Chart Datum for Hydrography

This paper sets out to discuss ongoing activities in the tide and chart datum components of hydrography and charting. It expands on the paper presented at FIG (Fédération Internationale des Géomètres) in Brighton 1998 by Mr Matt Higgins and is a revised and updated version of the paper Chart Datum for Hydrography by GPS delivered by the present authors to the Survey 99 Congress, Sunshine Coast, Queensland, Australia, 6-9 October 1999. Mr Higgins’ paper (co-authored by the present authors) introduced the concept of relating hydrographic chart datum and the ellipsoidal heights measured by global positioning system survey for the purpose of reinstating tidal benchmarks situated on offshore structures. That concept has been extended to hydrographic surveying to eliminate the need to know the tidal height for use when reducing the soundings to chart datum. CHART DATUM FOR HYDROGRAPHY Raymond J Martin and G John Broadbent Maritime Safety Queensland, Australia The ideas and any opinions contained in this paper are those of the authors and do not necessarily represent those of Maritime Safety Queensland or the other organisations referred to herein. Introduction The surveying system uses the real time kinematic (RTK) capability of the GPS in three dimensions. The geographic position and height of the survey ship are continuously measured in ellipsoidal terms simultaneously with the depth measurements soundings. The combination of the depth measurements and height results, in the depth of the sea bed relative to the ellipsoid in use. At present and because paper charts are still required, it is necessary to refer the surveyed depths (soundings) to the chart datum. The concept of the AUSHYDROID was developed to satisfy this requirement. Chart and Tidal Datum Chart datum is the height reference surface used in hydrography depths depicted on navigation charts are below chart datum and drying heights are above it. Similarly, a tidal datum is the elevation above or below which tidal heights are measured. Its height is selected using arbitrary rules and the frequency of occurrence of low tide. Lowest Astronomical Tide (LAT) is the lowest level which can be predicted to occur under average meteorological conditions and any combination of astronomical conditions. It has been adopted by the Hydrographic Service of the Royal Australian Navy (RAN) as the chart datum for all new charts and new editions of existing charts of Australian waters. In order to maintain the required nexus between tidal datum and chart datum, the height of LAT is selected as the tidal station datum when a station is newly established, or the datum of an existing station is revised. For the purposes of this paper LAT, tidal datum, and chart datum are synonymous. Tides The range of tide is site specific, i.e. the range varies from place to place because of the hydrodynamic effects of the horizontal movement of the tides as they rise and fall. Because it is the height of the lowest tide that can be predicted to occur at the tidal station, LAT is the lower extreme of the range of the tide. Accordingly the height of LAT, relative to mean sea level, changes from chart to chart. Table 1 illustrates the typical range of tide experienced in the waters of the east coast of Queensland, together with the height of Highest Astronomical Tide (HAT), the height of mean sea level (MSL) and the Australian Height Datum (AHD) all relative to the LAT. Place HAT (m) MSL (m)* AHD (m) LAT (m) Range (m) Brisbane Bar 2.71 1.25 1.24 0.00 2.71 Mooloolaba 2.13 0.95 0.99 0.00 2.13 Noosa Head 2.18 1.04 1.12 0.00 2.18 Waddy Point Fraser Is. 2.26 1.03 1.01 0.00 2.26 Bundaberg (Burnett Hds) 3.58 1.74 1.69 0.00 3.58 Gladstone 4.69 2.32 2.27 0.00 4.69 Hay Point 7.14 3.36 3.34 0.00 7.14 Shute Harbour 4.26 1.90 1.91 0.00 4.26 * The estimated mean sea level as at 01 January 2002. (The epoch for the MSL is 1992 to 2011 inclusive. An allowance of 0.3mm per annum for sea level rise has been included in this mean sea level estimate). Table 1: Tidal heights and ranges at selected tidal stations in Queensland waters Cart Datum or Hydgraphy page 10 No. 112 April 2004 THE HYDROGRAPHIC JOURNAL ppers The tides for Moreton Bay north from Brisbane to the open sea at Caloundra Head, a distance of some 80 miles, are based on the tidal predictions for the Brisbane Bar standard port tidal station. Table 2 illustrates the changes in the range of tide and the height of LAT over that area. For example, the range of tide at the Caloundra Head is approximately three quarters of the range at the Brisbane Bar tidal station. See [Broadbent (1997)] for further details of the tidal range and the heights of selected tidal datums in Queensland waters. To further complicate the situation, the tide does not occur simultaneously everywhere. Table 3 illustrates the time of tide occurrence at the places listed in Table 2 relative to the standard port of Brisbane Bar. It can be seen from the above three tables that the estimation of tidal heights relative to chart datum at places some distance from a tidal station is not a trivial matter. The advent of the GPS provides a means of eliminating the height of tide from the reduction of raw soundings to depths below LAT datum for presentation on charts. Global Positioning System As indicated in the introduction, satellite technology is impacting on the surveying systems used to support the creation of the navigation charts. The Australian mapping and charting authorities have embraced satellite technology and the geocentric geodetic datums associated with that technology. The best example of a satellite derived geodetic datum in the marine field is the WGS84 system which provides the GPS satellite derived latitude and longitude graticules on the charts. (The GPS satellite derived latitude and longitude graticule is in addition to the astronomically derived graticules). Australia adopted the new geocentric geodetic datum for Australia, the Geodetic Datum Australia, known as GDA94 on 1 January 2000. The GPS system operates within the WGS84 geodetic datum. However the WGS84 datum and GDA94 datum are, for the practical use of hydrography, coincident. During the early 1990s, a decision was taken to refer hydrographic surveys to the WGS84 system and such surveys undertaken by Maritime Safety Queensland have been referred to the WGS84 ever since. A change to the GDA94 has not been made. Place HAT (m) MSL (m) * AHD (m) LAT (m) Range (m) Brisbane Bar 2.71 1.25 1.24 0.00 2.71 Pinkenba – Brisbane River 2.82 1.27 1.24 0.00 2.82 Cabbage Tree Creek 2.60 1.19 1.31 0.00 2.60 Woody Point 2.58 1.15 1.23 0.00 2.58 Scarborough Boat Harbour 2.41 1.12 1.17 0.00 2.41 Bongaree Bribie Island 2.33 1.08 1.10 0.00 2.33 Tangalooma 2.49 1.11 N/A 0.00 2.49 Caloundra Head 2.04 0.95 0.99 0.00 2.04 Table 2 : Tidal heights and ranges at selected tidal stations on the chart Moreton Bay to Caloundra Place High Water Low Water Minutes before or after (-) the time of tide at Brisbane Bar Brisbane Bar Standard Port Standard Port Pinkenba Brisbane River 11 16 Cabbage Tree Creek 1 -1