Determining local vertical datum (LVD) heights

The nature of the local vertical datum (LVD) networks can lead to inconsistencies, depending on the source of the heights used. This can create challenges and is why New Zealand Vertical Datum 2016 (NZVD2016) should be used whenever possible.

New Zealand Vertical Datum 2016 (NZVD2016) allows access to the datum or reference framework from a GNSS (global navigation satellite system) observation, but you need a connection to a local vertical datum (LVD) benchmark to connect to an LVD. The LVD benchmark network is mostly limited to state highways and the early developments in major towns and cities, so the LVD is undefined for most of the country.

New Zealand Vertical Datum 2016 (NZVD2016)

The LVD benchmarks were usually measured using long, poorly constrained networks, which meant that any errors or movement could easily be propagated. Most of the LVD network of benchmarks was established between 1940 and 1990 and has not been reobserved since. This has allowed the benchmarks themselves to move with local land movement, tectonic motion, or due to nearby development. The introduction of NZVD2016, which is based on a reference surface rather than a network of points, has exposed some of the limitations of the LVD including errors that were previously invisible.

NZVD2016 includes a vertical datum relationship (VDR) grid which helps users shift existing data to NZVD2016. This is a better solution than supporting the use of LVDs as any errors or uncertainties are inherent in LVD height values themselves, and the first step to fix this is to move to a consistent datum.

Vertical datum relationship (VDR) grids

Published LVD heights may differ from heights computed using the transformation grids, which can themselves differ from observations using GNSS. Deciding which (if any) of these heights is best for a project is not always straightforward.

Mark reliability

Published heights are only as reliable as they were when they were last measured. If the mark was last measured some time ago, it becomes more likely the mark has been moved or replaced. There are some features that can be checked to confirm its reliability.

  1. Is the mark itself reliable?
  • Do nearby marks have any similar inconsistencies?
  • Does the mark type match what is in the database? 
  • Does the mark have a clear reference point?
  • Is the mark in an area of known deformation, such as landslides, earthquakes, or changing hydrologic conditions?
  • Does the mark feedback include comments from users who identified a problem at this site?
  1. Is the published NZVD2016 height reliable?

NZVD2016 heights have been computed using the results of the National Geodetic Adjustment (NGA). Marks in this adjustment have been tested and assigned orders based on the computation statistics, so users can have confidence that if they level between any 2 published NZVD2016 marks the observations will close within tolerances.

National Geodetic Adjustment

  • The mark order indicates the expected accuracy of the mark.
  • The observation date and previous observations indicate if there have been any known changes (any GNSS survey will be listed in the NZGD2000 observations).

The current NZVD2016 height of a mark can be checked by completing a GNSS survey. If any problems are found with an NZVD2016 height please report this to Toitū Te Whenua for investigation.

Providing survey mark feedback

  1. Is the published LVD height reliable?

LVD heights have come from many sources, but most 1V and 2V heights are from precise levelling, and 3V and 4V from less accurate methods such as trig heighting. The observation methods used to determine LVD heights are very good at determining the uncertainties on lines which have been directly measured, but it is difficult to check if the resulting mark orders are statistically accurate.

  • The mark order indicates its expected accuracy.
  • The observation date and previous observations indicate the likelihood of changes over time.

The current LVD height of a mark can be checked by taking survey observations between the mark and the nearest reliable benchmark. This may not be practical if the nearest reliable benchmark could be many kilometres away.

Published or calculated values

After ensuring the mark and its published height are reliable, the surveyor needs to use their judgement in deciding whether to use the published LVD height or the one computed using the VDR grid. You should consider how the data will be used. For example:

  • If this project is going to connect other works, what reference did they use?
  • Can the project be completed in terms of NZVD2016 and then converted to the LVD for submission?
  • Does the project connect to multiple benchmarks, and how do they fit?
  • Is the project more concerned with local or network accuracy (also known as relative and absolute accuracy)?

Updated metadata

Whichever LVD value you use, your metadata should include:

  • the vertical datum used
  • the control mark height 
  • the method used for determining the control mark height
  • the date the height was observed.
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