A Christchurch CAT sheet requires a vertical datum that may be different from the datum used on site. Read this first.

The datum and horizontal circuit CCC requires

A datum is an agreed reference system for measuring position. You need two: one for horizontal position (where something is on the map) and one for height. In New Zealand, local horizontal datums are called circuits.

  • Vertical (height) datum: NZVD2016. CCC is unambiguous: "Make sure you're using the NZVD2016 vertical datum. If the old datum is used, we won't be able to process your CAT sheet" (CCC, As-built survey and data requirements). NZVD2016 (New Zealand Vertical Datum 2016) is the official national height datum maintained by ToitÅ« Te Whenua / Land Information New Zealand (LINZ), used "to define heights in New Zealand and its offshore islands" (LINZ, "New Zealand Vertical Datum 2016").
  • Horizontal circuit: Mt Pleasant 2000 (NZGD2000). IDS Part 12 states: "For all supplied geospatial data, use the Mt Pleasant 2000 projection, New Zealand Geodetic Datum 2000 (NZGD2000)" (CCC, IDS Part 12). The SAG repeats this: "these are in Mt Pleasant 2000 projection (New Zealand Geodetic Datum 2000)." Mt Pleasant 2000 is the local "meridional circuit" projection for the Christchurch area (EPSG:2124), one of New Zealand's 28 circuits used for cadastral and local survey work (LINZ, "Meridional circuits").

Why there might be more than one height datum to think about

Until recently Christchurch used a local height datum called Christchurch Drainage Datum (CDD), in use since 1876. CCC has now retired it: CCC adopted NZVD2016 on 19 August 2024 and they state: "Data supplied to Christchurch City Council should only be in NZVD2016" (CCC, "Vertical Datum 2016"). Construction drawings drafted before this date will likely use the old CDD datum.

CDD and NZVD2016 are not interchangeable - CDD is a straight offset to the Lyttelton 1937 datum, with the offset being 9.043 m (CCC, "Vertical Datum 2016"). The reason for this offset was to avoid having underground assets with a negative height value (below 0 m in Lyttelton 1937). The use of NZVD2016 for survey heights is reinforced nationally: LINZ made NZVD2016 mandatory for cadastral survey from 1 July 2024 (LINZ, "Obtaining NZVD2016 Heights").

GNSS, accuracy and "location certainty"

GNSS (Global Navigation Satellite System) is the umbrella term for satellite positioning - GPS is one such system. GNSS observations naturally produce an ellipsoidal height in terms of NZGD2000; survey equipment can convert that to an NZVD2016 height by applying the geoid model (NZGeoid2016), or alternatively LINZ's tools do it for you (LINZ, "Obtaining NZVD2016 Heights").

The SAG dictates accuracy tolerances for your CAT, and there are a few to be aware of: for example pipe inverts to plus/minus 50 mm horizontally and plus/minus 10 mm in height, and manhole lids/bases to plus/minus 100 mm horizontally and plus/minus 30 mm in height - and the SAG explicitly notes: "these accuracies cannot be reached using GPS devices" (CCC, Survey As-built Guideline, Table 10). In other words, standalone GNSS isn't enough for inverts and chamber levels; conventional survey methods are needed for these.

Where an asset genuinely can't be reached (e.g. directional-drilled pipe with no accessible ends, or a lined pipe with no manhole access), the SAG allows a looser "Approximate" category - but you must set the "Location Certainty" attribute accordingly and justify it in the survey report, which is one of the tabs in the Excel CAT sheet.

A tool that helps

LINZ provides a free online coordinate converter (Concord) to convert between NZ datums, projections and height systems - useful for sanity-checking which datum your data is in (LINZ, geodesy.linz.govt.nz/concord).

Sources & links