This section comments on some of these techniques and provides a table of predicted tidal levels at the New Zealand standard ports that can be used for MHWS determination.
Determination of MHWS
A number of surveyors have sought clarification regarding the establishment of MHWS boundaries. Unfortunately there is no single definitive method that can be used to establish this type of boundary. The approach taken by the surveyor needs to be customised to the individual location and take into account, amongst other things, the hydraulic gradient, the type and value of land concerned and the survey accuracy 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 semi-urban beach front subdivision. Baker and Watkins (1991)1 give a number of different options for establishing MHWS boundaries that depend on the accuracy of the survey required, basically the higher the accuracy the more work that is required by the surveyor.
The Eduction and Range-Ratio Methods described in Baker and Watkins (1991) are suitable where high accuracy definitions are sought. These methods transfer a level of MHWS at a Standard Port tide gauge to the survey location using a local tide gauge and sea level observations. Either an existing gauge or one established for the duration of the survey can be used. An alternative practice of direct leveling from a standard port tide gauge can only be used in close proximity to that gauge.
Previous advice has been to use the MHWS levels obtained from the Nautical Almanac. Due to changes in the way that the levels have been calculated these values should not be used for the determination of cadastral or administrative boundaries. The Nautical Almanac table is produced for navigation purposes and only represents the tidal levels for the next 12 month period. These values will 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 the average predicted values over the 18.6 year tidal cycle. When tidal information is being used as part of a MHWS determination for cadastral surveys these values should be used.
The use of tidal level information is only one approach of determining MHWS. The actual approach used to achieve this determination needs to be customised to the individual location and take into account, amongst other things, the hydraulic gradient, the type and value of land concerned and the survey accuracy required.
WARNING: These values must not be used for navigation purposes.
|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|
The above levels are referred to CHART DATUM, which is the same as the zero of predictions in all cases.
The values for the MHWS, MHWN, MLWN and MLWS tidal levels for each Standard Port are the averages of the levels of all spring and neap tides predicted to occur under average meteorological conditions during the period 1 January 2000 - 31 December 2018 using the designated harmonic constituent set.
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.