FIRST ORD ANGLES

FIRST ORDER ANGULAR CONTROL

H.A. Johnson

Division of National Mapping, Department of National Development

Johnson, H.A. (1962) First Order Angular Control, Presented, Australian Survey Congress (6th : Adelaide).

Early Angle Measuring Instruments

Most surveyors have some knowledge of the evolution of angle measuring devices.

After hundreds of years of almost unnoticeable improvement, the discovery and incorporation of lenses allowed more accurate pointing, as well as more accurate graduation of scales.

More accurate machine dividing of circles brought down the 36" diameter horizontal circle of these giants of the theodolite world to more manageable diameters of 18", 10" and finally 8".

To present eyes, even the 8" diameter 2 or 3 micrometer theodolite would be a massive instrument, but slow reading and cumbrous as they now appear, they gave remarkable accuracy when carefully used.

80 years ago, these instruments and their painstaking surveyors were producing accuracies comparable with those being read. on most geodetic traverses in Australia today.

Their very slowness of reading is probably one of the main reasons for this accuracy, since it meant a surveyor was usually at his station over a longer period, with the longer period giving the conditions more chance to even out.

They were laborious, painful instruments to use and they demanded skill and care in handling at all times.

There were the runs of the micrometers to check, to see they accurately covered their correct span - exposed, protruding, vulnerable micrometers so easily bumped out of adjustment, or seriously damaged.

The telescope and crosshairs were brought to the intersection of the object with the utmost care that the object was not overshot, or it was necessary to give the instrument another complete 360° swing to avoid any backlash or wear which might have built up in the oil films, tangent screws, clamps or springs.

Lighting of modern instruments can really be appreciated only by those who at some stage used oil lamps, or even a dangling web of electric wires and bulbs to read the bubbles, micrometers, and to light up the crosswires.

Those were the days when the surveyor himself, alone used the theodolite and waved and warned off anyone approaching within 20 feet of it.

He almost certainly had a section of a net of triangulation to observe on which he was probably the only observer.

On his readily checkable and simply tabulated results rested his reputation as a skilful observer and surveyor.

Modern Instruments

In the early 1900s Heinrich Wild began his impact on the design of surveying instruments; whilst working with the Zeiss company at Jena.

The 1914-1918 war delayed production of his revolutionary lightweight theodolites, one type of which was produced by Zeiss soon after the war ended.

In 1921 the first Wild theodolite was manufactured in Wild's home country, Switzerland, after he had left Zeiss, embracing most of the time saving ideas

he had thought out as a practical surveyor, himself, in the field - one stance pointing and reading, split bubble reflection and adjustment of the vertical circle, internal focusing, double circle reading on glass circles, and a fraction of the weight and mass of earlier instruments.

Other firms gradually followed, and in my experience after 5", 6" and 8" old style Troughton & Simms theodolites, the 3½" CTS Tavistock appeared an impossible toy, until a glance at the first few sets of results showed otherwise.

I have never lost my affection for the 3½" and 5½" Tavistocks, especially the first 5½" Tavistocks of 65 lbs, the scars of carrying which (prior to the advent in Australia of the Yukon pack) I possess today.

Most of my own personal geodetic angular measurements (as against training and close supervision of other observers) were done on two of these 5½" Geodetic Tavistocks, the second one giving me the supreme pleasure in early 1939 of unwrapping it from its tissue paper.

That instrument was really cherished and set up on a mountain top, it never failed to give me complete confidence of its ability to do its part - the rest was up to the observer.

It appeared a stable astronomical instrument, with a good, adjustable length striding level, and was quickly and closely adjusted in both vertical and horizontal collimation.

Its telescope altitude limit of 63°, as with the Wild T3, restricted selection of latitude stars, but presented no great problem with present day catalogues.

There was a steady small but appreciable change of the vertical circle index value with temperature variation, but this could be readily balanced in a latitude programme.

During the last eight years there has been an opportunity to train a number of observers on Wild T3 theodolites and to examine the results obtained from these instruments over varying conditions and over much of Australia. These results are the reason for this article.

Triangulation

From 1954 to 1957 the Division of National Mapping carried out normal geodetic triangulation methods over good trigonometrical terrain, as encountered in the Flinders, Gawler, Macdonnell and Musgrave Ranges, fading off into low lying areas where the rays skimmed to and from low residuals and sandridges, and where for most of the day it looked impossible,

Here one examined the trend and fall of the country, seeking out the salt lake depressions and dry river drainage areas to straddle stretches of almost continuous running mirage and little promise of success.

It has been our policy, wherever early triangulation was established, to adopt such marks exactly, and in this matter the marking of the original trigonometrical work of the old South Australian Survey Department is outstanding in Australia and deserves mention.

Most of their cairns, particularly through the Flinders and Gawler Ranges are magnificent, craftsman built monuments 8 to 10 feet high, in some cases 100 years old and in many instances without a stone dislodged. On sandhills and gibber rises, lacking in larger stones, massive, pyramidal mulga cages were erected, and many of these still stand and some have been used by our parties.

These old marks were dismantled only so far as was necessary to find the original marked stone, and were then re-erected with a new internally strutted pole and vanes.

The old triangulation schemes were of great help on the 1954-1956 work in South Australia. Further lines were needed to make up many of the figures between Marree and the Fink, and "refraction lift", to give intervisibility between stations in the last light or immediately after dark, was often used. Strangely enough most of these "lifted" rays gave little trouble, and better than the results obtained along some apparently textbook lines.

It is an old trick, and several of these original rays had obviously been observed in similar fashion, conjuring up visions of acetylene lamps, till an old station owner mentioned that, as a boy, near Oodnadatta, he had helped burn a fire one night on such a point whilst a surveyor read angles to it from a distant station.

Tellurometer Flexibility

With the timely arrival of the Tellurometer in 1957, geodetic control into almost every part of Australia became certain and economical, and gave the opportunity of putting down one of the most accurate, extensive surveys in the world, not only for mapping purposes but for scientific investigation and observation.

Traversing is the most flexible means of obtaining ground control, and with sub-traverse legs as are now possible, reconnaissance presents few problems.

Except in very flat country, timbered with 20’-30’ trees, as was experienced on the Mataranka to Newcastle Waters section (where mobile towers were used on reconnaissance, and later 30ft windmill stands as observing towers) Division of National Mapping has experienced no real difficulties in selection and measurement of traverses.

Some aerial reconnaissance has been done and more is planned this year in Central Queensland, where timbered, flattish terrain will be met, but most of our work has been carried out on the ground in good traverse country with good cross country going, of which so much of Australia is comprised.

"Desert Going"

Inland Australia is well known to an increasing number of people these days, but to some those areas marked ..."Desert" may give a conception of moving, barren, endless sandridges, and Sahara like deserts.

This is not yet so, though it could well be brought about with a few more droughts of the last six year pattern, when mulga and other top feed for hundreds of miles die, and stock will make their regeneration difficult or impossible.

Most of the sandy soiled "Desert" areas are covered with open vegetation of 7’ to 30’ high eucalypts and acacias, various 2’ to 6’ shrubs, and spinifex - the latter so maligned by early explorers but which holds so much of Australia together.

There are occasional but extensive parks of the magnificent desert oak, and thick belts of mulga in some of the "Desert" areas, which are generally sandridge country where the ridges may be anything from 100 yards to several miles apart.

These ridges run in different directions in different areas, but probably have a more common SE to NW trend - the prevailing wind direction.

They form into labyrinthine patterns in some areas - often looking more difficult from the air and in photos than they are in fact on the ground to 4 wheel drive vehicles.

In other cases, they form up like the prongs of a fork, very easy travelling from the handle to the prongs, but somewhat more difficult in the opposite direction.

It is sometimes quite easy to cross them anywhere; or again to cross in one direction only but impossible to return in one's tracks against the wind formed cornices and soft steep slopes on the lee side.

Through these sandridges are occasional sedimentary residuals - sometimes wide, gentle undulations, other times sharp sided but flat topped and typical of the plain they once formed - sometimes isolated and fifty or more feet above the sandridges and commanding the country for miles in every direction, other times 10 to 20 feet below the crests of the ridges and with the sandwaves already rolling them under. But generally, they are about the same height or slightly higher, and always they are worth seeking out for permanent establishment of survey marks, and better going - and the interesting possibility of a rockhole.

In their seemingly limitless settings, they deserve their "Mount...," and "......Range", so often given them by the explorers,, lone prospectors, surveyors and geologists, who traversed these routes, or in close proximity to them, the tough way, 90, 100 and even more years ago - with no built in water tanks and refrigerators, no wireless to bring winged help either at one’s call or because one has failed to call, and with neither photographs nor knowledge of what was ahead.

Conditions are vastly different today, and it is well to remember what such people did so long ago.

Getting to areas and laying down fuel are major costs, and we have found where reconnaissance and supplies and equipment have been properly planned, marking and measuring operations follow swiftly, economically and thoroughly, through most of inland Australia.

Any well selected track (even single wheel tracks) into most areas, immediately allows the main planning of a Tellurometer traverse. Where photographs are available, as is the case now over so much of the country, the exact route of such a track may, and should, be chosen for good going and probable traverse stations, before the surveyor leaves the city office, with full confidence that the control can be taken through, and the logistics of the operation.

Most sandy country packs down mechanically with the passage of each vehicle, especially of the same track width, and with sufficient traffic and particularly traffic soon after rain, a good free running track can be fairly quickly established.

In the Canning Basin, in good going, and with the route pre-selected on photographs, 100 miles cross country has been covered in two days, with frequent stops to investigate intended traverse stations, and the 100 mile return trip, in the previous outward tracks, covered in one day.

Weapons Research Establishment has given special assistance in grading tracks along Tellurometer routes, previously selected on the ground by our surveyors, between Warburton and Carnegie, and Giles and Lake Mackay, and on aerial photos between Lake Mackay and Lake Tobin.

These tracks have greatly expedited and cheapened movement in the area, not only for survey and levelling, but for geological and geophysical parties.

Marking

There is no doubt that, without proper supervision, the first aspect of a survey to suffer, will be its marking and the second its records.

Remote or awkward stations, which should be the best and most prominently marked of all for future users, because of their very difficult or cost of access, are usually the poorest marked.

This Division has tried to make a special point of its marking, with due regard to getting materials into distant areas, permanence, ready recognition and future use by other survey authorities and to carrying out daylight observations.

In very flat country, near Mataranka to Newcastle Waters, and across the Nullarbor Plain, 20' and 30' galvanized windmill stands have boon erected on concrete emplacements, and after use as instrument stands, have been left as future marks.

On our first traverses, and where stone is limited, circular walls of stone 10-12 feet in diameter and 18"-24" high have been built. Where there has been no stone, circular trenches 12 feet in diameter, 24" wide and 18" deep, have been dug, with the spoil centred in a pile over the station mark, or in an outside symmetrical ring.

Where sandridges have been used, one or two of those circular recovery trenches have been laid off in the nearest firm flat anything from 100 to 800 yards distant.

Several football sized stones (even one) in an area of no stone at all, are very noticeable, and a returning truck has often brought them some miles to put over a mark.

Cairns have been erected, however, wherever possible, with centre poles and vanes - the most stable shape being 7'-8' diameter, 5'-6' high, pudding shaped, with 11' pole and four 3' x 2' vanes.

Where stone is insufficient for a normal sized cairn, the groundmark is placed 12" below the surface and the cairn reduced in size.

It was noticed after the first season that centre poles have a tendency to shake and rock several inches in a cairn. These poles were then strutted internally with four 6' x 3” x 2” struts, 11' steel poles, internally steel strutted, are now used.

Most rises exist because of a protective cap or hard undersurface; prodding and quarrying with a pick or crowbar in unpromising spots quite often unearths some stone, at least for a ring or a small pile.

Three recovery marks, usually of small lengths of unattractive ½” diameter copper tubing, are inconspicuously flushset in cement blocks, rock crevices or star drilled holes in solid rock, round each ground mark.

Mistakes in measuring these short distances have been overcome by angle and distance radiations from two out of the four positions comprising the ground mark and three recovery marks.

To avoid the heavy and unnecessary labour of dismantling these carefully constructed monuments, and to enable other observers to sight to them at any time, instrument standpoints are chosen eccentrically and usually form one of three recovery marks emplaced.

They in no way govern or lessen the prime importance of the station mark. Any subsequent occupant can set up wherever he wishes.

Most surveyors can quote examples, not only of thoughtless or careless damage, and loss of marks, but also of deliberate, wanton destruction of them.

Few, if any field surveyors have not spent time in searching for their starting point. There have been hours lost on wide flat tops looking for some sign of an old station.

And there have been other occasions in an archipelago of rough approach residuals all the same height, and in the Musgrave and Flinders Ranges, when climbing the wrong peak could occupy 3 or 4 hours and some exertion, where 100 year old marks could be seen 10 miles away.

Some of these new geodetic marks could remain unvisited for generations, but within 5 or 10 years, most future visits will be by helicopter.

In all cases, such visits are likely to be more expensive than the initial ones establishing the stations, and it is hoped no time will be lost in finding any such sought for mark by future users.

Observing Procedure

Particular care has also been given to the theodolite standpoint and stand. The standpoint is chosen on the best position at a station, and 18" x 3" x 3" Oregon pegs driven with the tops then bored for the ferrules of the legs - when ground is too stony, 15" x 1" internal diameter, pointed pipes are driven; or in sand 36" x 3" x 3" pegs are used; on sheet granite, leaf effect is always checked with a hammer.

Wild T3 Zero Setting

Face

00°

00’

05”

Swing

Face

210°

02’

15”

Swing

Face

180°

00’

05”

Swing

Face

30°

02’

15”

Swing

Face

240°

00’

25”

Swing

Face

90°

02’

35”

Swing

Face

60°

00’

25”

Swing

Face

270°

02’

35”

Swing

Face

120°

00’

45”

Swing

Face

330°

02’

55”

Swing

Face

300°

00’

45”

Swing

Face

150°

02’

55”

Swing

Face

10°

00’

05”

Swing

Face

220°

02’

15”

Swing

Face

190°

00’

05”

Swing

Face

40°

02’

15”

Swing

Face

250°

00’

25”

Swing

Face

100°

02’

35”

Swing

Face

70°

00’

25”

Swing

Face

280°

02’

35”

Swing

Face

130°

00’

45”

Swing

Face

340°

02’

55”

Swing

Face

310°

00’

45”

Swing

Face

160°

02’

55”

Swing

Face

20°

00’

05”

Swing

Face

230°

02’

15”

Swing

Face

200°

00’

05”

Swing

Face

50°

02’

15”

Swing

Face

260°

00’

25”

Swing

Face

110°

02’

35”

Swing

Face

80°

00’

25”

Swing

Face

290°

02’

35”

Swing

Face

140°

00’

45”

Swing

Face

350°

02’

55”

Swing

Face

320°

00’

45”

Swing

Face

170°

02’

55”

Swing

Unless on towers (where a special traversing clamp is used to hold the theodolite head) normal theodolite legs, with all bolts and screws frequently checked, are used.

Light canvas screens, fitted to a Dural framework, are always erected for vertical and horizontal work.

Vertical angles, truly simultaneous by radio or helio flash are usually read between 1400 and 1600 hrs. L.M.T., when the air is most evenly heated.

Horizontal angles are read in the afternoon as soon as the beacons are steady enough - generally an hour before sunset, but this depends on time of year, locality, weather (including clouds,) etc., and may be longer on some occasions. Horizontal work usually stops when it is too dark to see.

Through the method of double pointings and double readings each time a beacon in turned to in a zero, there are no repetitions of a zero, blunders of reading or pointing being checked at once. Rejections of sets are seldom made, oven if a rare 7" or 8" range is obtained in a set. Range in a set is usually under 4½".

A double pointing forms a zero, and six zeros a set. Six sets, which divide the circle into 10° divisions, can be read in an hour or less, to two stations.

The horizon is not closed, but sets are first initially swung right, then second sot initially swung left, etc.

It appears that this alternative swinging of sets can be important, even with modern instruments, when there are extremes of temperatures to be met on a 6 or 8 months season in central Australia, and oil can apparently either dry out or be too thick to cover all conditions during such a period.

A typical set of directions and reductions on two stations, as observed by a Wild T3, are shown below.

	Station: Boola Boola			1900-1913 hrs. Friday 31st Oct. 158.
		Carlabeencabba				Cheeta
		15.2"				36.7"
220°	02’	15.2"	30.4"	38°	32'	36.8' 13.5"	43.1"
		17.0				38.5
40	02	17.1	34.1	218	32	38.5 17.0	42.9
		34.4				57.0
100	02	34.9	09.3		32	57.2 54.2	44.9
		33.4				54.8
280	02	33.6	07.0		32	55.2 50.0	43.0
		55.4				16.1
340	02	55.6	51.0		34	16.2 32.3	41.3
		57.8				18.1
160	02	57.0	54.8		34	18.0 36.1	41.3
							16.5
00°	00'	00"				178º 30’	42.75"

Angular Discrepancies

For a very long period, angular accuracy on an extensive traverse was well ahead of the accuracy of measured distances, and Wild’s modifications to theodolites in the 1920s put it further ahead.

In 1939 the use by the Royal Australian Survey Corps of the electrical resistance method of standardizing base line tapes brought things more into line, and these base lines measured by this method were to be invaluable in proving the accuracy of the Geodimeter and the Tellurometer.

But even these were comparatively short lines of about 5 to 7 miles on very carefully selected ground.

Up till 1937 in Australia, one of the main functions of a trig station was its use as a visible from afar point of control, to be sighted on by theodolite or plane table and thus help to build up further control.

Accordingly, all trig stations were almost invariably and conspicuously marked by steel or wooden beacon, or by cairn, pole and vanes.

On geodetic work, angles were usually read to these beacons, when they steadied down, about one or two hours before sunset, depending on the time of year, elevation, latitude.

Where difficulty was expected, as on a long or backed line, or a SE direction, where haze first forms, efforts were made to man such a station with helio and/or Lucas electric lamp.

Also, if manpower permitted, other stations were manned by lights, since the observing time was greatly increased and this increase was especially important with slow reading two or three micrometer instruments. Triangle and figure closures so obtained were usually of high quality and well within 1st Order standard.

My own experience on some 4 or 5 stations in 1939, however, led me to doubt some of these late night results to lamps, since some big movements were found and seen along good, clear lines which apparently should have given no trouble.

There was no further opportunity to test this out till 1954, when with a small party Division of National Mapping began the geodetic triangulation from Broken Hill, through Port Augusta towards Darwin.

It had already been decided, not only through manpower considerations, but since it was thought late afternoon observations near sunset were more accurate, to keep as much as possible to daylight work, now all parties were equipped with fast reading, modern instruments.

Early morning was also fully tested, and though an occasional morning gave apparently satisfactory results, after a six weeks test, morning work was considered doubtful and discontinued. Other than late afternoon work has since been confined only to those rare, heavily overcast days, when conditions are sometimes good all day.

Broken Hill to Marree was in the grip of a serious drought in 1954, and trouble was experienced through haze and poor conditions. Whether it was due to inexperienced observers or to these poor conditions is not certain, but despite quite good rays, especially through the Flinders Ranges, this section has given our largest average triangle misclosure of 0.9".

Most of the troublesome misclosures were in the last 50 miles south of Marree, and it was interesting to see the spectacular movement of the Geodimeter beam, when for nearly a month in this same area and during the same period as the angular difficulties, this instrument tried many nights to hold the reflected beam, on its initially equipped plane mirrors, long enough to effect a measurement.

In 1954 the Geodimeter did away with the need for costly and elaborate base line measurement and extension, and began to prove in some instances how quickly scale fell away in triangulation chains.

1957 saw the arrival of the Tellurometer and the beginning of the wholesale onslaught by Commonwealth and State mapping authorities on the geodetic scheme which now covers Australia, and in the main will be completed by 1965.

It changed geodetic techniques, since it was no longer necessary to make only for the highest and often the roughest country, but was cheaper, easier and usually more useful to keep a traverse near a road or track in low hills and rises, and a near graze along a Tellurometer line was quite helpful - for the Tellurometer measurement. Control could be taken along a single line of low towers in flat, timbered country quickly, and impossible otherwise without a full scale high tower project.

Much interest was immediately paid to this instrument in Australia, which was fortunate, since it proved that traverses of geodetic accuracy were well within its capacity, and that, in fact, a continuous line of Tellurometer measurements through all triangulation chains was desirable to bring all geodetic work to the same standard

Until the middle of 1957, in Division of National Mapping, normal triangulation methods were used on geodetic work.

After the arrival of the Tellurometer, the highest and clearest rays, in the vicinity of any selected route, have always been used, either as main traverse stations, or as side points (usually occupied wherever economically possible) to give checks against angle and distance work, as well as to establish further control points, whilst the party was in the area.

The overall average triangle misclose to date is 0.75", and side equations, wherever obtainable, have indicated good work.

Numerous single direction T3 azimuths were taken initially along all the early triangulation and traverse routes (uncorrected for any Laplace conditions), and in general these gave remarkably close agreement when compared with carried forward geodetic bearings.

At the end of 1958, some very large swings encountered near The Bight, coupled with some unexpected fallaways in scale through apparently well closing, well shaped figures in several earlier sections of triangulation, made single direction work, including azimuth work, suspect in certain areas, more especially near the coast, and it was decided to see if T3 simultaneous Reciprocal Azimuths, observed along a line simultaneously from both ends, would help to counteract consistent swings.

Various examples follow of some of the bigger angular movements, which have been seen in recent years.

At Boola Boola, near Great Australian Bight

About 90 miles West of Eucla, and on a headland of the abrupt 250-300ft high escarpment, which is set back some 16-20 miles parallel with, and North of the coastline. It is a site where trouble could be expected, and is difficult to avoid, with the general, cool damp Southerlies blowing over the hot flat ground most of the day.

These original observations were taken at what might well be expected as a reasonably mild time of the year for this locality, and likely to give as satisfactory results as any.

They were observed on three late afternoons just before and after sunset to beacons, until Cheeta in the East, faded, and would be replaced by helio and/or by Lucas lamp as required till darkness obscured Carlabeencabba in the West.

Each result shown, thus 40.83”, 39.25” etc., is the mean of a set of 12 pointings. Only seconds are listed.

Prevailing South wind each day.

30th. Oct. 1958	31st. Oct. 1958	1st. Nov. 1958
Mainly Cloudy	Cloudy & Sunny	Chiefly Sunny.
40.83” 39.25 40.28 41.00 41.32	43.40” 39.27 41.78 41.28 42.75 43.58	36.90” 37.30 38.05 38.87 39.67 41.52 42.87 43.08 44.38 44.13
40.54”	42.01”	40.68”

It was this range in sets of 7½" together with discrepancies of 2" and 3" in several previous azimuths observed at different times elsewhere by T3 and T4 theodolites, which emphasized the need for better angular control, and indicated that a method of observing simultaneous reciprocal azimuths might supply this need and be worth testing out.

Pending this introduction to our control, a traverse party again visited Boola Boola, to clarify the value of this angle.

This was in early June, when conditions were expected to be different. Results then gave the largest swings so far encountered anywhere by our parties.

After the very wide values of the afternoon of 6th June, angles were read at varying times.

Rain prevented afternoon work on 8th June.

A summary of the results of the observed sets obtained on this re-occupation are listed below.

6 June 59 1650-1750	6 June 59 2050-2200	7 June 59 1407-1504	7 June 59 1652-1657	8 June 59 1852-2000	8 June 59 2101-2150
42.82”	40.97”	39.93”	42.87”	45.90”	47.17”
43.47	44.43	39.15	41.20	45.98	43.48
45.55	45.77	42.47	41.53	48.30	42.85
48.10	44.93	40.95	42.20	46.42	46.57
52.72	45.43	37.63	42.03	46.03
55.53	46.35	40.12	43.38	46.62
			44.62
			43.03
48.03	45.65	40.04	42.61	46.54	44.94

It will be noticed on the afternoon of 6th June, in one hour, there was a swing of 13", and that over all the observations at Boola Boola the swing amounted to nearly 19" - not in an occasional wild zero, but in these sets of 12 pointings in each set.

Change of zeros in each set were higher than usual, but generally any swing was comparatively a steady one.

Subsequent double ended simultaneous reciprocal azimuths indicated that the value of this angle was about 43.2”.

Mataranka to Newcastle Waters, N.T.

This section is in very flat country, thickly timbered with 25 to 30 feet trees.

Mobile, adjustable towers mounted on 3 ton trucks were used on the reconnaissance, after which 20 and 30 feet high windmill towers were erected for theodolite and Tellurometer stands, with outside, Dural scaffolding for the observer's platforms. Most of these towers have been left in position as survey marks.

On the initial angular work, in 1959, observed to lamps after dark, there was a steady accumulation of 40" in the 48 angles involved.

The section was re-observed in 1961 on the same towers, but to daylight lamps in the last 45 to 60 minutes before sunset.

The 30ft internal windmill towers were shaded all the afternoon by hessian draped the full length of the Western sides of the external scaffolding, to prevent twist in these stands. This shading was essential, or towers were still twisting at sunset.

Simultaneous reciprocal azimuths were also commenced about 10 minutes after sunset over 36 of the 48 lines.

The swing in the geodetic azimuths carried forward, using these horizontal angles, and compared with the various meaned reciprocal astronomical azimuths, gave a gradual accumulation of less than 4" in the 48 lines.

On this flat, belaboured, Mataranka to Newcastle Waters section, it was found that lamps on the short 2 to 12 mile lines involved, were observable for no more than an hour before sunset. At sunset the light was usually hard, sharp and excellent, but began to deteriorate very quickly after that.

An hour after sunset it was almost invariably a large woolly, rolling or flaring ball, and this was at times over very short lines and from 30ft high towers at each end, with fair clearance along the whole line.

It was noticeable, when one came down from the 30ft towers on completion of the Sigma Octantis observing, how much cooler it was on the ground.

It was also noticeable that though the lamps had deteriorated into poor, woolly sights, Sigma Octantis still remained, at 16° altitude, clean and sharp throughout.

Comparison of Single T4 with Single T3 and with Simultaneous Reciprocal T3 Azimuths

In the course of checking the Bight and Newcastle Waters - Mataranka traverses, T3 azimuths were observed along a number of lines already observed one way by the T4.

The T4 azimuths had been observed over 2 or 3 nights at each Laplace station, with the usual procedure such observations with this instrument are given.

With the T3 azimuth, 2 Face Lefts and 2 Face Rights make up each zero. Twelve of these T3 zeros were read at each end of a line simultaneously each night.

Along the Mataranka traverse there were two observers at each end, who changed over after 6 zeros, and making, also 12 simultaneous azimuths at each end of a line.

There is no striding level with the T3, so the plate bubble was read at each pointing to Sigma Octantis. Also, to avoid any misreadings, or stickiness or flat in this 6.5” plate bubble, it was re-levelled after each 8 pointings, that is, after each 2 zeros.

It is hoped to fit this Division’s T3 theodolites with 4" plate bubbles on future work.

The Bight traverse is in Latitude 32°, and the Mataranka - Newcastle Waters is 15°-18°.

Laplace stations were observed very closely along the Bight, and for the sake of obtaining some comparison in the two azimuth values along a line, an approximate Laplace correction has been applied to the observed azimuth at the reverse end. This is not precise but an estimation only.

Along the Mataranka work, where geoidal anomalies are small, and the latitude low, no Laplace corrections have been applied to the azimuths at either end in the listed results.

COMPARISON OF SINGLE T4 WITH RECIPROCAL T3 AZIMUTHS
Station		Date	Forward Azimuth		Reverse Azimuth

NEAR GREAT AUSTRALIAN BIGHT
Oak – Nurka	T4 T3	Feb. 60. Apl. 61.	301	22 32.25 32.18	31.82
Coppu – Black	T4 T3	Feb. 60. Apl. 61.	281	55 17.34 17.84	18.04
Cooper – Wantiby	T4 T3	Feb. 60. Apl. 61.	269	36 43.76 41.40	42.59
Black – Colona	T4 T3	Feb. 60. Apl. 61.	275	28 33.80 30.31	30.47
NME89 – 88	T4 T3	Feb. 60. Apl. 61.	78	50 35.22 34.74	34.59
NME94 – 93	T4 T3	Feb. 60. Apl. 61.	82	00 33.00 30.97	30.79
NME110 – 109	T4 T3	Feb. 60. Apl. 61.	89	44 46.89 44.95	45.13

BETWEEN MATARANKA AND NEWCASTLE WATERS N.T.
Stott - MacDrill	T4 T3	Aug. 58. June 61,	28	40 35.07 32.90	32.52
NMG109 - 110	T4 T3	Sep. 60. July 61.	348	37 08.28 08.96	09.73
NMG18 - 15	T4 T3	Sep. 60. Sep. 61.	6	37 56.04 56.01	54.07

NEAR MARREE S.A.
Attraction - Alford	T4 T3 T3	July 57 May 57 Oct. 60.	278	27 20.03 23.62 22.26	18.31

Laplace stations were closely spaced along the Bight; where anomalies were large.

From these closely spaced Laplace stations, corrections were estimated and applied to the reverse observed azimuths to bring Forward and Reverse azimuths into sympathy. This is not accurate procedure, but pending further Laplace work at the reverse stations, it possibly gives a closer indication of the reverse bearing. Latitude along the Bight is about 32°.

Anomalies on the Mataranka traverse are small, and latitude is 15°-18°. No Laplace corrections have been applied.

These Mataranka results are of interest. It is again mentioned they were commenced as soon as Sigma Octantis was visible - about 10 minutes after sunset. Despite the very wide disagreement at times between each end, the means of the two ends remained in close agreement with the geodetic values as carried through the daylight angles down the full length of the traverse.

On this Mataranka work, wherever a big difference between ends was found, a second night was observed - as was also done where the results differed appreciably from the previous T4 value.

WILD T3 SIMULTANEOUS RECIPROCAL AZIMUTHS
RESULTS OF TWO SEPARATE NIGHTS

Station	Date Forward Azimuth	Reverse Azimuth

NEAR THE BIGHT S.A.
Wantiby - Browns	22.4.61 295° 58' 19.14” 1.5.61 18.61”	14.14” 15.25”

BETWEEN MATLRANKA & NEWCASTLE WATERS N.T.
NMG77 - 78	30.8.61 3 13 09.19 31.8.61 04.94	05.18 12.05
NMG80 – 81 (T4)	26.8.61 322 08 02.62 27.8.61 02.52 -.8.58 07 58.65	02.96 01.32
NMG82 – 83	23.8.61 349 24 30.79 24.8.61 32.67	36.45 36.04
NMG94 -95 (T4)	2.8.61 6 26 48.78 3.8.61 51.06 -.9.60 47.80	51.20 51.71
NMG99 - 100	26.7.61 5 40 52.72 29.7.61 58.88	59.76 56.05
NMG112 - 113	29.6.61 331 31 34.30 3.7.61 36.10	39.04 38.77
NNG88 - 89	12.8.61 28 05 04.28 13.8.61 04.02	08.33 08.80

The result of two night's work at several stations seems to confirm the need for simultaneous work.

Thus at NMG77-78 :

Date Forward Az. Reverse Az. Mean.

30.8.61 3° 13' 09.19” 05.18” 07.18”

31.8.61 04.94” 12.05” 08.50”

If reciprocal, (but not simultaneous) azimuths had been observed on the 30th and 31st results could have been 10.62” and 05.06”. Similarly, along NMG99-100, there has been a big change over in swings.

Simultaneous Azimuth Observations with T3 & T4 Theodolites

WILD T3 and T4 THEODOLITE TESTS AT KHANCOBAN (NEAR COOMA)
From Cochranes Gap (1814ft.) to Scammels Lookout (3724ft.) – Distance 11 miles.
Approximate Values Cochranes, Lat. 36° 10' : Long. 148° 04’ : Azimuth 148° 30' 57”

Date	9.Nov.'61.	10.Nov.'61	11.Nov.'61	12.Nov,’61	13.Nov.'61
Instrument	T4/56091	T4/56091	T4/56091	T4/56091	T4/56091
Observer	A	A	A	A	A
Time	2030-2330	1920-2125	1926-2126	1920-2135	1927-2122
No. Zeros & Range	8 (2.14”)	11 (3.77”)	12 (3.11”)	13 (1.82”)	12 (2.63”)
Mean	57.55	57.30	57.34	57.58	57.83

Instrument	T4/37448	T4/37448	T4/37448
Observer	B	B	B
Time	2000-2300	1920-2100	1930-2115
No. Zeros & Range	12 (2.31”)	12 (1.42”)	12 (2.79”)
Mean	56.39	56.48	56.65

Instrument	T3/26687	T3/18516	T3/18788		T3/29886
Observer	C	C	C		C
Time	2006-2050	1922-2003	2035-2124		2044-2134
No. Zeros & Range	6 (2.35”)	6 (4.07”)	6 (3.30”)		6 (2.84”)
Mean	57.72	56.46	58.05		57.50

Instrument	T3/26687	T3/18516	T3/18788		T3/29886
Observer	D	D	D		D
Time	2235-2315	2025-2100	1930-2013		1940-2022
No. Zeros & Range	6 (2.41”)	6 (2.93”)	6 (5.89”)		6 (2.94”)
Mean	56.14	55.55	55.47		58.80

Instrument	T3/29886	T3/29886	T3/18516	T3/26687
Observer	E	E	E	E
Time	2004-2048	1920-2003	2035-2124	1923-2012
No. Zeros & Range	6 (3.23”)	6 (4.02”)	6 (2.38”)	6 (2.84”)
Mean	55.60	55.57	55.86	57.61

Instrument		T3/29866	T3/18515	T3/26687
Observer		F	F	F
Time		2024-2100	1930-2015	2036-2130
No. Zeros & Range		6 (2.08”)	6 (3.55”)	6 (1.92”)
Mean		55.70	55.39	56.17

Instrument		T3/26687	T3/26687	T3/18788
Observer		G	G	G
Time		1920-2100	1930-2123	1920-2126
No. Zeros & Range		12 (4.73”)	12 (4.95”)	12 (6.23”)
Mean		55.42	57.39	58.18

Although the simultaneous reciprocal method was expected to show up discrepancies, the big differences between T3 and T4 results along some of the Bight lines gave an uneasy feeling that the T3 might be unable to produce the required accuracy, possibly through sluggishness or coarseness of the plate bubble. It was noticeable that the widest discrepancies along the Bight between the T3 and T4 results, show the T3 value lower in each case.

A test was therefore arranged for the first opportunity, along a good line and under good conditions in the Snowy Mountains, with two T4’s and four T3's and seven observers participating.

The following is a Summary by Instruments of the Wild T3 results.

SUMMARY BY INSTRUMENTS (WILD T3)

T3 Inst. No.	26687	18516	18788	29886
9 Nov.'61	57.72 : 56.14			55.60
10 Nov.'61	55.42	56.46 : 55.55		55.77 : 55.70
11 Nov.'61	57.39	55.86 : 55.39	58.05 : 55.47
12 Nov.'61	57.61 : 56.17	58.18
13 Nov.'61				57.50 : 53.80

Overall Means	56.74”	55.82”	57.23”	55.63”
SUMMARY OF WEATHER CONDITIONS. 9th Nov.'61 Cool, clear, light Westerly wind 10th Nov.'61 Cool, some cloud, Westerly breeze 11th Nov.'61 Cool, clear, calm first half of obs., then high S.E. breeze 12th Nov.'61 Mild, calm, clear 13th Nov.'61 Mild, some light cloud. S.E. breeze

The T4 observers were experienced observers, and all T3 observers (except one who was however an experienced observer) had just completed 7 months continuous azimuth work.

The T4's, as one would expect, have given very consistent results, proving fairly conclusively the stability of the atmosphere under these high and good conditions.

It is not possible to indicate whether the observers or the T4 instruments account for the 1.0” difference in the results, but extensive investigation in Finland during 1946-1956 showed one precision instrument to have a constant 1" error.

On 12th and 13th November, instrument T4/37448 with observer 'B' and T3's 18516 and 18788 and all T3 observers, over one or other of these two nights, moved to Scammels Lookout, and simultaneous reciprocal azimuths were observed between the two stations.

Results at Scammels Lookout are not final, pending check on the Longitude observed there and they will have to be shown at a later date. It appears there will be little difference in the forward and reverse bearings, when the accurate longitude is known.

This reverse bearing should then give a better idea of the real value of this line.

On these present figures from Cochranes Gap, the T3/observer combinations seemed to indicate a low trend as compared with the T4’s.

At Scammels Lookout T3 and T4 results (not shown here) are generally closer to each other.

There is quite a scatter among the T3 sets (each set 24 pointings), and it is worth noting that two observers ('C’ and 'D' on 13th November 1961) can get a reasonable range in their 6 zeros, of 2.84” and 2.94”, yet differ in their means by 3.7” on the same instrument, under apparently stable conditions as indicated by the T4 result.

General

In comparison with old style triangulation, angular accuracy can fall away quickly on Tellurometer traverses for various reasons such as :

(a) availability of lower and often poorer angular rays;

(b) absence of triangle checks;

(d) late night observations to lamps, in low hot areas particularly;

(e) necessity to use observers at times with insufficient experience. The new instruments make angle reading look simple, but do not change the old trigonometrical adage of "not so much knowing how to read an angle as when to read it"

There is a good deal of agreement now that the most accurate period to observe horizontal angles is likely to be round sunset, and this seems even more so in hot areas of low relief.

The period is short, and valuable time is lost to an observer, even on a sunny day, as the helio fades out 5 to 10 minutes before sunset and a lamp is not yet visible over any fair length line. This is one of the main reasons why this Division observes, wherever possible, to opaque beacons.

It is summed up in the following extract from "A Singular Geodetic Survey" by Lansing G. Simmons, U.S. Coast and Geodetic Survey Technical Bulletin No.13, September 1960, regarding work on a precise triangulation and Geodimeter net, observed from Bilby towers near Cape Canaveral.

"There is no known way of correcting for horizontal refraction. Experience seemed to prove that the better a light appeared in the telescope, the better were the observing conditions. The problem is just about as simple as that. Good-appearing lights produced good results.

Almost always, everywhere, the lights look the best just after sundown. Turning angles right after sundown, rather than calling, identifying, and communicating with lightkeepers, resulted in improved observations".

Our experience fully confirms this statement, though the opaque beacons possibly allow our observers a somewhat longer and more balanced period of accuracy.

The foregoing summaries of various results give an indication of discrepancies which have arisen on geodetic work where it would seem reasonable care has been taken.

They appear to involve refraction, as well as observer and/or instrument errors.

These discrepancies are for all to see, but on a less spectacular scale they doubtless occur on many lines.

Not every line gives refraction trouble, but with single line traverses now going anywhere, more refraction effect is likely to be met, and there may be certain areas (such as the Mataranka - Newcastle Waters) where it can possibly occur on most lines after dark.

It is therefore encouraging to see in such poor conditions, the close agreement between daylight angles and simultaneous reciprocal azimuths near Mataranka, since even in Central Australia, not every night is favourable for astronomical work, and normal angular work is always likely to be required.

Such azimuths, Laplace controlled at each end are now being established at least every fourth station on the National geodetic framework, and it is thought current astronomical tests will prove that a less elaborately observed Laplace station will supply sufficient deflection data for use at least at the reverse ends of such Laplace azimuth lines.

Geodetic surveys supply both mapping control and scientific data for geophysical investigations.

With Tellurometer measured lines, well controlled in azimuth, and due to its terrain, latitude, weather and. shape, Australia in the next few years appears to have the opportunity of producing the most extensive area of accurate and useful scientific survey in the world for such investigations and for space research.