Changing face of survey instruments
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Article with a lot of information to the accessories written by A.H.Ward, Heerbrugg, Switzerland
Wild Heerbrugg , July 1967

 THE CHANGING FACE OF SURVEY INSTRUMENTS

 ”Changing face” is a procedure with which all surveyors are familiar. When this simple action of reversing, or transmitting, the theodolite tele­scope is made and readings are taken on both faces, the mean of the two values will be free from the small residual discrepancies that exist in all theodolites. It is, therefore, an essential operation when the maximum accuracy is required.

Changing face, however, is an express ion that is not limited to the physical relationship between the telescope’s position and the vertical circle. All designers and makers of modern equipment are continually producing instruments with new shapes, performances and functions. These constitute a different, but vitally important, manner in which survey instruments themselves have changed face over the years.

For the first real break through from the old-fashioned open type theodolite, we must go back to the period at the beginning of this century when Heinrich Wild began his distinguished career as an instrument designer. Dissatisfied with the existing theodolites, which were particularly unsuitable for his triangulation work in the mountains, he abandoned his career as a surveyor and took on the task of developing an instrument that would stand up to rugged field work, that would be easier to use and that would provide a higher reading accuracy.

Although many new ideas and refinements have been incorporated into theodolites and levels since the remarkable Dr. Wild’s death in 1951, the success of his efforts can be seen in virtually every present day theodolite, level, tacheometer and alidade. The story of Heinrich Wild’s career is given in detail in Dr. Strasser’s article in the Survey Review (”Heinrich Wild’s contribution to the development of modern survey instruments”, G.J. Strasser, April, 1966).

His main objectives were to reduce the size and weight of the instrument and its container, to provide protection and stability to all parts and to improve operational and reading efficiency. These aims were to be achieved by a simplification of design that would enable the observer to work without superfluous movement, whilst allowing him full control and easy manipulation of all knobs and screws.

It is difficult for a young surveyor to appreciate exactly what was involved in the handling of the classical open theodolite. He is accustomed to a compact, light, easy-to-handle instrument and this bears little re­semblance to the vernier model of fifty years ago.

When using a present day theodolite, there is no need at all for the observer to move his position when sighting to a target and then reading the circle values. With Wild’s well-known coincidence reading system, the diametrically opposite values of the horizontal and vertical glass circles are meaned by the simple turning of a micrometer knob. This ingenious invention is now taken for granted, but it was not really so very long ago that the ”A” and ”B” verniers of the metal circles had to be read (with the aid of a magnifying glass) and this involved walking around the instrument after the target had been fine-pointed. The time needed to do this was quite considerable and, of course, the danger of knocking something on the way round was always there, particularly when the set-up was a difficult one on a slope or a rocky outcrop. The booking errors and subsequent meaning errors were also serious drawbacks to this old type of instrument.

In addition to the easier and more accurate circle reading, a modern theodolite can boast of its short, powerful telescope with its coated lenses and internal focussing. This allows much speedier operation, dispenses with the problems caused by having a variable or moving measuring point (which changed with every re-focussing) and provides complete protection for the telescope tube and its contents a glance at the triangulation theodolite of less than 50 years ago will show how necessary such changes were. The amazing thing is that, even today, it is possible to find manufacturers who still make the old-fashioned open type of instrument and proudly advertise them.

The facility with which adjustments are made also represents a great advance, together with the simple method of testing and correcting the residual collimation errors. The cross or similar pattern that is etched so finely and with mathematical precision on the reticule plate of a modern telescope requires only the minimum of adjustment and the manufacturer’s guarantee that the ”horizontal” and ”vertical” cross hairs are exactly at right angles to each other overcomes the difficulties associated previously with the alignment and setting of real hairs it was no joke when the hair broke and a new one had to be substituted.

The protection of the footscrews against dust and moisture, together with the smoothness of their run, enables the levelling-up of the instru­ment to be made much more easily than with the old type of open screw which did not always allow an even, regular movement to be made. It is interesting to note, in passing, that there is still a tendency in some countries to use four footscrews instead of the logical three. It is difficult to understand the reasoning behind this strange relic of anti­quated instrumentation. It is so much easier to level up with only three footscrews, the initial pre-levelling being an especially simple procedure. For this, the observer soon masters the technique of centering the circular bubble by means of the virtually simultaneous manipulation of the three screws. This becomes such a routine task that it is almost ”automatic” and is completed more or less without thought.

This mention of an unusual means of levelling a theodolite brings to mind a method which has failed to find general approval, the use of a ball and socket fitting for levelling-up an instrument. There are various types and, although they may facilitate slightly faster levelling (even this has been disproved by tests), they certainly go completely against the accepted maxims of instrument design.

Another idea which has received a mixed reception has been the system of ”centering above the footscrews”. In principle, this is an excellent feature as it can be annoying, on steep slopes particularly, to set up the theodolite, pre-level, center with the optical plummet and then, after the accurate levelling with the plate level, to find that the instrument is not exactly over the ground mark and needs to be shifted slightly over the tripod head to compensate for the small difference between the approximately vertical line of sight of the optical plummet and the exactly vertical one obtained by the correct levelling of the instrument. By separating the movements so that the instrument slides over the already precisely horizontal levelling head there is no need for the repetitive steps that are needed with standard theodolites. The ”above the footscrews” method achieves its purpose but it does so by adding weight and height to the instrument, thus making it slightly top-heavy and losing some of its required stability and rigidity. There is always the tendency for dust and sand to come into contact with the sliding surfaces, which must be plane and smooth for the system to work properly. This may also occur with the ball and socket type of fitting, already mentioned.

To be perfectly fair to all types of instrument, it must be stressed that no existing system is completely satisfactory and there is no doubt at all that the question of developing an efficient but robust system of levelling- up is one of the biggest problems facing instrument manufacturers and it is difficult to visualize a solution that will be completely satisfactory. This may, in fact, be one part of instrument design that will bear the mark of ”compromise” for many years to come.

To revert to Dr. Wild’s contribution to the changing face of survey instruments and to continue with the items he introduced to the theodolite, mention must be made of the cylindrical axis in place of the classical conical type. This was a great step forward, as there was no need to adjust this new type of axis. The use of glass for the circles was also a milestone in the progress made since the First World War. It is possible to obtain and retain the high uniform quality of the circle graduations only when glass is used. The added safeguard of projecting the circle images through the hollow parts of the instrument so that they are seen in a reading microscope placed alongside the telescope, offers the double advantage of improving the simplicity and comfort of reading and also enables the circle housing to be sealed tightly.

It is an accepted feature nowadays to have the plate level placed cen­trally between the standards, so that it will not run-off rapidly when the alidade is rotated. This was not always so, however, and it was in 1923 that such a positioning of the plate level was first made when the T2 made its appearance. Some time before that Dr. Wild made the first use of his famous split-bubble system, in which the two ends of a spirit bubble are projected through a system of prisms so that they are viewed simultaneously.

Other important features which distinguish modern theodolites from their earlier counterparts are the more or less general use of the optical plummet and the various devices for ensuring forced-centering interchangeability between theodolites and their auxiliary equipment, such as traverse targets, subtense bars and the many accessories that are used to aid the surveyor. The optical plummet is a most popular feature of a theodolite, provides an excellent functional service and is infinitely preferable to the plumb bob, which has a tendency to sway, especially in windy conditions. The use of a plumbing or centering rod is another device that is finding more support amongst some surveyors, particularly when working in broken and sloping ground. The majority of surveyors, however, seem to prefer the optical plummet and this device is suitable in most situations.

The interchangeability of instruments and accessories is a comparatively new innovation and there is no doubt that this facility has revolutionized the art of traversing. Whereas it was necessary previously to plumb the instrument or a target over each traverse point at each and every change of instrument station, it is now commonplace to use the forced-centering three tripod system. After observing, for example, from station 2 to targets set up in tribrachs in tripods at stations 1 and 3, respectively, the surveyor releases the theodolite from tribrach number 2 and walks forward with it to station 3. Here the assistant releases the target and the surveyor replaces it with the theodolite. As the tribrach remains locked to the tripod, the instrument takes up exactly the same position that the target had done originally. In the same way, target 1 is set up in the tribrach at station 2 and the original tripod number 1 plus the target from 3, are carried forward and set up at station 4. Work progresses in this way, with the theodolite and the targets always progressing forwards by one station at a time and the rear tripod and tribrach being leapfrogged forward. Additional speed and convenience can be obtained by using an extra tripod and tribrach. One big advantage of this system is that there is no need to mark each station on the ground. As the instrument or target or subtense bar is always positioned over exactly the same point the traverse measurements and calculations can be made without permanent marks being placed and it is only at the stations that are to be used for future work that pins need be placed.

With regard to the form that the forced centering device should take there is little doubt that the best method is the three stud system, in which the three receptive holes in the tribrach match the studs on the bottom of the theodolite. The fit is perfect and provides for complete and absolutely safe interchangeability between the, instrument and its accessories.

The most striking differences between, new and old theodolites are in the basic, dimensions of the instruments and their containers. The neat, compact and robust models of today are far more convenient than the cumbersome open type of single second theodolite, which weighed  24 lbs. and was packed in a wooden box with the same weight. The T2 weighs only 12 lbs. And is carried in a strong. metal container, weighing 5 lbs. a total of 17 lbs. as compared with the previous 48 lbs.

The changes that have been made to levels are equally impressive. A level’s accuracy was once judged in terms of the length of its telescope the longer it was the better it was! All of the bad features that existed in the open theodolite were present also in the open type level and all have been corrected in more or less similar ways. Telescopes now have coated lenses and internal focusing arrangements. The axes are cylindrical  and the footscrews protected and adjustable. Cross-hairs are etched in the reticule plate and collimation errors are simple to adjust. There is no longer any need to remove telescope from its ”Wye” standards in order to reverse its position as a means of  making collimation test measurements. Simple techniques have been evolved for such tests and, with the Wild N2 level, provision is made for the telescope to rotate about its longitudinal axis, thus allowing staff readings to be made in two ”face” position, for easy checking.

Apart from the constructional alterations, however, which have been made in keeping with  the parallel  changes in theodolites, the most important improvement in spirit levels has been the introduction of the split-bubble coincidence system. No matter how sensitive a spirit bubble may be it is impossible to ”center” it unaided more accurately than to one fifth of the normal 2 mm division on the vial. With a split bubble system, however, in which the observer sees the two ends of the bubble side by side and has to bring them into coincidence so as to form a smooth, regular curve, it is possible to make an 8x improvement in this setting accuracy. With the N3 level, the sensitivity of the tubular bubble is 10” per 2 mm. By means of the split bubble viewing system the line of sight can be leveled up (using the tilting screw until coincidence of the bubble ends is obtained) to within the equivalent of one fortieth (one eighth of one fifth) of the 2mm division - i.e. one fortieth of 10”, which is one quarter of the second. If such a split bubble device is not incorporated in the instrument it will be possible to reach this high degree of accuracy only by increasing the sensitivity of the bubble. This would mean a bubble with a sensitivity of about 1 1/4” per 2 mm. Not only is this more difficult to produce but it take much longer to center, as each movement of the tilting screw produces a correspondingly larger movement of the bubble.

There is, however, no point in increasing the sensitivity of a level, and the precision with which it can be leveled-up, if the observer is still obliged to estimate staff divisions beyond the usual 0.01 ft. graduations. For this reason instruments such as the N3 make use of a parallel plate micrometer. After the line of sight has been made absolutely horizontal, it is unlikely that the ”horizontal cross-hair” will be seen at an exact staff graduation line and it will be necessary to estimate the portion of an interval by which the hair is displaced from the graduation. With a metric staff the graduation is usually in centimeters, so estimation will be to the nearest millimeter and with a foot staff the graduation is usually in hundredths of a foot, with estimation to a thousandth. By activating the N3’s parallel plate, by means of the micrometer knob, the horizontal line of sight can be displaced slightly above or below, but always parallel to itself. If the knob is turned until the cross-hair is set to an exact staff graduation the combined sum of this staff value plus the reading on the micrometer scale will give the required staff value for the pointing. With the metric model of the micrometer, readings can be made in this way to the nearest hundredth of a millimeter, as compared with the estimation to a millimeter without the micrometer. The foot model allows direct reading on its scale to 0.0001 ft. (which is the graduation interval) and estimation can be made to half an interval. If the utmost precision is to be obtained from a first order level, it is quite pointless to use a wooden staff, which is subject to significant changes because of varying temperatures and humidity.

To overcome this difficulty it is essential that observations with a precision level should be made to invar staves. The upper end of the invar strip is connected to the staff by means of a tension spring, which absorbs any small changes in the length of the frame, so that the strip always retains the same overall length. The coefficient of linear expansion of the invar is virtually nil, so that the individual graduations on the strip always retain the same absolute positions.

It is easier to ”straddle” a line graduation with a wedge-shaped cross-hair than it is to set a single hair on to the normal type of staff graduation. The wedge is set so that its limbs are symmetrical about the line graduation and, if preferred, this setting can be made so that the wedge lines are tangential to the rounded ends. Experience has shown that it is easier, and more accurate, for the observer to use this method of straddling a definite mark than it is for him to set a single line (which is black) to the almost imaginary division between a white section and a black section of an ordinary staff.

The overall precision that can be obtained when using the wedge-shaped reticule lines of the N3 to straddle the line graduations of the PNL invar staves is so high that levelling of this order becomes almost free from error. In one mile of double run levelling the attainable m.s.e. is ~0.0008 ft. (or ± 0.2 mm in one kilometer) - about one hundredth of an inch. Clark quotes figures for the English Geodetic Levelling (1912 - 21) of a maximum closing error of 0.4771 ft. in a perimeter of 290 miles and the smallest of 0.0073 ft. in a perimeter of 60 miles, The m, s, e. of ± 0.0008 ft. per mile is equivalent to ± 0.0008 Ö290 and ± 0.0008 Ö60 feet, respectively, for these two circuits - i. e. ± 0. 0135 feet (as compared with 0. 4771) and ± 0. 0062 feet (as compared with 0.0073). These figures relate to the attainable m.s.e. of course, and the actual results could be up to three times these amounts with the likelihood, however, that they would be within the limits given. If the 290 miles circuit is taken as a fair example, then the 0.4771 feet disclosure actually obtained about 50 years ago and the ± 0.0135 feet that the modern precision level can give indicates the enormous progress that has been made in the construction of levels and their auxiliary equipment.

Whilst the almost incredible accuracy of the large, powerful levels represents a standard that is essential for primary work or for the increasingly high demands of industry and constructional engineering, it is not always needed and, in many fields of use, the observer prefers speed to absolute precision. For such a user, the automatic level, with some form of compensating device to ensure that the line of sight is always horizontal, is the solution to his problems. The saving in time that is made with an automatic level is rewarding in itself, but the added security given to the observer, as he no longer has to center a tubular level bubble, enables work to proceed with considerably less mental strain in addition to the reduction in eye fatigue.

An increase in attainable m.s.e. is normal with all automatic levels, which are now accepted as being capable of more precise work than classical levels in their own class, For the observer who wants speed of operation combined with almost geodetic accuracy’s, it is now possible to attach a parallel plate micrometer to the objective end of the NA2 and to obtain results only a little below those obtainable with the larger instrument.

A similar saving in time, combined with a convenient dispensing of one of the easily forgotten tasks of theodolite surveying, is obtained by using a theodolite with an automatic vertical index. Whenever vertical angles are read it is essential that the index be set to read 90 with horizontal sights. It would be interesting to find out just how many times in his career the average surveyor fails to use the index setting screw when reading a vertical angle! The use of a liquid compensator, through which the circle image is seen, provides an ingenious solution to this important problem. After the instrument has been leveled up, there is no further index setting required. The vertical circle image is viewed, therefore, after it passes through the equivalent of an optical wedge, formed by the liquid. The circle reading is the required value automatically compensated by the elimination of the index setting error. Other instruments successfully employ pendulum type compensators.

To revert to the modern level, the improvements in design and operator comfort have not been restricted to the larger and very precise instruments used for high order work. The trend, in fact, has almost revolutionized the appearance of the small level, which is now a compact, robust and light instrument. For everyday levelling tasks the engineer has ‘a wide choice of dumpy and engineers’ levels, with or without a horizontal circle, and he now has the most useful alternative of having a telescope giving either an inverted image or an upright image. The high quality of modern lenses makes it possible for the optics required to produce an upright image to be added to the telescope without causing any lowering in performance. The same facility is also available for many types of theodolite. To the engineer who uses his instrument occasionally and who is unaccustomed to the traditional inverted image the erecting eyepiece is a great boon.

Another innovation that should be mentioned is the widespread use of a friction-braked action, which has replaced the horizontal clamp in many levels particularly in the smaller, builders type and in the automatic models. The action is adequate to hold the instrument from unwanted movement and it is possible to make slow motion settings, as the horizontal tangent screw is still included in the design.

A new feature which has made an appearance in the Wild range of levels is the incorporation of a directional arrow in the ”split-bubble”. When the tilt of the level is such that the ends of the bubble are out of the field of view an arrow appears indicating in which direction the tilting screw has to be turned. This almost ridiculously simple idea, it could be called a gimmick, is a great time saver.

Two similar points need mentioning here, although they are actually in­corporated into theodolites instead of levels. Both are simple ideas that do nothing to increase the accuracy of the instrument but merely provide a time-saving and a convenient aid. The first is once more a simple arrow on the telescope focussing sleeve, indicating the direction of turning to obtain focussing to infinity (there being an infinity sign, ¥ ,also). This, by implication, shows how to turn the sleeve for longer sights, with a turn in the opposite direction helping to focus down to the shortest distances. The second is an improved version of the change-over or inverter knob  the device whereby the observer switches over to viewing either the vertical or the horizontal circle when using an instrument of the T2 or T3 type. Until now, the knob has had a line engraved in it When the line was horizontal it was the horizontal circle that was being read and, when vertical, it was the vertical circle. To avoid confusion, particularly at night, this knob is now made with a raised bar on it, so that the circle in use may be identified by touch.

The trend in the past 40 to 50 years has been to reduce the weight and size of levels and theodolites, to improve their basic design and to make them easier to use and maintain. These objectives have all been attained and there has been no lowering in the precision of the instruments quite the reverse in fact. This face changing has not been restricted to the appearance and performance of the classical type of instrument, however, Developments just as startling have been made in the auxiliary equipment used with the theodolite and level and many completely independent in­struments are now available.

The optical plummet, for example, which is itself an innovation to the theodolite, has already progressed to the status of being an instrument in its own right. With a powerful telescope and a changeover prism allowing either upward or downward sighting the plummet can be centered accurately above or below a mark well beyond the very limited range of the device built into a theodolite. The device can then he removed from its tribrach and a theodolite set up in its place, without any loss of centering accuracy. The advantages of such a system are enormous and can be put to good use when observations on a survey tower are required or for building purposes, when it is essential that the construction lines are absolutely vertical to quote two examples.

For more limited ranges it is possible to mount an optical plummet to the telescope housing in order to plumb beneath a roof mark (or to place such a point). A similar attachment, only this time a split bubble level device, enables an ordinary theodolite to be used as a tilting level. The vertical tangent screw is manipulated until the ends of the split bubble are in coincidence, which indicates that the telescope itself is absolutely horizontal.

Another device that can be used for plumbing purposes, although its main use is the transfer of bearings from one mine level to another, is the objective pentaprism. This merely clamps over the objective end of the theodolite telescope, with a counterweight at the eyepiece end, and its rotational housing provides a line of sight in a plane perpendicular to the telescope direction. The use of such an attachment naturally calls for special observational techniques but it is not difficult to imagine the con­venience of establishing a line of sight and then, with the horizontal and vertical movements of the theodolite clamped, diverting that line of sight either upwards or downwards, as required. It is possible to plumb with an accuracy of 1/70’000 with a pentaprism of this type, although care has to be taken to eliminate the inevitable eccentricity between the instru­mental axis and the deflection axis of the prism itself.

A wide variety of gadgets are available from all instrument manufacturers for use in astronomical work. These consist of eyepiece prisms and diagonal eyepieces to enable steep pointing to be made (and the circles to be read), by deflecting the line of sight to the observer’s eye so that he can stay in a comfortable viewing position, together with various attachments with which observations or readings are actually made. These are items such as a prismatic astrolabe attachment, the Roelofs solar prism and the Horrebow and Striding Levels. Of these, the Roelofs prism is the most interesting. Fitting over the objective end of the theodolite, it produces four overlapping images of the sun, leaving a small diamond-shaped gap in the middle. As the prism has a built-in sun filter, it is possible to sight directly to the center of the sun and to use the reticule cross-hairs to bisect the gap between the four images. This makes sun observations as easy as those to stars and, of course, the subsequent calculations are less complicated than when the sun is sighted by setting the cross-hair to each edge in turn. This involves the reduction of the observations to the sun’s center which, in itself, is a lengthy procedure liable to computing errors. The solar prism eliminates the hardship from, solar observations, improves their accuracy and simplifies the office drudgery previously associated with this type of work. The added attraction that sighting is now direct, instead of slightly eccentric as with earlier models of the prism, makes this little attachment even more attractive.

For simple magnetic orientation a tubular compass can be screwed to the right hand standard. By a ”split bubble” coincidence setting arrangement the two ends of the magnetic needle can be viewed simultaneously but only when the alidade has been rotated so that the telescope is pointing towards the Magnetic North will the two ends be viewed in coincidence. This is a very easy way to establish a single magnetic setting for the horizontal circle but, if it is intended to take independent compass bearings for each target pointing during a round of angles, it is necessary to use a circular compass. This rests on a bridge which fits on the theodolite standards which have to, be modified slightly by the addition of a fixing bracket. This bracket has two holds into which the stud-like legs of the compass bridge fit, the other legs straddling the opposite standard. The circular compass is attached when required and a reading is made each time a target is sighted through the theodolite telescope. With each type of compass attachment, the needle is lowered to its pivot only when released by the observer thus avoiding unnecessary damage to the pivot point. Neither type can be used with an all steel instrument, such as the Wild T2.

Other theodolite attachments which have provided new facilities recently, are the parallel plate micrometer and the autocollimation eyepiece. Similar in design and function to the parallel plate used with precision levels, the theodolite attachment is intended for use in industry for the accurate measurement of small lateral shifts parallel to the line of sight. An in­teresting feature of the new Wild model is that the attachment may be put on in three pre-set grooved positions to measure shifts parallel to the horizontal line of sight, or parallel to the vertical line of sight, or at 45 inclined angle from the horizontal. It is also possible to hand set the inclination at 10 intervals, by reference to a graduated ring.

The autocollimation eyepiece is also intended mainly for industrial use and its applications include many types of alignment and measuring problems. The ordinary bayonet-type eyepiece is removed and replaced by the auto-collimation eyepiece, which is connected to a battery box as a power source for illuminating the reticule. By means of a beam splitter positioned between the eyepieces and the reticule it is possible to view not only the reticule cross itself but also an inverted mirror image of the cross, as reflected from a plane mirror placed in front of the objective.

If the telescope is focussed to infinity and if the plane mirror is at right angles to the line of sight, the real reticule and its inverted, mirror image will unite symmetrically at the focal point in the plane of the reticule plate. This simple device has had a big impact in the interesting field of industrial surveying, where it has solved many problems connected with accurate positioning and alignment. It is also possible to build in a beamsplitter between the objective lens and the reticule plate. A second reticule plate is then fixed to one side of the telescope, parallel to its axis, in such a way that it lies in the focal plane and its image meets the mirror, together with the line of sight of the telescope. The second reticule has a negative cross, i.e. a translucent cross in an opaque setting, and this is illuminated by an outside lamp. The final image produced by this arrangement is of the reflection of the illuminated cross on the original reticule plate cross. The contrast permits easy viewing and facilitates the bringing into coincidence of the two crosses, which is achieved only when the telescope is aligned exactly at right angles to the mirror. This second arrangement is more convenient, especially as there is a complete absence of outside light,, which can be disturbing, but the telescope has to be fitted at the manufacturing stage with two precisely adjusted reticule plates. The first system is easily fitted as it is merely a question of exchanging the bayonet type eyepieces.

For optical distance measurement there are several possibilities, which really narrow down to two principles observation to a vertical staff or to a horizontal staff. ”Normal” tacheometric work, using the stadia hairs of the theodolite, is well known and understood’ by all surveyors and .engineers, and does not call for any description. There are, however, some interesting variations on this theme and they are worth discussing, as this field, is one of great importance.

The distance measuring wedge (or double image prism) is clamped, to the objective end of the theodolite telescope, with a counterweight at the eyepiece end to restore the balance. This wedge produces a parallactic angle, equivalent to the popular tacheometric multiplication constant of 100. As only part of the objective is covered by the glass wedge, a direct view of the special horizontal staff is obtained, together with the deflected view caused by the wedge. The intercept out on the staff is equivalent to 1/100 the slope distance between instrument and staff. The reading system is simple and, as the rays forming the distance measuring angle are at the same height, they pass through the same atmospheric layers, thus minimizing the effect of differential refraction which is one of the big advantages of any distance measurement to a horizontal staff. At 100 yards an accuracy of 1/10’000 can be expected with such an attachment (about a third of an inch). For traverse work in congested areas, where taping is inconvenient, it has the advantage that distance and angle measure­ments can be made at one setup. It is possible to obtain distances reduced to the horizontal by extending this system to make use of a pair of prisms, which, by rotational movement controlled by the tilt of the telescope; change the deflection angle so that the intercept is reduced to its ”slope distance” value multiplied by the cosine of the angle of slope.

Also working with a horizontal target is the subtense bar method of distance measurement. This is an old method, involving the accurate measurement of the angle subtended at the theodolite by two points on the subtense bar which are spaced at an exactly calibrated interval. Instead of having a fixed parallactic angle giving a constant factor of 100 and measuring the intercept cut by this angle, the subtense bar works more or less in reverse and has a constant intercept (usually 2 meters) and a variable parallactic angle that needs to be measured. One very convenient feature of this old and trusted system is that the result (obtained by converting the ‘angle subtended by the subtense bar into the distance from instrument to bar) is already reduced to the horizontal. Although old in principle there has also been a big face change given to the subtense bar and a modern version will be expected to have the interval between its carefully designed and easily observed aiming marks, rigidly controlled by means of an invar wire kept in tension by a coil spring. It will also have a levelling-up bubble, a sighting device so that the staffman can direct the bar towards the observer and a collimating device enabling the observer to check for himself that the bar is set up correctly. Fitting, with forced centering, into a tribrach it will also be fully interchangeable with the theodolite and other ancillary equipment that might be necessary for the survey in question thus ensuring that every possible means of getting accurate results is used.

The theodolite itself has been redesigned by many manufacturers in order to make it more suitable for optical distance and height measurements. The main trend has been towards the development of self-reducing tacheometers using a vertical staff, although a few makers have produced instruments that use the horizontal staff principle. These double image self-reducing tacheometers have become very popular on the European continent, particularly for use in cadastral survey. The regulations of many survey departments do not allow such instruments to be used for cadastral surveys, although there are many non title tasks that can be performed most efficiently with a double image tacheometer. Similar to the distance measuring wedge attachment already described, a prism system is built in to the telescope. Whilst producing a double image of the special horizontal staff (one direct and one deflected in the relationship of 100 to 1) the prisms can rotate in opposite directions, proportional to the angle of slope of the line of sight. This proportional rotation reduces the staff intercept in such a way that the readings obtained are the required ones namely distances reduced to the horizontal. Vernier settings of the staff graduation images, controlled by a micrometer drum, give readings to centimeters. The precision of these double image tacheometers is of the order of about 1:10’000 at a distance of 100 yards. The Wild RDH model has the added facility of a changeover device enabling height differences to be measured in a similar way.

For the average type of tacheometric survey, however, the high precision obtained from such an instrument rarely justifies the additional weight and the extra organization required in the field. Observations to a horizontal staff are always much more accurate but it is much easier for the staffman to hold a staff vertically by hand than it is to set up a bar horizontally and to direct it so that it is at right angles to the line of sight. For this reason it is the self reducing tacheometer for use with a vertical staff that has become really popular in recent years and, without doubt, this type of instrument has done more to remove everyday drudgery from a normal survey task than any other piece of equipment.

It is unnecessary to embark on the rather lengthy description of classical stadia line tacheometry (the expression ”old-fashioned” is a valid one) but it is easy to appreciate how much easier this work has been made by the development of a modern self-reducing tacheometer. In an instrument such as the Wild RDS the stadia lines have been replaced by flat curves, which are etched on a plate mounted in the diagram housing on the right hand standard of the tacheometer, which is actually a modified version of the T16 theodolite. These curves are designed to take into account the tacheometric formulae - i. e. the staff intercept multiplied by 100 cos2 B and 100 cos B, sin B for the reduced horizontal distance and the vertical interval, respectively. When sighting a vertical staff the telescope field of view shows the image of the staff, which is projected to the diagram curve plate, from which a com­bined image is projected through a system of prisms and lenses onto the reticule plate, which itself has only a vertical ”cross-hair”. The angle at which the telescope is inclined , B, dictates the portion of the diagram curve that is viewed and the spacing is such that the appropriate corrections are always made so that the intercepts give the required, reduced values.

The observer actually sees three curves. The lowest of these is almost straight and is referred to as the Base or Zero Curve. The top curve is also very flat and is known as the Distance Reading Curve. Irrespective of the angle of slope the intercept cut by these two curves represents one hundredth the reduced horizontal distance. Between these curves the Height Reading Curve is seen and against it there is a simple factor. There are actually several of these height curves, each with its own fact or and, as the telescope is tilted progressively steeper, it will be seen that one curve will ”run off” and that another will take its place. The function of these curves is similar to that of the distance curve. If the intercept cut by the base curve and the height curve is multiplied by 100 and again by the simple factor that appears on the curve, the result gives the vertical interval between the instrument axis and the point on the staff read by the zero curve. As the factors are ± 0.1, 0.2, ½ and 1, respectively, it will be realized that this involves virtually no calculation at all and that the field work and office work are already reduced considerably by such a system.

This is not the end of the story, however. As the same result is obtained irrespective of the telescope’s angle of slope it is logical that a further saving can be made by setting the base curve to a convenient staff reading with a foot staff, 5 feet would probably be a suitable value. In this way the two intercept subtractions are simplified and the ”height of target” portion of the final calculation is also made easier. Unlike the alternative methods described for stadia hair tacheometry, only one pointing to the staff is needed, however, and after setting the base curve to a suitable value all that is required in the field is to read the distance and height curves and to book the values, together with height curve factor. The subsequent office work is almost nil. It is reduced even further by the final refinement that makes tacheometry almost foolproof. By setting the base curve. to a value such as 5 feet, the subtraction is simplified but, to eliminate all subtractions completely, a setting to zero would be required. This would normally be impossible and , in fact, undesirable. It is rarely possible to see to the bottom of a vertical staff and the influence of the rapidly changing refractive layers close to the ground would cause serious inaccuracies in the results. A special staff, however, overcomes this problem by having its zero mark three feet above the bottom of the staff and, in addition, it has an extendable leg which enables the main staff to be raised until the zero mark is the same height above ground level as the instrument is above the ground. At each instrument station the observer calls out the height of instrument to the staffman (or staffmen, if additional speed is required) who merely ”jacks up” the staff until the correct height is reached. This is done in a matter of seconds. The extension leg slides along a notched guide rail and is clamped in position at the required value.

Everything is now simplified to the extreme and observations are made under the best possible conditions for vertical staff work, as the lowest measuring ray is to a point on the staff that is about 5 feet above the ground. The sequence, after levelling up the instrument and adjusting the staff height, is to set the base curve to the staff zero mark and to read the distance and height curves and the height factor. This is followed by no subtractions at all! As the base curve is on 0. 0 the other readings are directly equivalent to the ‘intercepts. The vertical interval (obtained as height curve reading x 100 x factor ) is already the required height difference between instrument and target stations, as the height of instrument has already been equated to the height of target. Not only: is the field work reduced to a minimum but the subsequent office work: also disappears almost completely.

This principle has been incorporated in the latest version of what many people misguidedly consider to be an out dated instrument that should be relegated to the status of a museum-piece namely the plane table alidade. Memories of almost back breaking days spent bending over a plane table, cutting in control points and items of detail may be rather painful to the surveyor of only a few years back but most, if not all, of the unpleasantness associated with plane-tabling has now been eliminated.

With a diagram similar to the RDS’s, the RK1 self-reducing alidade enables the observer to measure reduced horizontal distance and the vertical interval in the minimum of time and with virtually no calculations. If the same type of staff with extendable leg is used, the height of staff target can be made equal to the instrument height, thus giving direct height differences between station and target ground levels. The observation is made through a powerful telescope, with the added advantage of a rotatable eyepiece inclined at 45 to the line of sight a major feature in easing the physical discomfort of this type of survey. The mental strain is reduced by the simplicity of reading and the lack of computations. After sighting the target staff, and this is also made easier by means of a slow motion swivel-action knob for the fine-pointing, the readings are made in the same way as with the self-reducing tacheometer and the parallel plotting arm is shifted sideways so that it passes through the plotted position of the instrument station. This arm holds a plotting scale which is then slid in its groove until the value of the measured distance is against the station point. The fine pricker at the distance zero mark is then depressed giving a sharply defined, unique position for the point of detail to which the observation has been made. This dispenses with the former requirement of drawing a line to the point in question and then fixing it either by cutting it in with one’ or two more rays from other parts of the board or by scaling off the distance, if it was known, using a boxwood scale and a pair of dividers (which eventually reduced the paper surface to a series of large and often irregularly shaped holes). To be able to mark each point with a single, fine hole, using one of the interchangeable plotting scales, improves both the speed and accuracy of the work. After fixing the position, the elevation can be written alongside it the sighting, the measurements and the plotting being done more or less at the same time, with the plan building up as the field work progresses. When the target point is inaccessible or beyond the range for normal staff observation the alidade can be used for classical plane-tabling, with the great advantage of having a powerfully magnified image, plus a vertical circle that ‘can be read in the telescope’s field of view. In the Face Right position there are no self-reducing curves but, instead, ordinary fixed interval stadia circles, with a 200 multiplication constant, allowing readings to. be made to a vertical staff far beyond the normal limit.

With the availability of such self reducing alidades, plane tabling has taken on a new lease of life and is finding increasing popularity in large scale work, such as road reconnaissance surveys and the revision or completion of plans. Maps produced by aerial survey methods, using photogrammetric plotting machines, always need a field completion survey to fill in the gaps that. cannot be ”seen” in the stereo models. By taking a paper print from the machine plot, pinning it or taping it to the plane table board and by setting up over one of the many points already plotted, using other plotted detail for orientation, the gaps can be filled in on the spot, with excellent opportunities for the checking of other features. As an educational means the plane table is ideal, as the student can do his work by more or less any of the normal survey methods and can see the results of his efforts building-up graphically on the board. Using a modern alidade he has even greater possibilities of doing good work, thereby making it easier for him to understand the principles of survey techniques.

Considerable time and space has been devoted to a discussion on the old and new methods used for the optical measurement of distances and, of course, there are other devices that have already appeared on the market for use over relatively short ”tacheometric” distances. These include the ”code theodolite” type of instrument designed to give an automatic readout. which the observer does not have to book down in the field.

The idea behind this is to photograph the circles and their positions when pointing to the targets of a special type of subtense bar, As yet these instruments are difficult to justify as their cost, plus very expensive ancillary equipment, is too high and the organizational procedure too elaborate to be considered by the normal individual or even a large department. As the circles are not numbered in a conventional manner but are symbolized (to give the ”code”) the photographs, after developing and printing, have to be fed into a special reader for decoding and the results, in punched tape or card form, must then be processed through an electronic computer for the bearing and distance (and possibly the height difference) to be calculated. The overall expense of the equipment and manpower involved is naturally very high and, as already stated, is beyond the limits of the average surveyor or organization. Additional criticism of this type of equipment is that the surveyor cannot ”talk back” to his ingenious code theodolite and is unable to lay off an angle or distance with it, as there is no way of giving reciprocal instructions. The Breithaupt digital theodolite uses a pick-off device but this is used for horizontal circle readings only, as it would require a second rather bulky cylinder for the vertical readings.

The various descriptions and comments that have been given cover most aspects of the ways in which conventional survey instruments have changed face in recent years. It would be quite wrong, however, to conclude without mentioning other recent developments which have certainly given a new look to survey instrumentation and methods, but which are independent items in their own right. All surveyors are familiar with the electronic distance measuring devices of the Tellurometer and Geodimeter varieties the former a microwave instrument and the latter an electro-optical one. Although the two types of systems differ to the extent that, in one, a radio microwave is transmitted and in the other a light wave, there is similarity in the measuring principles in that both use reflectors (either active or passive) to return the modulated wave from the ”remote” to the ”master” stations and that the distance is obtained by reference to the travelling speed of the wave and the actual time elapsing between the transmission and return of the signal. This time interval is measured in different ways by the various makes of equipment but invariably involves the measurement of phase shift. In the case of the Distomat, a built-in computer converts the interval into a fully digitalised distance which is displayed in a readout window, without any partial results’ to be booked and combined. Experiments are now being made using lasers as a means of providing accurate distance measurements, but as yet, the art has not been mastered sufficiently for consideration as a practical survey method. The ”Tellurometer and Geodimeter” types of equipment however, have long been accepted as survey tools and the many different makes on the market have already contributed a great service to the engineer and the surveyor. Without doubt these instruments will continue to be improved, but they are already at the worthwhile limit of their practical accuracy, as it is impossible to measure the atmospheric conditions with anything like the same precision as the basic distance, with the result that an inadequate correction for refraction is made.

Another new device is the gyro attachment, which ‘is mounted on a special holding bridge which, in turn, is fixed to the standards of an ordinary theodolite. This light-weight attachment has a. small gyro motor, which is suspended from a thin, wire band and which runs at 22’000 r.p.m. The combined action of gravity and the horizontal component of the earth’s rotation pulls the gyro out of its initial random spinning plane in space and creates a reaction which results in the gyro’s spin axis taking up an oscillating position in the meridian plane. By means of cleverly devised observing methods it is possible to find the middle of these oscillation movements and, thereby, to establish the direction of True North. As the gyro is not affected by magnetic anomalies it is possible to use the attachment more or less anywhere (except close to the Poles) and at any time of the day or night. In 20 minutes of working time an orientation Is obtained to the True North, with a m.s.e. within ±30 seconds of arc. This development has great possibilities, especially for use underground in the mining industry.

In conclusion, a brief mention of future trends would not be out of place. Amazing improvements are already being made in other fields where the latest electronic and laser devices are making reading and operational conditions easier and more accurate. Exactly how these ideas and developments will be applied to our conventional survey equipment or to the design of entirely new instruments is difficult to forecast. The time will, come, no doubt, when all surveying will be of a fully-automatic push button, digitalised read out form and the operator will become more of an electronic technician than anything else. For the time being, it is possible to reflect on the fact that modern design has already made great progress in changing the face of survey instruments and that these changes have all been of great benefit to the surveying and engineering professions. At the same time it is comforting to know that whatever further advances are made and however ”automatic” we may become, it is difficult to visualize a peg or other marker being positioned in any other way than by the field man himself, who will continue (we hope) to set up his own equipment himself.

Let us therefore, accept all that the designers have in store for us and content ourselves with the satisfying knowledge that, as always in the past, the intention is not to replace the instrument man but merely to assist him.

 

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