Q&As on Datums & the Rules for Cadastral Survey 2010

These questions and answers relate to geodetic datums and the new Rules for Cadastral Survey 2010. They were formed during a series of education seminars on the new Rules held around the country in September 2009.

Other questions and answers on the new Rules are also available.

If you have a question, please email newcadastralrules@linz.govt.nz.

New Zealand Geodetic Datum 2000 (NZGD2000) orientation

Will the Surveyor-General provide guidance on what is an adequate origin of bearings?

No. Because of the different types of survey equipment and methods used, the surveyor will be in the best position to determine firstly how best to obtain an origin, and secondly how to satisfy themselves that the orientation is correct.

Is it sufficient to establish bearings using observed GPS vectors (rather than using three old marks)?

The Rules focus on ensuring the orientation is in terms of NZGD2000 rather than in terms of three old marks. Determining the orientation by the correct use of GPS can meet the orientation requirements of rule 4.1. There is no requirement to connect to three old marks for orientation – old marks are only necessary in the context of survey definition, and the number of old marks for this purpose is not specified.

Is a surveyor required to locate three old marks for an origin of bearings when using a theodolite?

No. The Rules focus on ensuring the orientation is in terms of NZGD2000 rather than the method of determining the orientation. A surveyor may use any method they consider adequate to ensure their survey orientation is correct.

Can a surveyor assume that the orientation shown on an existing survey as NZGD1949 is in terms of NZGD2000?

No. Notwithstanding that the datum orientation for NZGD1949 and NZGD2000 is virtually the same, the bearings on some existing surveys have not been accurately obtained. A surveyor will need to satisfy themselves that the bearings of the previous survey are in terms of the local NZGD2000 projection (rule 4.1).

Can a surveyor assume that the orientation shown on an existing survey as NZGD2000 is in terms of NZGD2000?

No. A surveyor will need to satisfy themselves that the bearings of the existing survey are in terms of the local NZGD2000 projection (rule 4.1).

How does a surveyor satisfy the geodetic orientation requirement when they do not have GPS or there are no NZGD2000 control marks present (especially in rural areas)?

The method of achieving this is left to the surveyor who in some circumstances may need to carry out additional work (eg by observing rays to distant trig beacons) or in some cases acquiring or hiring suitable equipment such as GPS.

Where an area is lacking control marks, and there is a long lead in time for the survey, the surveyor can request LINZ to provide additional control marks

Do CSD’s containing only compiled data have to be oriented in terms of NZGD2000?

No. The requirement to orientate in terms of NZGD2000 does not apply where there is no new field measurement (rule 4.1(b)).

Do CSD’s with only non-primary parcels need to be oriented in terms of NZGD2000?

No. Only surveys defining a new primary parcel point are required to be orientated in terms of NZGD2000 (rule 4.1). Surveys of new non-primary parcels may use the orientation of the underlying primary parcel.

Where will the 12 month transition requirements for NZGD2000 be published?

The transitional requirements will be published in a Surveyor-General’s Ruling. The Ruling will be available on the LINZ web site prior to the introduction of the 2010 Rules.

Will dispensations from the NZGD2000 orientation requirement still be granted after the 12 month exemption ends?

Such dispensations will only be approved in very exceptional cases where the Surveyor-General is satisfied that there are no significant risks to the cadastral outcomes or to circumstances not covered by the Rules.

Vertical datum

Will there be a single vertical datum?

The official vertical datums will be the New Zealand Vertical datum 2009 (NZVD2009) that covers the whole country, and the 13 local mean sea level vertical datums. These will be listed in a Surveyor-General’s Ruling to accompany the 2010 Rules.

Can heights be in terms of a datum other than an official vertical datum (eg a local authority datum)?

Where there is no official vertical control mark within the distances specified in rule 4.3, the surveyor can use any other datum (including an assumed datum).

In some circumstances local authorities are maintaining heighted marks that are not recorded in the LINZ database. Are they acceptable height values?

If the survey is required by rule 4.3(a) to be in terms of an official vertical datum and the local authority height is nominally in terms of that datum, then the surveyor will be taking responsibility for the correctness of the height. However, if the survey is not required to be in terms of an official vertical datum, then the local authority height could act as an alternative or assumed vertical datum under rule 4.3(b).

If I use GPS to determine the ellipsoidal heights of marks, can I then use the LINZ coordinate conversion service to transform them to normal-orthometric heights that are in terms of an approved vertical datum?

Yes, the New Zealand Vertical datum 2009 (NZVD2009) is defined by the NZGeoid09 geoid surface that is implemented in the online coordinate conversion service on the LINZ website. This service enables heights (and coordinates) to be transformed between NZVD2009, 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 NZGeoid09 / transformation accuracy. The Rules specify the accuracy of between heighted points, not heights. Over the scale of most CSDs the effect of NZGeoid09 errors on height differences is likely to be small. The major source of error will be from the GPS determined heights.