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Frequently Asked Questions about NZ Geodetic Datum 2000 and Cadastral Survey
The answers to the following frequently asked questions are derived from sections 3.4, 3.5 and 3.7 of version 4.2 of the Surveyor-General's Cadastral Survey Guidelines. They provide an alternative way of identifying some of the key requirements relating to the implementation of NZGD2000 for cadastral survey - particularly on the application of Rule 24, Surveyor-General's Rules for Cadastral Survey 2002/2.
Note that references in the answers below to sections of the Guidelines apply to version 4.2. Note also that the Cadastral Survey Guidelines should be referred to for a more complete clarification of the requirements.
Are the requirements for connecting cadastral surveys to NZGD2000 greater than the requirement that previously applied for connection to NZGD49?
The requirements for when a survey should be connected are very similar. However, there are some changes as to how it is to be connected. In both cases, surveyors are expected to connect to geodetic marks within 1 km of the survey. See section 3.5.1.1 and its subsections in the Cadastral Survey Guidelines. Note in particular, that the application of the 1km rule has been clarified in section 3.5.1.1.1.
The Senior Advisor to the Surveyor-General (SASG) has discretion to relax this requirement in some cases where it would otherwise be unreasonable or impracticable. In this case, a dispensation should be sought before lodgement. Details of any dispensation granted should be included with the survey report. See section 3.5.1.1.8 of the Cadastral Survey Guidelines.
Some of the differences in how the survey is to be connected are as follows:
Did the requirement to use NZGD2000 start as soon as Landonline rolled out in each Land District?
Not as a requirement although it became an option at the time of rollout. For most Land Districts, the requirement to connect applied once the survey conversion project was completed within that Land District. Subsequently, the requirement for surveys to be in terms of NZGD2000 was applied to all Land Districts from 1 October 2002. However transitional provisions do apply. See sections 3.5.1.1.5 and 3.5.1.1.6 of the Cadastral Survey Guidelines for the transitional provisions.
Were any of the surveys lodged before roll out of Landonline in terms of NZGD2000?
No. NZGD2000 only became available for cadastral surveys once Landonline had rolled out in each Land District.
Can NZGD49 surveys still be accepted into Landonline?
Yes. NZGD49 survey can be accepted into Landonline and may be required in the following cases:
- the survey is not within 1 km of NZGD2000 geodetic marks (6th order or better) but is within 1 km of NZGD49; or
- the survey is within 1 km of both NZGD49 and NZGD2000 geodetic marks and the Senior Advisor to the Surveyor-General has, for reasons of practicality, agreed to a dispensation waiving the requirement for it to be in terms of NZGD2000, provided that the survey is in terms of adjacent NZGD49 surveys; or
- the survey is within 1 km of both NZGD49 and NZGD2000 geodetic marks but the transitional provisions apply - i.e. new survey work commenced before the date notified by the Surveyor-General where use of NZGD2000 is a requirement (1 October 2002).
Refer to sections 3.5.1, 3.5.1.1.5, 3.5.1.1.6, 3.5.1.1.8 and 3.7.1 of the Cadastral Survey Guidelines.
Can Old Cadastral Datum (OCD) surveys still be accepted into Landonline?
Yes. OCD survey can be accepted into Landonline. This may occur where:
- the survey is not within 1 km of either NZGD49 marks or NZGD2000 geodetic marks (6th order or better); or
- the survey is within 1 km of either NZGD49 or NZGD2000 geodetic marks but the Senior Advisor to the Surveyor-General has, for reasons of practicality, agreed to a dispensation waiving the requirement for connection to a geodetic datum; or
- the survey is within 1 km of NZGD2000 geodetic marks but not within 1 km of NZGD49 surveys; and the new survey work commenced before the date notified by the Surveyor General where use of NZGD2000 is a requirement (1 October 2002).
See sections 3.5.1, 3.5.1.1.2, 3.5.1.1.5, 3.5.1.1.6, 3.5.1.1.8, 3.5.4, 3.7.1 and 3.7.3.3 of the Cadastral Survey Guidelines.
How do I identify NZGD2000 geodetic control marks?
NZGD2000 geodetic control (5th order or better) can be identified in the Landonline Spatial View with the:
"NZGD2000 Order 0-2",
"NZGD2000 Order 3" and
"NZGD2000 Order 4-5"
mark layers turned on, or in the Geodetic Database on the LINZ web site.
Currently 6th order and 7th order marks are in a common layer
"NZGD2000 Order 6-7".
The 6th order points are distinguished by being non-boundary marks and by the mark symbol having a small "6" inside the triangle. Note that 6th order NZGD2000 marks are not in the Geodetic Database.
LINZ proposes, later in 2003, to re-organise the mark layers in Landonline so that two layers will be used to cover geodetic control marks (5th order and better) and a third layer will show only 6th order marks. This will make it easier to identify the NZGD2000 geodetic marks (6th order and better) that can be used for an origin.
See section 3.7.1.1 of the Cadastral Survey Guidelines.
For an explanation of horizontal orders of NZGD2000 coordinates, click here
Does the requirement to put a survey in terms of NZGD2000 mean that I always have to connect to geodetic control marks?
Not necessarily. NZGD2000 coordinated traverse marks can also be used for NZGD2000 origins provided that they have had 6th Order 2000 coordinates assigned to them.
Traverse marks that are well connected to NZGD2000 control and which meet the accuracy standards of the Survey Regulations 1998 are assigned 6th Order 2000 coordinates in the following circumstances:
How many NZGD2000 marks do I have to connect to for an NZGD2000 survey?
If there is only one NZGD2000 geodetic mark within 1 km of the survey, then it should be connected to the survey by a closed loop (which can include adopted observations). See sections 3.5.1.1.3 and 3.7.3.2 of the Cadastral Survey Guidelines.
If there are two NZGD2000 geodetic marks within 1 km of the survey, then they should both be connected to the survey by bearing and distance observations (which can include adopted observations). See sections 3.5.1.1.4 and 3.7.3.2 of the Cadastral Survey Guidelines.
If there are three or more NZGD2000 geodetic marks within 1 km of the survey, then it may be highly desirable to connect to three of them but the survey is only required (in the absence of a dispensation) to be connected to two of them. See section 3.5.1.1.4 of the Cadastral Survey Guidelines. Note however that the requirement under Rule 7, SG Rules for Cadastral Survey, for the origin of bearings to be proven by a third mark still applies.
What does "connected to" mean in the above question? Do I have to traverse through these origin marks?
Not necessarily. A survey may connect to two or more geodetic origin marks by an observed or adopted tie (bearing and distance). If there is only one NZGD2000 geodetic mark within 1 km and is included in the survey, then the connection to it must form a closed loop. See section 3.5.1.1.1 of the Cadastral Survey Guidelines. Also refer to section 3.7.3.2 for the requirements for ties by adoption.
If NZGD2000 geodetic marks in the survey are inter-visible, then observing between them would be desirable to prove their reliability and confirm the NZGD2000 bearing origin. However it is not essential provided the survey complies with normal good survey practice. See section 3.7.3.1 of the Cadastral Survey Guidelines.
Do I have to determine bearing swings between NZGD49 and NZGD2000?
Only if calculations lead you to believe that there is a significant bearing difference. In general it is expected that NZGD49 bearings will not be significantly different from NZGD2000 bearings, within the limits of accuracy required by Rule 26. In most cases, these differences are of the order of a few seconds, which is insignificant for cadastral survey.
However there are some cases where differences of the order of 30" have been reported. These could be due to:
- local variations in the orientation of NZGD49 surveys;
- errors or inadequacies in the NZGD2000 geodetic control; or
- movement of the NZGD2000 geodetic control marks between when they were originally surveyed and when they are re-occupied for the cadastral survey. Note that only a minority of the 5th Order 2000 control marks have been recently resurveyed. The majority of the NZGD2000 5th Order 2000 control marks were coordinated by readjustment of historical observations captured from survey datasets. It is therefore particularly necessary in utilising these marks, that mark reliability is proven.
Despite these exceptions, in the absence of any indication of a discrepancy the surveyor is entitled to adopt NZGD49 bearings as if they were NZGD2000 bearings.
See section 3.7.2.1 of the Cadastral Survey Guidelines.
Furthermore, because of this presumed equivalence, it is not mandatory to provide a panel note stating that a bearing correction of 0" has been applied to convert NZGD49 bearing to NZGD2000 bearings. However such a note may be of benefit to other surveyors in the future and is therefore desirable. If a bearing correction other than 0" is determined however, a panel note (in terms of section 6.2.18 of the Cadastral Survey Guidelines) will be required and the survey report will need to provide evidence of how the non-zero correction was determined.
See section 3.4.1 of the Cadastral Survey Guidelines.
If an underlying NZGD49 survey shows a bearing correction applied to older Old Cadastral Datum (OCD) bearings, can I adopt this correction for a new NZGD2000 survey?
Yes. This correction can be adopted as representing the correction from OCD to NZGD2000 (provided no significant difference between NZGD49 and NZGD2000 has been detected), subject to the usual commonsense rules for adoption of this data. For example, on a large underlying OCD survey, a bearing correction determined for a small part of that survey might not be applicable to other parts of the survey hundreds of metres or even kilometres away. This should be proven in the usual manner if outside the original area of proof. This proviso is also true for adoption of underlying bearing corrections on NZGD49 surveys.
Therefore, where it is reasonable to adopt an underlying bearing correction, the OCD - NZGD49 correction can be adopted as being the OCD - NZGD2000 correction. The usual panel note (in terms of section 6.2.18 of the Cadastral Survey Guidelines) will still be required reporting the source and value of the adoption of the previously determined bearing correction. However it will not be necessary to state or justify the acceptance of NZGD49 and NZGD2000 bearings as being consistent with each other.
See section 3.4.1 of the Cadastral Survey Guidelines.
If I can observe between the NZGD2000 control marks but no previous observation of these lines is available, how do I get a bearing origin?
The preferred method would be to use the NZGD2000 marks for coordinate origin but to obtain the bearing origin by re-observation of any available NZGD49 traverse lines. Note that the NZGD49 bearing of these lines can generally be adopted as being not significantly different from NZGD2000 bearings (see section 3.7.2.1 of the Cadastral Survey Guidelines). These origin lines chosen and re-observed should comply with the usual good survey practice principles - using terminal marks where possible or sufficiently long lines and checking on another line.
Calculation of an origin of bearings by coordinates, while not the preferred method, is also an option and in some cases, will be the only viable method of obtaining geodetic bearings. If there are no previous observations between the intervisible NZGD2000 marks and no other NZGD49 or NZGD2000 surveys that can be used for a geodetic origin of bearings, then a calculated origin of bearings based on NZGD2000 geodetic coordinates would be used.
Note however that adjusted coordinates are a summary of a best fit of a number of observations and may contain adjustments that affect observed bearing alignments. Note also that bearings calculated from coordinates of marks on a straight alignment may show significant differences between some short sections of the alignment and the whole line between terminals. Therefore, where coordinates are used to compute bearings between NZGD2000 control marks, these lines should be sufficiently long, and preferably between alignment terminals, to ensure that adjustment of the coordinates is not likely to significantly affect the bearings within cadastral limits.
Also note the requirement of Rule 8, Surveyor General's Rules for Cadastral Survey 2002/2 that "The origin marks and traverse of a survey must be configured in such a way that the bearing and positional accuracy are maintained over the whole survey."
See section 3.7.2.2 of the Cadastral Survey Guidelines.
If the nearby NZGD2000 control marks are not inter-visible, how do I get an origin of bearings?
You are entitled to consider NZGD49 bearings and NZGD2000 bearings as being in terms with each other (unless evidence specifically indicates that they are out of terms). This means that a survey based on an NZGD49 origin of bearings will usually be able to (and generally should) specify that the bearings of the new survey are in terms of NZGD2000.
The 5th order 2000 control marks are almost all existing cadastral marks so even if they are not intervisible with each other, they will usually each be intervisible with other cadastral marks in their localities. Therefore an inter-visible NZGD49 bearing origin will usually be available adjacent to NZGD2000 control marks even though it may not be to other NZGD2000 marks. Re-observation of such an NZGD49 line will generally enable it to be accepted as providing an NZGD2000 origin and this bearing origin will also be able to be tested for consistency with the orientation inherent in the NZGD2000 coordinates.
See section 3.7.2.1 of the Cadastral Survey Guidelines.
What do I do if there is NZGD2000 control nearby which is not inter-visible and all the underlying bearings are Old Cadastral Datum (OCD)?
In this case, a bearing correction will need to be calculated for the OCD bearings. However, this is hampered by the lack of an intervisible NZGD2000 origin of bearings.
If there are three or more NZGD2000 marks which are well distributed in relation to the survey, a bearing correction between OCD and NZGD2000 can be determined by comparison of the missing line between the origin marks derived from OCD traverses and the bearings calculated from NZGD2000 coordinates. This is the ray trace method. The availability of at least three NZGD2000 geodetic marks allows the consistency of this bearing correction to be checked.
If it is only practicable to connect to two NZGD2000 geodetic marks, the use of GPS may be necessary for an independent confirmation of the orientation of the origin marks and the NZGD2000 orientation of observed OCD lines. If this is also not practicable, or if there is only one NZGD2000 geodetic mark, and the bearing correction between NZGD2000 and OCD is not capable of independent confirmation, a dispensation may be sought from the Senior Advisor to the Surveyor-General (refer to section 3.5.1.1.8 of the Cadastral Survey Guidelines) to lodge the survey with bearings and coordinates in terms of Old Cadastral Datum.
Even where only one or two NZGD2000 marks are surveyed, three existing origin marks will still be required to prove the reliability of the origin of bearings in terms of Rule 7 of the Surveyor-General's Rules for Cadastral Survey.
An origin of coordinates can also be obtained from one NZGD2000 geodetic mark and an NZGD2000 origin of bearings may be able to be obtained by bearing-only observation to distant NZGD2000 geodetic marks, or from an NZGD49 origin of bearings.
See section 3.7.3.3 of the Cadastral Survey Guidelines.
Where the bearing swing for an underlying Old Cadastral Datum (OCD) survey has not been determined on an earlier survey, how should it be calculated?
By following the same procedures outlined in section 3.4.1 of the Cadastral Survey Guidelines for determining a bearing swing between OCD and NZGD49.
Should I calculate and apply a scale factor to distances to put them in terms of NZGD2000?
No. The scale differences between NZGD49 and NZGD2000 are not significant for cadastral survey. The distances shown on cadastral surveys are nominally reduced to the datum ellipsoid and the different levels of the NZGD49 and NZGD2000 ellipsoids across New Zealand, in theory, imply differences for ellipsoidal distances of up to a few parts per million. In practice, this is insignificant for cadastral survey. It is also irrelevant in those cases where, in line with standard practice, the distances on new or older surveys have been reduced to sea level, rather than to the ellipsoid.
Therefore, no general scale factor should be applied to distances on underlying survey datasets when adopting these for an NZGD2000 survey.
In special cases, where application of a scale factor to adopted distances is considered by the surveyor to be strongly supported by the evidence - based on comparison of several lines of different lengths, this will be considered to be evidence of a consistent scale error in the distances on the underlying survey rather than a datum scale difference. In this case, if it is necessary to use distances from the underlying survey to provide definition or to prove marks, the resulting distances should be identified as calculated rather than adopted. This will need to be justified in the survey report.
See section 3.5.5 of the Cadastral Survey Guidelines.
Note that the new NZGD2000 meridional circuits have the same projection scale factors as the NZGD49 circuits. Calculations to generate coordinates or to calculate missing lines from circuit coordinates will need to apply projection formulae (including projection scale factors) in the usual manner. However, this does not apply to the adoption of distances shown on survey datasets and traverse sheets which are sea level distances, not projection distances, and can therefore be adopted without correction.
See section 3.5.2 of the Cadastral Survey Guidelines.
Have all NZGD2000 geodetic control marks been resurveyed into the new datum?
No. All marks of NZGD2000 4th order or better have been resurveyed in the last few years. A minority of NZGD2000 5th order marks have also been recently surveyed - usually by GPS. Most of these resurveyed marks are existing cadastral marks.
However, the majority of 5th order marks were not resurveyed but instead were coordinated by readjustment of historical observations captured from survey datasets. It is possible with some of these marks, that the mark had already been destroyed or disturbed at the time that a new NZGD2000 coordinate was generated for it. It is therefore particularly necessary in utilising these marks, that mark reliability is proven.
See section 3.7.1.2 of the Cadastral Survey Guidelines.
How do I tell which 5th Order control marks have been resurveyed (and therefore recently confirmed as being in position) and which were only coordinated by readjustment of historical observations?
At present, it is not immediately obvious which marks have been resurveyed but this can be deduced in Landonline or from information on the LINZ web site. The 5th Order 2000 marks that have been coordinated by readjustment of historical observations have an Ellipsoidal Height of 0.000 or the height field is null on the 'View Mark/Node' form. This can be used to distinguish them from the recently GPS surveyed marks which have a non-zero ellipsoidal height.
Implementation of an enhancement to this information in Landonline and the web-based geodetic database is underway. The intention is that in the 'Mark Physical State Information' panel of Landonline's View Mark/Node form, the following changes will be made:
- Under 'Description' a note will be added to say that the mark has been coordinated from historic data.
- Under 'Condition' the note will change from Not Specified to Unknown.
- Under 'Date' the date shown will reflect the date of the original survey from which the observations were captured and not the date of readjustment (as now).
The last of these three bullet points is expected to prove the most useful in that it will enable surveyors to judge for themselves, how likely it is that the mark is still in place and undisturbed.
See section 3.7.1.2 of the Cadastral Survey Guidelines.
I understand that geodetic control marks in Landonline, periodically have their coordinates updated as new information comes to hand. If this happens after I have coordinated my survey, will I have to re-coordinate it and generate new traverse sheets?
No. Surveyors are no longer required to always use the latest available coordinates where geodetic control marks are used for an origin of coordinates. New coordinates for marks in the survey in terms of NZGD2000 will be generated in Landonline following approval of the survey. The coordinates provided by the surveyor play only a transitional role during survey processing.
This also means that where the coordinates in Landonline have changed (which happens from time to time) during the conduct of a survey, it is not necessary to re-generate traverse sheets that were based on earlier published coordinate values.
Note however, that if the origin of NZGD2000 bearings was derived by calculation from coordinates and the coordinates have subsequently changed, it will be important to confirm by recalculation that the calculated origin of bearings has not been materially affected. If the coordinate shifts are small and/or consistent, or where the origin marks are sufficiently well separated, the change in calculated bearings will often not be significant. However, note that this is one reason why the calculation of the origin of bearings from coordinates is not LINZ's preferred method of determining a NZGD2000 bearing origin.
See section 3.7.1 of the Cadastral Survey Guidelines.
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