Overview

Throughout history, the expanse of one’s land or ownership has been defined by boundaries. Whether it be natural boundaries such as those formed by water or rock faces, or artificial boundaries such as lines between corners, walls of buildings or fences, boundaries serve to limit one’s possession and hence there have always been issues in regards to the location of a boundary.

In today’s society, the most frequently and bitterly contested boundaries are water boundaries. The edge of water forms an excellent natural boundary in that it is easily defended and easily recognised. However, when attempts are made to accurately locate water boundaries, complex technical and legal problems may occur. This is because the surfaces of most water bodies are constantly changing due to tide and meteorological conditions. A water boundary is therefore a dynamic boundary and unlike most other boundaries that are two dimensional, a third dimension (height) and a fourth dimension (time) must be considered (Cole, 1997).

In New South Wales the boundary of all land having frontage to tidal water is, by Common Law at the position and level of Mean High Water (MHW) (Department of Lands, 1976).

The Surveying and Spatial Information Regulation 2006 defines MHW or the Mean High Water Mark (MHWM) as “the water line of mean high tide between the ordinary high-water spring and ordinary high-water neap tides”. There have been various views in the past on what the term ‘ordinary’ actually depicts and indeed what range of uncertainty there is in defining such a dynamic boundary. Various information and resources are available to a surveyor to assist in locating this boundary but how accurate will the determination be? Is the location of this boundary likely to change regularly depending on different techniques used by different surveyors at different times? Geoff Songberg (2004), states that an accurate MHWM determination cannot be achieved, not because the techniques or observations are inaccurate or that there are too many errors in the data, but rather because mean high water and other tidal effects fluctuate over time. With a wide range of methods available to be used to obtain MHW level at a certain location and to determine the MHWM, this thesis endeavours to understand how accurate, or inaccurate for that matter, two select techniques can be, amid the varying effects of the water bodies, tides and tidal patterns.

The two different techniques that were analysed in this thesis are not methods for marking out a water boundary as such, but rather are seen as effective techniques that can be used as part of the MHWM determination process. The two techniques both encompass height transfer using a water body, but each is used for a different purpose.

The first technique that was examined - the Range Ratio method, involves height transfer along a river or estuary to obtain the height for the MHW plane at a specific site. This was complemented by the examination of a second technique – the Radio Transfer method, which uses the water body to transfer height, from one point across or along the waterway to another location.

The Range Ratio method is a prescribed technique outlined in Part 6 of the Integrated Survey Manual (1976) as a method of MHWM determination. This contrasts from the Radio Transfer method which is an unofficial method not widely recognised nor familiar throughout the surveying community. This provided a good platform for testing and analysis in order to discover the accuracies of, and variables involved in transferring heights along or across water using this method.

The analysis of water bodies and tides in particular, also acted as a major part of this thesis. The importance in understanding the influence which tides and the effects on tides, had on the testing procedures along with the outcomes, was imperative in the overall examination of the two methods. Five key elements were earmarked as having the greatest impact on the results obtained from this testing. These elements were astronomical and non-astronomical effects on tides, predicted tides, tidal lag and tidal gradients.

Aims

The aim of this thesis project was to obtain an understanding of the absolute and relative accuracies of the Range Ratio method and the Radio Transfer method. The conclusions drawn were aimed to be able to provide a surveyor with information and confidence when undertaking a MHWM determination. The project makes recommendations for both methods as to when each technique should be used and in what circumstances and makes further recommendations as to the type of tidal and meteorological conditions that are conducive to the best outcomes from each method. Finally, this project involved understanding what considerations are to be taken when presented with certain tidal and meteorological conditions, as observations cannot always be taken in a perfect environment due to time and resource limitations.

Scope

This project involved a comprehensive field component for the individual testing of both Range Ratio and Radio Transfer methods. The field survey consisted of establishing control through two techniques at each observation site along with observing at the select locations over multiple epochs. Details of control establishment and observation techniques are documented in this report. A detailed analysis of both techniques has also been conducted with Range Ratio data compared between different epochs as well as to historical data. Radio Transfer data has been compared to derived elevations as well as between epochs. This project does not attempt to make any judgments on alternative techniques for transferring height along, across or around water bodies nor does it investigate the accuracy of a defined MHWM boundary.

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