in the cadastre as part of the geodetic control network.
If a survey is required by rule 4.3(a) to be in terms of an official vertical datum and a local authority height is nominally in terms of that datum, then the source of the origin of heights is required to be recorded in the CSD [r 8.1(f)].
Where a boundary is a surface that is mathematically described by having a reduced level at one or more boundary points, the boundary is considered to be a stratum boundary [r 6.8].
The online coordinate conversion service on the LINZ website enables heights (and coordinates) to be transformed between NZVD2016, NZGD2000 (ellipsoidal heights), and the 13 major local vertical datums.
The accuracy of the converted heights will be a combination of the original height accuracy and the NZGeoid2016/transformation accuracy. The Rules specify the accuracy between heighted points, not the heights themselves. Over the scale of most cadastral surveys, the effect of NZGeoid2016 transformation errors on height differences is likely to be small. The major source of error is likely to be from the surveyor's determination of heights.