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COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL DEVELOPMENT

BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS.

RECORDS

1953, No.6

MAGNETIC SURVEY OF

COMPASS-SWINGING SITE,

R.A.A.F~ AIRFIELD,

POINT COOK, VICTORIAi

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F.W. WOOD and P.M.McGREGOR.

7

COMMONWEALTH OF AUSTRALIA

DEPARTMENT OF NATIONAL DEVELOPMENT

BUREAU OF MINERAL RESOURCES GEOLOGY AND GEOPHYSICS.

RECORDS 195'3. No.6.

'. I

. MAGNETIC SURVEY OF

COMPASS-SWINGING SITE,

R.A.A.F. AIRFIELD,

POINT COOK, VICTORIA.

by

F • W. WOOD and P .M .McGREGOR •

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ABSTRACT (iil)

1. INTRODUCTION 1.

2. METHOD. 1.

(a) Layout of grid 1.

fb) Observa.tions 1.

3. RESULTS 2.

4. CONCLUSION 2.

IltLU2TRATION.

P1a.te 1. Plan of Compa a ~wlnging site showing magnetic

compass deviations.

(il)

A declinometer survey WaS made over

a proposed compass-swinging base at the R.A.A.F.

Airfield at Point Cook, Victoria in January, 19,2,

to determine its suitability.

The results show that there are local

magnetic disturbances which cause deviations in

excess of the permissible maximum of one quarter

of a degree. The site is therefore unsuitable.

(111)

J.. INTRODUCTION

In response to a request from the Department of Air, a survey was made in January 1952, over an area within the R.A.A.F. Airfield at Point Cook, Victoria, to determine its suitability as a compass-swinging base. '

The site was chosen in collaboration with the Navigation Officer at Point Cook. A grid of closely-spaced stations Was then set up over the area ,selected, and measure­ments were made to determine the departures from normal

, magnetic declination within the area. The magnetic measure­ments were made with cookel Troughton and Simms theodolite -declinometer Ne. V012025 w th ccmpass attachment No. 155. T~e instrument had previously been tested for accuracy at the Toolangi :Magnetic ObS9Tvatory and found to have an error not exceeding two minutes of arc. For laying out the grid of observation stations', the same theodolite was used.

The observations were carried out between 16th and 18th January, 1952 by P.M. McGregor and J.A. Brooks, geophysicists. Acknowledgement is made of the very appreciable assistance given by the R.A.A.F. and by members of the staff at Point Cook airfield, particularly F1t/Lt. Cassidy, in arranging accommodation for the observers and assisting in laying down the grid.

The site selected for this survey is in the south­western part of the airfield and its position relative to the perimeter track in this part is shown in Plate 1. The main group of aircraft hangers and buildings is about 2 chains east of Line H of the survey grid. It should be noted, also, that a small arma­ment hut, a detonator hut and a quantity of steel drums and cases were situated about 20 yards west of stations 7 and 8 on Line A.

2. METHOD.

(a) LAYOUT OF GRID.

To give systematic and complete coverage, a grid, or network of observing stations, was set up over the site of the proposed compass-swing base. It cohsisted of eight lines (A to H) each of eight stations (1 to 8) marked by pegs. A plan of the grid is given in Plate 1.

The lines were 60 feet apart and the stations were spaced at intervals of 60 feet along each line. A reference station was set up at the centre of the grid. Eight azimuth pegs were placed

, on the extensions of the eight lines at a distance of about 250 yards beyond each of the eight end pegs. A ninth azimuth peg was placed midway between the azimuth pegs D and E for sightings from the reference station.

(b) QBSERVATIONS.

In order that an appropriate correction could be made to the reading at each station, for the effect of the diurnal variation of declination, observations were made at the reference station both before and after each line was surveyed. The time of each observation at the reference station was noted and a curve was plotted showing the changes in the observed direction of the compass during the period. From this curve it wa.c; possible to determine the the necessary correction to apply to each observation at the grid stations.

•

(i)

(1i)

(iii)

Each station observation consisted of:-

a "mark" reading obtained by sighting on the appropriate azimuth peg).

a "compass"reading obtained by sighting on the end of the freely-swinging magnet;

a record of the time of the compass reading.

The difference between "compass" and "ma.rk" readings at anyone station represents the angle between the magnetic meridian at that station and the direction of the lines of . the grid.

The difference angle D (see Plate I ), corrected for diurnal variation is, for convenience, called the "relative declination l1 at the station.

If the whole of the area surveyed consisted of uniformly magnetized or non-magnetic rocks, the relative declination at all grid stations would be the same and would he equal to that at the reference point. The deviation of the value obtained at each grid station from that obtained at the reference station would then be zero. The degree of uniformity of an area is, therefore, conveniently expressed as the amount of this deviation in minutes of arc at each station.

,3.. RESULTS.

The deviations observed at all sixty-four stations are shown on Plate 1. The number alongside each grid point represents the compass deviation in minutes of are, positive indicating easterly and negative indicating westerly deviation. .

The portions of the area within which the deviation exceeds one-quarter degree are enclosed in contour lines and hachured. Deviations exceeding + i and - 2 degrees were recorded in the c~ntral and northern portions of the area.

4. CONCLUSION,

The proposed compass-swinging base is unsuitab.le because deviations over much of the area exceed one-quarter degree.Speci­mens of basalt which covers much of the Point Cook district show quite strong magnetic polarization and it seems probable that almost any site of the required size in this area would include areas of magnetic disturbance •

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iMAGNETIC SURVEY OF

I, R.A. A. FAIRFIELD,

COMPASS- SWINGING SITE,\

POINT COOK , VICTORIA

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