CHART DATUM FOR HYDROGRAPHY Raymond J Martin and G John Broadbent Maritime Safety Queensland, Australia

Transcription

1 page 9 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. Abstract 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 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. 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 Mooloolaba Noosa Head Waddy Point Fraser Is Bundaberg (Burnett Hds) Gladstone Hay Point Shute Harbour * The estimated mean sea level as at 01 January (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 THE HYDROGRAPHIC JOURNAL No. 112 April 2004

2 page 10 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. Place HAT (m) MSL (m) * AHD (m) LAT (m) Range (m) Brisbane Bar Pinkenba Brisbane River Cabbage Tree Creek Woody Point Scarborough Boat Harbour Bongaree - Bribie Island Tangalooma N/A Caloundra Head Table 2 : Tidal heights and ranges at selected tidal stations on the chart Moreton Bay to Caloundra 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. 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 Cabbage Tree Creek 1-1 Woody Point 0 2 Scarborough Boat Harbour 5 5 Bongaree - Bribie Island 0-15 Tangalooma Caloundra Head Table 3 : Tidal times at selected tidal stations relative to the tide times at the Standard Port tidal station 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 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. Surveying Considerations The GPS measures position in three dimensions. It has always provided horizontal position and height. While the real-time differential mode of GPS (DGPS), using single frequency (L1) receivers, provided the hydrographers with precise (1-5 metres) horizontal control, the height component was not sufficiently precise to meet the vertical control specification (Orders of survey 1, 2 or 3) defined by the International Hydrographic Organisation or the specification for Special Order surveys defined by Maritime Safety Queensland. The advent of GPS real-time kinematic (RTK) in recent years has realised a significant advancement of GPS to provide three dimensional navigation at the centimetric level of accuracy. RTK is currently provided in various levels of configuration including single frequency GPS (L1), dual frequency GPS (L1/L2) and dual system (GPS+GLONASS). While most RTK systems are limited to baselines of kilometres, the development of dual frequency RTK to minimise the effects of ionospheric errors and the use of multireference stations is now increasing the effective operational range. The recent developments of the GPS real time kinematic (RTK) mode, using dual frequency receivers, has increased the precision of the height component. The precision is now to a point at which the RTK mode can be used to determine the height of the boat from which soundings are taken and the required order of survey precision can be met. Use of the water surface as the reference for reduction of soundings is no longer necessary. Real time measurement of the height of the boat also removes the need to: measure tidal height, including making any allowance for the time and range difference between the tide at the recording station and at the survey site measure heave (vertical rise and fall of the vessel due to swell and wave action) squat (vertical rise or fall of the vessel due to changing hydrostatic pressure around the hull as the boat moves through the water) The authors are not yet satisfied that the their surveying system is fully capable of heave measurement. There are issues surrounding system schecks following the on-the-fly ambiguity resolution following a loss of lock and cycle slip detection. Accordingly the RTK is employed in sheltered waters at present. All that is necessary is to know the relationship between the vertical datum surface used in the GPS (the WGS84 ellipsoid) and the required chart datum, i.e. datum separation. No. 112 April 2004 THE HYDROGRAPHIC JOURNAL

3 page 11 The Datum Separation Model Concept The WGS84 provides the universal seamless vertical reference surface proposed by O Reilly, Parsons and Langelier [O Reilly, C, Parsons, S and Langelier, D (1996)]. Their proposition is that an ellipsoidal datum be adopted as the universal (global) master reference surface for hydrography and that client datums such as Lowest Astronomical Tide, chart datum, mean sea level, geoid, etc. be related to the master reference surface. The client surfaces undergo periodic revision and refinement and accordingly may be regarded as being unstable. On the other hand the master reference must be capable of being, and must be held fixed for a very long time i.e. stable. For Australia, the geocentric ellipsoidal datum WGS84 - GDA94 is preferred. The transformation between client datums is simple - it is sufficient to know the relationship between each client datum and the master datum. The various AUSGEOIDs are a case in point. The AUSGEOID is a very sophisticated model of the height separation between the geoid and the WGS84 ellipsoid. The AUSGEOID has undergone a number of refinements but each revision is uniquely identified and remains referred to the WGS84. This is merely extending the AUSGEOID concept to the marine and hydrographic environments. The height separation between the Lowest Astronomical Tide chart datum and the WGS84 is referred to as AUSHYDROID or L for short. The AUSHYDROID The AUSHYDROID is a model of the height of chart datum relative to the WGS84 ellipsoid. The AUSGEOID and the AUSHYDROID share similar features. They each have: Figure 1: Location diagram The determination of the AUSHYDROID is a two step process: determination of the height of LAT relative to the WGS84 preparation of an interpolation process by which the height of LAT is estimated everywhere within a chart The Height of Lowest Astronomical Tide The height of LAT is now routinely calculated for all tidal stations at which there are sufficient tidal recordings. Most tidal stations have a LAT height reference. Both the Department of Natural Resources and Mines (DNR&M) and Maritime Safety Queensland (MSQ) have commenced GPS surveys of the ports in Queensland to establish the height of LAT relative to the WGS84. This is a top down, statewide approach providing ellipsoidal heights at the tidal stations at three levels: Statewide level (DNR&M) - the port benchmark at all major ports (Note - DNR&M is using the GDA94 as the reference datum. The WGS84 is for practical purposes coincident with the GDA94) Port level (MSQ) - the multiple tidal stations established to control the hydrography within each port Short-term tidal stations (MSQ) - on a per project basis a grid of points at which their respective data are known an interpolating process by which the value of the data can be estimated at any place within the grid the WGS84 as the reference surface The AUSGEOID has points at which the height of the geoid is known. The AUSHYDROID has points at which the height of LAT is known. It is important to recognise that neither the AUSGEOID nor the LAT are parallel with the WGS84 or with each other. Each is a client of the master WGS84. Figure 2: Moreton Bay Figure 3: Gold Coast and Southern Moreton Bay THE HYDROGRAPHIC JOURNAL No. 112 April 2004

4 page 12 Level 1 stations are included in the State control network - fieldwork is complete and the calculation of the final heights is in progress. The surveys for level 2 are in progress. The ports of Karumba and Gladstone are complete. The surveys are continuing in the vicinity of the mouth of the Brisbane River and to the south at the Gold Coast. The results are shown in Tables 4 and 5. Tidal Station Elevation of N L Benchmark Brisbane Bar West Inner Bar Beacon Gauge Stn A Brisbane Bar - Whyte Island Gauge Pinkenba - Brisbane R Stn 46047A Cabbage Tree Creek Stn A Woody Point Stn A Table 4: Mouth of the Brisbane River Tidal Station Elevation of Benchmark N L Gold Coast Seaway Runaway Bay Marine Operations Base Southport Grand Hotel Jetty ProudPark Coomera River Brygon Creek Paradise Point The Broadwater Paradise Point Coomera River Saltwater Creek Sanctuary Cove Coomera River Couran Point Jacobs Well Woogoompah Island Cabbage Tree Point Table 5: Gold Coast Seaway Notes to Tables 4 and 5 1 The ellipsoidal elevation of the benchmark at the tidal stations in Table 4 is the surveyed height resulting from a minimally constrained adjustment of the network, holding fixed the benchmark for the station Brisbane Bar. 2 The ellipsoidal elevation of the benchmark at each tidal station in Table 5 is the surveyed height resulting from a minimally constrained adjustment of the network, holding fixed the benchmark for the station Gold Coast Seaway. 3 N is the geoidal separation according to AUSGEOID98. 4 L is the measured separation between the ellipsoid and the LAT datum - the AUSHYDROID i.e. the height of the zero metre LAT relative to the ellipsoid. LAT at Offshore Stations It is necessary to know the height of both the LAT and the ellipsoid at a wide range of places, including offshore, in order to establish the AUSHYDROID with a high degree of precision. In Maritime Safety Queensland, the ellipsoidal height of tidal stations is surveyed as a network by GPS. The network is calculated as a minimally constrained adjustment holding fixed the port benchmark [Martin (1997)]. For example in the port of Gladstone, Maritime Safety Queensland uses six tidal stations each with an elevation for the LAT and the ellipsoid. Three of these stations are offshore on navigation beacons; a forth is on Facing Island. The six stations are the minimum required to determine the tides and chart datum in the port and they form the grid of known points on which the AUSHYDROID in the port is based. Interpolation of Lowest Astronomical Tide across a Chart There is no ready answer to the question of interpolation. Various hydrographic authorities employ different methods to reduce soundings to chart datum. For example: The NOAA Office of Coast Survey uses a tidal zoning process in which the survey area is divided into a number of zones. Each zone is assigned a range ratio and time difference which is applied to the tidal height at the reference tidal station [Collier W, Glang, G and Huff, L C, (1999)]. The Hydrographic Office RAN has a multi tidal station interpolating technique employing a weighted interpolation. The weight applied at a point is proportional to the distances to the adjacent tidal stations and to the area of the triangular sector subtended by those stations. Maritime Safety Queensland uses a zoning process with the zones aligned along the ship channels. The tide is measured at two stations situated at each end of the zone. The height at every point along the channel is interpolated by distance from the stations. All of these procedures require a knowledge of the tidal height at one or more stations. Accordingly none of them is satisfactory because the objective is to avoid using the tidal height and the water surface as a reference in the sounding process. Because it is recognised that the interpolation process defines the chart datum at places between tidal stations, Maritime Safety Queensland uses the zoning process 3) above and the known height of the AUSHYDROID at both tidal stations. The height of the AUSHYDROID at every point along the channel is simply interpolated by distance from the stations. Discussion The RTK system latency - telemetry and processing - needs to be accounted for. This can amount to about one second. However most RTK systems provide two modes of operation: No. 112 April 2004 THE HYDROGRAPHIC JOURNAL

5 page 13 Synchronised RTK - this is the most accurate. Data from the base station is received and matched by time tag to the remote data. This is used for static stop and go type of operations where accuracy is essential (survey control etc). Fast RTK this has has a low latency (generally > 50ms). Because it is using older data at the remote. However there remains some degradation of the accuracy. For hydrographic applications, an update rate of 10hz is considered the minimum with data latency minimised as much as possible. In the next few years the Global Satellite Navigation System (GNSS) will undergo significant developments that will have a positive impact on the performance of fast ambiguity resolution, particularly over long distances. The modernisation of the existing GPS system will include the addition of two new civilian signals being broadcast on future Block IIF satellites. A new European funded satellite navigation system, Galileo, with a constellation of 30 satellites is to be launched later this decade. These two GNSS systems, along with the Russian GLONASS system will offer up to 58 satellites for world wide coverage. The combined systems will improve the performance of ambiguity resolution by allowing for satellites to be used in the solution, and the use of a third frequency to develop various linear combinations and new or modified mathematical models [Mowlam et al (2003)]. It is anticipated that improved ambiguity resolution will increase the certainty of the height measurements provided by the system. Conclusion It has been indicated above that RTK may be used to provide the height of the sounding vessel and thereby allow for the effects of tides and the vertical motion of the vessel caused by swell. At present RTK is good enough for use in sheltered waters to account for the tide, it is difficult to say at this stage if RTK can be used successfully for correcting the effects of wave action and wind chop. The GPS and the geodetic datum in which it operates provides the means to achieve the objective of eliminating the need to use water levels and tidal heights in order to refer soundings to chart datum. biographies Ray Martin is a Senior Hydrographic Surveyor with Hydrographic Services, Maritime Safety Queensland. He is a Licensed Surveyor and member of The Hydrographic Society. He joined the Department of Harbours and Marine in 1986, principally in the role of a Licensed Surveyor to perform the cadastral requirements of the Department as well as engineering and hydrographic surveys. Since the amalgamation of Harbours and Marine into the Department of Transport and the subsequent creation of Maritime Safety Queensland from the Maritime Division out of the department, his role has been more involved with the establishment and maintenance of horizontal and vertical survey control for various hydrographic survey operations and the data capture of foreshore information for coastal management and environmental studies. During the past twelve years he has been heavily involved in the acquisition, deployment and integration of GPS based survey systems into the survey operations of the Hydrographic Services. (ray.martin@msq.qld.gov.au). John Broadbent is Manager of Tides, Maritime Safety Queensland. He holds a Certificate in Engineering Surveying (Queensland Department of Education 1970) and Bachelor of Applied Science (Surveying) Queensland Institute of Technology 1979). In addition he is a Registered Surveyor with the Surveyors Board of Queensland (Engineering and Hydrographic endorsements), an Accredited Hydrographic Surveyor Level 1 (Coastal Zone) and Queensland delegate to the working group of the Australian Permanent Committee on Tides and Mean Sea Level. In addition John is a member of the Institution of Surveyors Australia (Queensland Division), the Spatial Sciences Institute, the Institution of Engineering and Mining Surveyors (Queensland Division), The Hydrographic Society. John commenced training as an hydrographic surveyor in 1965 with the Queensland Department of Harbours and Marine. He worked extensively along the coast and in the ports of Queensland on surveys for coastal management and port development and, after graduating in 1980, undertook the role of hydrographic surveyor in charge of the tide section. That role continues to the present within Maritime Safety Queensland after the amalgamation of Harbours and Marine into the Department of Transport and the subsequent creation of Maritime Safety Queensland from the Maritime Division of the latter. The role includes co-ordination of the recording of tides (which in Queensland are undertaken by several agencies), tidal analysis and prediction, and maintenance of the tidal and chart datums for the ports of Queensland. (tides@msq.qld.gov.au) ACKNOWLEDGEMENT This paper was presented at the 7th South East Asian Survey Congress, 3-7 November 2003, Hong Kong and is reproduced with the kind permission of the organisers. Refererences over page THE HYDROGRAPHIC JOURNAL No. 112 April 2004

6 page 14 REFERENCES Broadbent, G J (1996). Tides and Hydrography, Proceedings of Institution of Surveyors and Institute of Engineering and Mining Surveyors Unification Technical Seminar, Brisbane. Broadbent, G J (1997). Tidal Datums, The Queensland Surveyor, Volume 1997 Number 5, pp Collier, W, Glang, G and Huff, L C (1999). Managing Tide Information for Hydrographic Surveys, Proceedings of the Eleventh Biennial International Symposium of Tthe Hydrographic Society, Special Publication No 40, University of Plymouth, 5-7 January. Higgins, M B, Broadbent, G J and Martin, R J (1998). The Use of the Global Positioning System for the Maintenance and Investigation of Tidal Datum: A Case Study in Queensland, Proceedings XXI International Congress of the International Federation of Surveyors, Brighton United Kingdom, July. Johnson, P (1997). IHO Standards for Hydrographic Surveys (S-44) Working Group - Part 1, Proceedings of the Third Australasian Hydrographic Symposium, Maritime Resource Development, Special publication No. 38. Fremantle, Western Australia, pp Martin, R J (1997). Guidelines for GPS Connections to Tide Stations - Queensland Transport, Brisbane. Martin, R J and Broadbent, G J (1999). by GPS, Proceedings of the Survey 99 Congress, Sunshine Coast, Queensland, Australia 6-9 October. Mills, G B (1998). International Hydrographic Survey Standards, International Hydrographic Review, Vol, LXXV(2), September, Monaco. Mowlam, A, Joosten, P and Collier, P A (2003). The influence Galileo and modernised GPS on fast ambiguity resolution over extended distances, Proceedings of the 6th International Symposiumon Satellite Navigation Technology including the Mobile Positioning & Location Services, Melbourne, Australia, July. O Reilly, C, Parsons, S and Langelier, D (1996). A Seamless Vertical Reference Surface for Hydrographic Bathymetry with Specific Application to Seismic Areas, Proceedings of the International Symposium on Gravity, Geoid, and Marine Geodesy, Tokyo, Japan, 30 September 5 October. Parker, B D and Huff, L C (1998). Modern Under-Keel Clearance Management, International Hydrographic Review, LXXV(2), September, Monaco. Wells D (1997). Course Notes from the Asia-Pacific Coastal Multibeam Training Course, Nautronix Ltd and the Western Australian Region of the Hydrographic Society, Fremantle, Western Australia, 4-8 December. New Special Publication No. 3 Hydrographic Surveying as a Career SPONSORS NEEDED A new format, full-colour version of this invaluable, popular Special Publication is due to be published over the coming months. In addition to being available via the Society s website, SP 3 will continue to printed in booklet form, with the addition of new loose-leaf appendices. Hydrographic Surveying as a Career is provided free of charge to careers offices and fairs, trade exhibitions and to students, of all ages, interested in pursuing a career in hydrography. Sponsors are currently being sought for this very worthwhile publication. The valuable financial assistance offered by sponsors will naturally be acknowledged within the publication. If your company would like to discuss sponsorship of this new version of SP 3 please contact: The Hydrographic Society PO Box 103, Plymouth, PL4 7YP, United Kingdom Tel and Fax: +44 (0) helen@hydrographicsociety.org No. 112 April 2004 THE HYDROGRAPHIC JOURNAL