MOORKAIE and Related Trigonometrical Points 60 Years On

Compiled by Paul Wise, September 2014.

The first Nat Map geodetic activity was undertaken in 1949, but it was not until 1954 that Nat Map really commenced work on the geodetic survey of Australia near Broken Hill. Over the next ten years or so the geodetic survey of Australia was extended by Nat Map north and west. Chains of first order trigonometrical stations were created that formed the basis for Australia’s first homogeneous network of positions.

In 1954 the Broken Hill area was isolated from any existing triangulation chains. The nearest schemes being the Army primary triangulation in the Peterborough - Orroroo - Carrieton area of South Australia and the New South Wales Lands Department triangulation near Cobar, New South Wales. Nat Map’s initial task was to carry a first order triangulation survey west from near Broken Hill to connect to the Army triangulation work near Orroroo.

As part of this triangulation in March 1954, some 22 kilometres north-east of Broken Hill in the Coonbaralba Range Nat Map built its first cairn over a ground mark. This trigonometrical (trig) station was named Moorkaie (today NSW 3190). The map section at Figure 1 refers.

Figure 1 : Section of 1:250,000 scale map showing the location (green pin) of Moorkaie triangulation station (NSW 3190). The trig was probably named after the nearby Moorkaie Hill.

On completion, the Moorkaie rock cairn was 6 feet in diameter and about 5 feet high. The cairn supported an Oregon centre pole 11 feet long and 4 inches square to which Masonite vanes were attached. Post-war shortages of building material meant the usual sheet iron used for the vanes was in short supply so Masonite was utilised instead. Figure 2 shows the finished 1954 Moorkaie monument.

Figure 2 : First cairn built by National Mapping at Moorkaie triangulation station, near Broken Hill, March 1954 - H.A. (Bill) Johnson is second from the left and E.J. (Ted) Caspers is on the far right.

After a period of sixty years with weather, cultural development and possible vandalism the state of the Moorkaie cairn was unknown. However, such a mark not only in time but also on the ground has great significance in Nat Map’s history and it was considered that the fate of the Moorkaie cairn should be discovered if possible.

Through a series of events and with the assistance of Chris Rawlins of Broken Hill and the permission of The Springs station manager the following pictures (Figures 3-7) of Moorkaie were recently obtained. Despite modern day Google Earth and GPS, Chris’s finding of the right track to the top took persistence and local knowledge.

Moorkaie first order trigonometrical station – 6 September 2014
West to Broken Hill	Site on approach
Broken Hill in right background	Site also occupied by Microwave Tower today
Figures 3-7 all courtesy of Chris Rawlins

The most obvious change to the Moorkaie trig is that the original Masonite vanes have been replaced by modern galvanized steel plate painted black. More difficult was to determine whether the centre pole had also been replaced. The fact that the shape of the cairn is just a little different and the patterns in the rocks of the cairn in the original and recent photos do not seem to match, indicates the likely dismantling and rebuilding of the cairn to replace the centre pole. In addition, the centre pole now has guy wires. These wires would likely have held the new centre pole vertical while the cairn was being rebuilt.

Save for the rocks in the cairn, what is visible of today’s Moorkaie (NSW 3190) trig is not 60 years old. Nevertheless, the trig stands on the site of the original and it is very likely the original Nat Map ground mark under the centre pole is still there. Importantly, the site is on private land and not easily accessible giving Nat Map’s first survey cairn a continuing measure of protection and longevity.

Chris’s local knowledge also took him to two other trigs that were part of the 1954 network; Sundown (NSW 4224) and Round (NSW 3960).

The hill on which Sundown trig was located in 1954 is now part of the 180 hectare Living Desert Flora and Fauna Sanctuary. The hilltop now accommodates 12 sandstone sculptures with vehicle access to the site. Amongst these sculptures stands a survey observation obelisk. The plaque on the obelisk indicates that around 1995 the 1954 Nat Map cairn was removed and the ground mark covered by the obelisk. Figures 8-13 refer. The plaque makes no mention of National Mapping’s earlier role!

Sundown first order trigonometrical station – 8 September 2014
NSW Sundown Reference Mark	Nat Map 1954 Sundown original Reference Mark 1
View from Sundown to Moorkaie trig	Enlargement highlighting Moorkaie cairn
Plaque on Sundown observing obelisk
Figures 8-13 all courtesy of Chris Rawlins

At the site of Round trig the 1954 Nat Map cairn has also been replaced by a survey observation obelisk but being away from the public eye the obelisk is unmarked and decaying! Figures 14-18 refer. A tribrach atop both Sundown and Round obelisks allows surveying equipment to be accurately centred over the original ground mark if required.

Round first order trigonometrical station – 8 September 2014
View from Round to Moorkaie trig	View from Round to Sundown trig
Broken Hill in the background	Survey tribrach atop Sundown and Round obelisks
Figures 14-18 all courtesy of Chris Rawlins

To enable the 1954 Nat Map triangulation to be checked as the work progressed the length of one of the observed lines had to be known. As mentioned above the Broken Hill area, however, was isolated at that time from any existing triangulation chains. The Broken Hill primary network therefore required the incorporation of a classical base line and base net triangulation. Such a scheme included a line, the base line, that could be measured (chained) with a surveyor’s chain or steel band. Then by means of the scheme’s triangles with their observed angles this distance was incorporated into the primary triangulation by computation. Figure 19 refers. At a suitable location later in the triangulation another base line and base net triangulation would be used as a check on the work. In 1954, as the work progressed quickly the connection was made to the existing Army triangulation work. The existing Army base line near Carrieton, South Australia then provided the necessary check and negated the need for another complex base net triangulation by Nat Map.

Figure 19 : Map section showing relevant primary triangulation and base net triangulation (insert) around Broken Hill. Blue lines indicate primary triangulation, the green line that ultimately measured by Geodimeter, yellow lines the base net triangulation and the magenta line the measured sub-base line. For clarity a yellow line, S. Base - Sub-Base terminal, has not been included.

The line North Base - South Base (Figures 19-26 refer), was included in the Broken Hill base net to be the chained base line. However, the complexities involved in measuring such a highly accurate base line would have caused a significant delay to the observing program so it was decided that this line would be measured later. The fact that the Geodimeter might be available later that year and had the ability to measure one of the longer lines of the primary triangulation relatively quickly and accurately was also a major consideration in postponing the chaining of the distance North Base - South Base. For computational and check purposes a short, sub-base line was thus chained. The linear value of this line enabled preliminary computations of sufficient accuracy to be undertaken to ensure that the angular observations met first order standards.

The starting point for the sub-base line was the point North Base. The location of the exact end point (Sub-Base terminal) is not in any easily available records and only shown diagrammatically (as reproduced in Figure 19). This line was established using a 300 foot steel band resulting in a sub‑base length of 2699.787 feet (822.895 metres). The distance of nearly 2700 feet indicates that the 300 foot steel band was laid out end-to-end nine (9) times in succession with correct tension applied and temperature noted each time and the steel band likely supported to eliminate sagging. As this chaining was done in early 1954 the summer temperatures would have caused the steel of the band to expand meaning the correct distance between the start and end point was not 2700 (9 x 300ft) feet but something less. The expansion of the steel in the band per degree of temperature would have been known and the expansion correction easily calculated and applied to give the true length of the line.

As Reg Ford recorded in his report on the Geodetic Survey of Australia:… the line North Base - South Base was never physically measured, instead when the Geodimeter became available later in the year one side of a triangle (the line Felspar – Twenty Mile) was measured…Later on, for comparison purposes, the Geodimeter value of the line Felspar – Twenty Mile was used to calculate a value for the measured sub-base; the comparison between the value so obtained and the actual measured distance was approximately 1:50,000 (or about 17 millimetres different).

The plaque at North Base today reads as if the North Base (NSW 774) - South Base (NSW 775) line itself was used as the base line whereas clearly from Reg’s statement above it was not; at least not for the 1966 geodetic adjustment. Records show that sixty years ago Nat Map emplaced a half inch diameter brass rod as the ground mark at both North and South Base. The South Base ground mark today is a steel star picket as can be seen in the photos (the North Base ground mark is not readily accessible). The replacement of at least one if not both South and North Base ground marks as part of later work by New South Wales might mean that this base line was used in the NSW State survey. For the record, the line North Base - South Base was 4891.927 metres long on an azimuth of 227° 36’ 40.66” (AGD66).

North Base Monument – 13 September 2014	South Base Monument – 13 September 2014
South Base steel star picket ground mark
View to South Base from North Base	View to North Base from South Base
Looking to North Base about 5km from South Base
Figures 20-26 all courtesy of Chris Rawlins

Nat Map’s selection of the line Felspar – Twenty Mile for measurement by the Geodimeter would have been based on the facts that Felspar was an easy drive-on for safely transporting the heavy Geodimeter equipment and that the distance to Twenty Mile, some 14 kilometres, would have been within the Geodimeter’s capabilities. The other lines of the network were longer, either exceeding the Geodimeter’s capabilities, and/or being less easily accessed by vehicle.

The North Base – South Base line must have also been considered as unsuitable for measurement by the Geodimeter. Although access would have been very easy the Geodimeter’s light path, however, would have been close to the terrain resulting in probable atmospheric transmission issues affecting final accuracy.

Remarks

These trigonometrical points are some of the oldest ever established by National Mapping as part of the geodetic survey of Australia. They also form part of the only classical base net triangulation ever undertaken by Nat Map. The Geodimeter’s introduction subsequently eliminated the need for base net triangulation schemes. A suitable line of any triangle could be measured with the Geodimeter (and later the Tellurometer) as the need arose.

Over the years it would appear that Nat Map’s original 1954 stations have been upgraded and monumented by predecessors of the now New South Wales Land and Property Information, a division of the Office of Finance and Services. This work means that some of Nat Map’s earliest first order geodetic marks will continue to exist for some time yet.

Acknowledgements

The efforts of Chris Rawlins of Broken Hill who accessed the sites and took the photos is greatly appreciated. More of Chris’s photos can be viewed via the link on this page.

Thanks also to David Langford of K Tank and The Springs stations for his assistance with access to the Moorkaie site as well as all others who were involved in any way.

Details of National Mapping’s 1954 operations were drawn from Reg Ford’s 1979 work: The Division of National Mapping’s Part in the Geodetic Survey of Australia-Activities Based on the Melbourne Office 1951-1969; this work can be accessed on the XNATMAP website at: http://www.xnatmap.org/adnm/ops/prog/rafgeosvy/index.htm