Transfer of Datum forHydrographic Surveys

Hydrographic Meteorological and Oceanographic Force Element Group

Gabriela Balla Manager of Tides and Geodetic Control

Australian Hydrographic Service

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.

Sounding Datum and Chart Datum: Definitions

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.

Chart Datum: oops!

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.

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.

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 4.0 0.3 3.7

Port B 2.6 0.1 2.5

Survey area ( Port C) lies between these two ports and is 20km from Port B.

By linear Interpolation, Port C’s Mean Spring Range (MSR):MSR = 2.5 + 20/50x1.2 = 3.0m

The ratio of ranges between Port C and Port B = 3.0./2.5 = 1.2

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)

Established Datum Transfer – Methods

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

Semi-diurnal Transfer MethodSemi-diurnal Transfer Method

O b s e r v e d 3 M H W

3

2 r R O b s e r v e d M e a n L e v e l

T r u e S p r in g M e a n L e v e l 2

1 M rR

M ’ O b s e r v e d M L W M 1 m ’

0 S o u n d in g D a t u m E s t a b l is h e d d

P o le 0 N e w P o le

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

Semi-diurnal Transfer Method (cont)Semi-diurnal Transfer Method (cont)

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)]

Where the true Spring Mean Level is not known, this formula reduces to:

d = m' – [(M' x r)/R]

Semi-diurnal Transfer Method (cont) Semi-diurnal Transfer Method (cont) Example AH533Example AH533

Semi-diurnal Semi-diurnal TTransfer Method ransfer Method Example AH533 (cont)Example AH533 (cont)

Diurnal Transfer Method – AHO-preferred

Established Pole New Pole

3 4

2 3

Observed Mean Sea Level True Mean Sea Level

1 2 H h

Z’ Zoo (Zooh/H) z’

Datum 0 1

Sounding Datum

Datum

d

0

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

Diurnal Transfer Method (cont)

From the previous slide it can be seen that:

d = z' + (Zoo – Z') – [Zoo x (h/H)]

Where the True Mean Sea Level (Zoo) is not known, this formula reduces to:

d = z' – [Z' x (h/H)]

Diurnal Transfer Method (cont)Example

H = 0.79 (the sum of the heights of the 4 principal constituents at the established gauge

h = 1.25 (the sum of the heights of the same constituents at new gauge)

Z‘ = 1.50 (the height of MSL above Chart Datum at the established gauge (Zo from analysis))

z‘ = 2.60 (the height of MSL above the zero of the new gauge (So from analysis))

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)]

= 2.6 + (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:

Direct Comparison

1.5Low Water SD on new poleHeights at 1.0Standard Port

0.5

CD 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Observed LW heights at the new tidal station are plotted against predicted, or better still, the observed LW heights at the Standard Port.

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.

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

“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

Validation – Ratio of Rises

AHO has the ability from harmonic constants to compare the new location to the standard port to determine

D = Ratio Range*Std Port + MSL Offset

Time difference

Establishing anEstablishing an Independent Independent DatumDatum

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.

Establishing Datums in Rivers and Estuaries

There are methods for establishing tidal datums in:

- A river- River entrance and estuary- Areas of impounding

Establishing Datums in Rivers and Estuaries (cont)

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.

Establishing Datums in Rivers and Estuaries (cont)

General Principles to keep in mind

The sea bed topography

River and tidal flow boundary

River entrances with large sand banks

Impounding zones

Off-shore Datums: Co-Tidal Charts

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

Assumptions- Truly accurate near HW/LW ->errors may occur at other

times, particularly near half-tide- Changes between the lines are linear

Refer to Admiralty NP122(2) for instructions

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

Connecting Chart Datum to the Australian Height Datum (AHD)

Connecting Chart Datum to the Australian Height Datum (AHD): Benchmarks

Critical to ensure that levelling between gauge and marks are made

Levelled into the Land levelling network and or connection to Ellipsoidal Height for Geoid referencing.

QUESTIONS?