Guidelines on the Use of GNSS for Tide Calculations

March 2024 Rev. 0.1 – Update of preface, reference links and layout; no change to content

June 2021 – Initial Publication

Introduction

As with horizontal positioning, vertical positioning is also referenced to specified datums.  Unlike land surveying though, where vertical measurements (elevations) are made from and to a fixed surface, vertical measurements offshore (depths) are taken against a moving, dynamic surface. Due to the influence of tides, vertical measurements taken at the same location will yield different results at different times of the day, and at the same times on different days. As such, all absolute depths recorded from survey activities need to be adjusted for tide and reduced to a known constant vertical datum. The tidal effect will result in depth differences from a few centimetres to several meters depending on the location.

There are various methods to enable the reduction of these depths to a datum, including:

• Predicted modelled tides (such as Polpred)
• Use of standard ports, tide harmonics and co-tidal charts
• Use of shore mounted, offshore, or seabed mounted tide gauges
• Use of GNSS.

This document will provide an overview of how Global Navigation Satellite Systems (GNSS) can assist in accurately accounting for tidal changes, reducing to typical datums and considerations for implementation of GNSS tide measurement. For an overview on GNSS themselves, please refer to IMCAS015 – Guidelines for GNSS positioning in the oil and gas industry.