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Transfer of Datum for Hydrographic Surveys Hydrographic Meteorological and Oceanographic Force Element Group Gabriela Balla Manager of Tides and Geodetic Control Australian Hydrographic Service

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Sounding Datum and Chart Datum: Definitions Sounding Datum is the plane to which soundings are reduced during a hydrographic survey. It is the datum used when compiling a survey “fairsheet” and should be connected to chart datum via a landborne benchmark (BM). Chart Datum is the datum plane adopted for the published chart and is the level above which charted depths, tidal predictions and tidal levels are given in the Australian National Tide Tables (ANTT), AusTides and on the published chart. Ideally, sounding datum for a survey should be the same as the chart datum.

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Sounding Datum and Chart Datum: Definitions

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Chart Datum: Lowest Astronomical Tide (LAT) Chart Datum should be: So low that the water level will seldom fall below it; Not so low as to cause the charted depths to be unrealistically shallow; That it should vary only gradually from area to area and from chart to adjoining chart, to avoid significant discontinuities. In accordance to a resolution of the International Hydrographic Organisation (IHO), Australia adopts Lowest Astronomical Tide (LAT) as the Chart Datum.

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Chart Datum: oops!

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Establish, Recover or Transfer of a Datum? Establish: Today, most surveys are undertaken in areas close to where an established datum already exists. Recover: Where areas have been previously surveyed, the original datum should be used utilising existing Benchmark records and levelling. Transfer: Where no datum exists in the survey area, but datum values exists nearby, the datum values can be transferred. Tidal datum should be transferred at intervals along the coast and distances between tidal stations will vary with different tidal conditions. Where tidal conditions change gradually, along an open coast, the maximum distance between tidal stations is 16 km. Where tidal conditions change rapidly, stations should be 1.6km or less apart.

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Issues with Datum Transfer Always investigate the known ranges of tide at places on either side of the survey before deciding whether a transfer is necessary or not. Serious errors in the reduction can be introduced by using tidal observations at a port which is too far way from the survey and has a different tidal range.

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Example of an Error introduced during Datum Transfer Two Standard Ports along a coast are 50km apart MHWS (from ANTT) MLWS (from ANTT) Mean Spring Range (MSR) Port A Port B n Survey area ( Port C) lies between these two ports and is 20km from Port B. n By linear Interpolation, Port C’s Mean Spring Range (MSR): MSR = /50x1.2 = 3.0m n The ratio of ranges between Port C and Port B = 3.0./2.5 = 1.2 n If the tide falls to datum at Port B, the range is about 2.7m and the range at Port C will be about 3.2m (2.7x1.2)

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Established Datum Transfer – Methods n The primary methods of datum transfer will depend upon the character of the tide: -Semi-diurnal -Diurnal -Direct comparison of low water heights -Ratio of rises

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Semi-diurnal Transfer Method Where: R = the observed range at the established gauge r = the observed range at the new gauge M' = the height of observed mean level above CD/SD at the established gauge m' = the height of observed mean level above the zero of the new gauge M = the height of the true Spring mean level above chart datum at the established gauge d = the height of sounding datum above the zero of the new gauge

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Semi-diurnal Transfer Method (cont) n Therefore, from the diagram it can be seen that the height of the sounding datum above the zero on the new gauge (d): d = m' – (M' – M) – [M x (r/R)] n Where the true Spring Mean Level is not known, this formula reduces to: d = m' – [(M' x r)/R]

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Semi-diurnal Transfer Method (cont) Example AH533

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Semi-diurnal Transfer Method Example AH533 (cont)

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Diurnal Transfer Method – AHO-preferred H = the sum of the heights of the 4 principal constituents at the established gauge h = the sum of the heights of the same constituents at new gauge Z‘ = the height of MSL above Chart Datum at the established gauge (Zo from analysis). z‘ = the height of MSL above the zero of the new gauge (So from analysis) Zoo = the true (average) height of MSL above Chart Datum at the established gauge (obtained from ANTT) d = the height of sounding datum above the zero of the new gauge

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Diurnal Transfer Method (cont) n From the previous slide it can be seen that: d = z' + (Zoo – Z') – [Zoo x (h/H)] n Where the True Mean Sea Level (Zoo) is not known, this formula reduces to: d = z' – [Z' x (h/H)]

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Diurnal Transfer Method (cont) Example n H = 0.79 (the sum of the heights of the 4 principal constituents at the established gauge n h = 1.25 (the sum of the heights of the same constituents at new gauge) n Z‘ = 1.50 (the height of MSL above Chart Datum at the established gauge (Zo from analysis)) n z‘ = 2.60 (the height of MSL above the zero of the new gauge (So from analysis)) n Zoo = 1.15 (the true (average) height of MSL above Chart Datum at the established gauge (obtained from ANTT)) d = z' + (Zoo - Z') – [Zoo x (h/H)] = (1.15 – 1.5) – [1.15 x (1.25/0.79)] = 2.6 – 0.35 – 1.82 = 0.43m above the zero of the new gauge From the equation on the previous page:

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Direct Comparison n Observed LW heights at the new tidal station are plotted against predicted, or better still, the observed LW heights at the Standard Port. n The point where the line of “best fit” cuts the axis of the new location is the height datum above (below) the zero of the new location.

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Establishing Datum for “Sketch” surveys The height of Sounding Datum (d) above the zero of the new gauge is obtained from: d = m – 0.5r where:r = [r/R] x R m = observed Mean Level height at the new gauge r = observed range at the new station R = predicted range at the Standard Port R’ = required range at the Standard Port

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“Sketch” survey example Crocodile Beach (survey) Darwin (Standard Port) Observed H.W = 5.60m Predicted H.W = 5.52m Observed L.W = 1.95m Predicted L.W = 0.65m Observed Range ( r ) = 3.65m Predicted Range ( R ) = 4.87m Observed ML (m) = 3.78m Predicted ML = 3.09m Approximate ratio of ranges = [r/R] ->3.65/4.87 = 0.75 From ANTT, Table I. LAT at Darwin = 0.0m and HAT at Darwin = 8.1m, hence R’ = 8.1 The equivalent Range on the pole at Crocodile Beach will be: r = [r/R] x R -> 0.75 x 8.1 = 6.07m and the required ½ range at the pole on Crocodile Beach will therefore be: 0.5r -> 0.5 x 6.07 = 3.04m The height of Sounding Datum above/below the zero of the pole: d = m – 0.5r -> 3.78 – 3.04 = 0.74m above the zero of the pole

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Validation – Ratio of Rises n AHO has the ability from harmonic constants to compare the new location to the standard port to determine n D = Ratio Range*Std Port + MSL Offset n Time difference

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Establishing an Independent Datum By reference to the land levelling system where the relationship between Chart Datum and the Australian Height Datum (AHD) is known at neighbouring places. By harmonic constants – rarely used these days. By mean sea level – where the tidal range is small.

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Establishing Datums in Rivers and Estuaries n There are methods for establishing tidal datums in: -A river -River entrance and estuary -Areas of impounding

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Establishing Datums in Rivers and Estuaries (cont) n As a tidal wave enters the estuary it is constricted: -Causes a gradual increase in range so high waters begin to rise higher and low waters to fall lower as the wave proceeds up the estuary. -This continues to a point where the topography of the sea bed no longer permits the lowest low water to continue falling. -Another complicating factor in the upper reaches of the river is the effect of varying quantities of river water coming down stream.

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Establishing Datums in Rivers and Estuaries (cont) General Principles to keep in mind n The sea bed topography n River and tidal flow boundary n River entrances with large sand banks n Impounding zones

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Off-shore Datums: Co-Tidal Charts n Relate to waters some distance from the shoreline but depth of water doesn’t allow a tide gauge to be positioned -Co-tidal charts are constructed n Assumptions -Truly accurate near HW/LW ->errors may occur at other times, particularly near half-tide -Changes between the lines are linear n Refer to Admiralty NP122(2) for instructions

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Off-shore Datums: Semi-Diurnal Co-Tidal Charts Solid co-tidal lines: show time corrections based on tide time at A Pecked co-range lines: tide range ratios on the tide at A At B: high water 30 min before A Tidal range is 0.65x the range at A

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Connecting Chart Datum to the Australian Height Datum (AHD)

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Connecting Chart Datum to the Australian Height Datum (AHD): Benchmarks n Critical to ensure that levelling between gauge and marks are made n Levelled into the Land levelling network and or connection to Ellipsoidal Height for Geoid referencing.

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QUESTIONS?

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