Determination of MHWS
There is no definitive method for establishing mean high-water springs (MHWS) boundaries – it requires a tailored approach that considers factors like hydraulic gradient, land type and the survey accuracy that is required. For example, the definition of the coastal boundary of a large rural class III or IV survey is often based on physical evidence and will not require the same degree of rigour as a high-value beachfront subdivision.
Baker and Watkins (1991, referenced below) suggest several methods for establishing MHWS boundaries, with greater accuracy tending to require more work by the surveyor. Direct levelling can only be used near a standard port tide gauge, so the most common methods for high accuracy definition are the education and range-ratio methods. Both involve the transformation of a MHWS level from a standard port tide gauge to the survey location using a local tide gauge and sea level observations (with a gauge established for the duration of the survey if required).
Using MHWS levels from the Nautical Almanac is no longer recommend for cadastral or administrative boundaries due to their navigation-oriented nature and annual variability. The Nautical Almanac only represents tidal levels for the next 12-month period, and these change from year to year by up to 15 centimetres over the course of the 18.6 year tidal cycle.
Predicted tidal levels for surveyors
The following tidal levels are average predicted values over the 18.6-year tidal cycle. These are suitable for MHWS determination in cadastral surveys, but this is only one way to determine MHWS – the most appropriate method should be determined by a cadastral surveyor.
See guidance for tidal boundaries
Tidal levels at New Zealand standard ports for cadastral and engineering purposes
Warning
These values must not be used for navigation purposes.
| Standard Port | MHWS (m) | MHWN (m) | MLWN (m) | MLWS (m) | Definition of Chart Datum | Harmonic Constituent Set |
|---|---|---|---|---|---|---|
| Auckland | 3.30 | 2.78 | 0.95 | 0.41 | 5.233m below BM 98-21 SO 69501 (DD1N) | 2 Jul 1996 |
| Bluff | 2.82 | 2.43 | 1.04 | 0.57 | 8.620m below Bluff Fundamental BM (ABCC) | 1 Jan 2002 |
| Dunedin | 2.18 | 1.80 | 0.39 | 0.07 | 3.728m below BM WW 83 (AFEQ) | 1 Jul 2002 |
| Gisborne | 2.02 | 1.76 | 0.70 | 0.44 | 4.091m below BM GB 01 (ACVP) | 15 Feb 2005 |
| Lyttelton | 2.49 | 2.05 | 0.65 | 0.27 | 4.478m below BM UD 40 (B40V) | 1 Jul 2000 |
| Marsden Point | 2.72 | 2.29 | 0.86 | 0.42 | 4.816m below RNZN BM (DJM9) | 2 Jul 1996 |
| Napier | 1.84 | 1.46 | 0.40 | 0.06 | 4.837m below BM H40 (B3XM) | 1 July 1997 |
| Nelson | 4.26 | 3.24 | 1.41 | 0.46 | 5.733m below BM N1 (AC4T) | 1 Feb 2001 |
| Onehunga | 4.19 | 3.39 | 1.38 | 0.51 | 5.593m below BM CC 65 (ADLT) | 1 Jul 2003 |
| Picton | 1.62 | 1.12 | 0.58 | 0.13 | 2.716m below Elaine Cairn (BQFK) | 24 Aug 2005 |
| Port Chalmers | 2.14 | 1.77 | 0.47 | 0.16 | 3.816m below Pin 1 SO 17533 (DR0F) | 1 Jan 2003 |
| Port Taranaki | 3.58 | 2.78 | 1.14 | 0.34 | 6.709m below New Plymouth Fundamental BM (AGMH) | 1 Jan 1996 |
| Tauranga | 1.88 | 1.60 | 0.44 | 0.13 | 4.103m below BM BC 84 (B309) | 1 July 1996 |
| Timaru | 2.42 | 2.05 | 0.77 | 0.45 | 5.759m below BM UD 42 (B2Y9) | 1 Jan 2004 |
| Wellington | 1.77 | 1.45 | 0.70 | 0.45 | 3.565m below BM K80/2 (ABPC) | 2 Jan 1996 |
| Westport | 3.25 | 2.57 | 0.94 | 0.25 | 7.351m below BM Harbour Masters Office (DJMC) | 1 Aug 2000 |
| Whangarei | 3.12 | 2.64 | 1.02 | 0.52 | 5.182m below BM DD99/23 (A2Q9) | 2 Jan 2003 |
Reference
Baker and Watkins (1991) "Guidance notes for the determination of Mean High Water Mark for land title surveys" NZIS Professional Development Committee; published in Kearns, Kerr and Smith (1997) Chapter 5 Law for Surveyors - Boundaries and boundary definition, Dept of Surveying University of Otago/NZIS available from School of Surveying, University of Otago.