, . , P H O T O 6 1 1 A M M E T I I I A lr .4

L u f t b i l d a u s w e r t u n g • R e s t i t u t i o n des r u e s a d r i e n n e s R e s t i t u t i o n f r o m a i r p h o t o g r a p h s

Brilish Air Surveying Inslrumenls' By Major-General Mac Leod, D. S. O., M.C., Ordnance Survey, Southampton.

The development of air survey in Britain has been influenced by several special factors. The Surveys of Great Britain, oil the scales of 1 : 2500 and six-inches to one mile, were completed in 1895, and the revision of the published maps on these and other scales is now so Elaborately organised, and the technique so highly specialized, that the :introduction of any new technical method is a matter of considerable administrat ive difficulty. Moreover, the present specialized revision technique, though perhaps a little slow, is very cheap; so cheap, that it is: only in a few limited areas that air photographv can compete with it in cost. While therefore air survey technique was "developing", it was not needed in Great Britain for carrying out new surveys on the basis of tr iangulation or other forms of control, but could only be useful for interpolat ing new detail in an existing, and very accurate map, on a very large scale.

Outside Great Bri tain the position was qmte different. The British Colonies are for the most part unmapped, and to a great extent undeveloped. Their revenues are small and none are able to spare any money from their current income for developmental work. A special fund, called The Colonial Development Fund, front which to advance money from the Imperial Exchequer to the ColoniEs for developmental work, was formed a few years ago, bnt surveys have only very recent ly been recognised as coming with{n this category. There has, in consequence, never been any money available for survey work, whether done by air photography or in other ways, nor has there been in the Colonies any demand for maps on those comparatively large scales for which air photo- graphy has hi{herto been best adapted.

The situation may be summed up thus: In Great Britain a good map already existed only requir ing br inging up-to-date, but there was no available tr iangulat ion, the points of the original t r iangulat ion of Great Britain being marked by bur ied stones extremely difficult and costly to recover. In the British Colonies, on the other hand, while there was a pressing need for topographical maps on small scales, no demand existed for large scale maps of any kind, nor was any t r iangulat ion available.

I do not th ink it is correct to say that Brilish surveyors have at any time failed to appreciate the potential value of air photography, but they have had to consider its use, and development, in terms of actual conditions and demands. In Great Britain they have been concerned to devise methods of revising the I :2500 map which would he cheaper than the existing methods, without lowering its accuracy; while in the Colonies they had to t ry and evolve a technique whidt would yield a small scale topographical m a p of reasonable accuracy without having first to establish a complete system of ground "control ' .

This will explain why there has been so little pressure in Great Britain towards the development of automatic plotting machines°

A further obstacle in the development of air surveys in Great Britain has had to be surmounted. At quite an early stage a rul ing was given by the appropriate authorities

In Nunnner 1, 1940, wird eine deutsche Zusammenfassung erscheinen.

146

that tile Royal Air Force and other Service agencies should not mldertake air photo- graphy for survey purposes, either in Great Britain or in the Colonies; it was considered that such work should be !eft to private enterprise. The openings for private enterprise were, however, limited by tile conditions stated, namely lack of demand in Great Britain and lacl¢ of funds in the British Colonies; and although one or two civil firms have managed to build up an organization for taking air photographs, none of them has found it possible to spare any money for research work; for tile same reasons it has been difficult to interest British ins t rument makers in air survey plotting instruments; indeed it is only by a fortunate chance that any British ins t rmnent firm has been induced to consider the question at all.

Some fifteen or sixteen years ago, when I was serving on the Staff of the Ordnance Survey, Dr. Stroud, of the firm of Barr& Stroud, happened to visit the Ordnance Survey Offices, and I took tl~e opportuni ty to show him a mechanical tilt f inder I had constructed and to discuss with him some of the geometrical and other problems connected with the application of air photography to survey work. He was very interested, and on his return to Glasgow discussed my instrument and the problem it attempted to solve, with his par tner , Dr. Bart. Dr. Barr also became keenly interested in the subject and eventual ly undertook the construction and a large share in the design also, of the first plotting ma&ine made for the War Office, afterwards jokingly christened "Big Bertha".

At that time the photographic side of air survey was in a very uncertain state; it was impossible to fot'ecast what sort of air cameras or lenses would be available, and ldotters had to be capable of catering for a great variety of conditions. This complicated the design so mud1 that this part icular ins t rument became too cumbrous to be of any practical use. A certain amount of experimental work was done with it, but its chief influence was perhaps only to emphasize the desirabil i ty of controlling the conditions of exposure in tl~e aircraft in order to simplify the subsequent stages of operation.

This may perhaps account par t ly for the preference British surveyors have always had for air snrvey methods depending on straight strips of overlapping vertical photo- graphs, and &eap simple plotting instruments.

Before dealing with the methods of plotting evolved in Britain it will be convenie~:t to refer first to apparatus for taking the photographs. The first essential for any kind of aerial photographic surveying is a suitable camera, and since it may take an hour or more of flying to carry the camera to tile point at whi& work commences, the camera should allow the greatest possible area to be photographed before the aircraft has to return to its base. Fi lm cameras are therefore preferable to those using glass plates. For the same reasoia the angular field of the lenses should be as wide as possible consistent with the view remaining a true perspective of the ground, but since the plotting of the photographs requires a knowledge of the exact geometrical properties of this perspective the dimensions of the camera and the relationship between the plate and the optical axis of the lens must be precisely determined. In British air survey cameras the fihn is pressed befor exposure against a flat glass plate, rigidly fixed in the focal plane of the lens, and engraved with fiduciary marks - - or in some.cases a fine r6sean - - whi& photograph on the fihn at eadl exposure. Tile lens, camera body and pressure plate form in effect a single unit whi&, after calibration, remains unaltered in dimensions.

The standard British Service Air Camera, known as the F. 24, is a fihn camera taking a ,5 X 5 picture and capable of being' fitted with lenses of six-indl and four- in& focal length. These are similar in design to the Eagle Cameras lnamtfactured by tile Williamson Manufacturing Company, which are also fihu cameras, made in several sizes. The Eagle lV takes a picture 1S X 24cm. The EagleVI takes a 5 X 5 picture, and its optical system is specially designed so that, after calibration, the camera can be operated,

147

P l a t e 1. T h e S e v e n - l e n s C a m e r a

serviced, and mechanically adjusted, without disturbing either the lens or tile pressure plate. There are several other models of Eagle camera, either already on the market, or in process of manufacture, the largest taking a 9 X 9 inch plate.

Lenses.

It is only during tile last six or seven years that lenses specifically designed for air survey have been produced in Britain. The Ross wide angle "Xpres" lens of 7-inch focal length is the first of these deserving of special mention. This lens gives sensibly sharp definition at an aperture of F/4 over a square plate of 7-lad/ side. The maximum distortion measured radial ly from the centre is at 35,6 ° , and is then no more than 0,03 mm. At 35 ° it is zero 1.

Excellent though this lens is, the light traw.'lling along the oblique rays to the margins of the plate is very mu& less than that falling upon the centre. Subsequently therefore the firm of Ross produced another lens in whirl1 the performance in this respect was improved without any appreciable sacrifice of its other attributes. This lens is known as the Xpres E. M.f. (extra marginal i l lumination) lens.

The latest production of the Ross firm is a wide-angle Sarvey lens designed to work at an aper ture F /5 ,5 and having an angular field of more than 90 °.

The field of these new lenses must, I think, be the practical Jimit which can be readied with a single lens. Any further increase in covering power can only be obtained by the use of multi- lens cameras.

1 See the 1935 Report of the Air Survey Committee.

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7,Lens Camera (see plates 1 and 2).

Tile use of mult i- lens cameras or mult i-camera assemblies to extend the field of view is as old as air photography. The British camera of this type is a 7-lens model with one lens dt the centre and six others placed at the corners of a regular hexagon; it is manufactured by Messrs. Barr & Stroud. The focal length is just under two-inches. and the rays entering the six outer lenses are deflected by prisms, fixed immediately in front of the lenses, which bend the rays enter ing the side lenses through an angle of 45 °. In front of the prisms is a yellow light filter which forms a circular glass cover over the whole assembly. The lens is designed to fit the fihn holder and magazine of the s tandard F. 24 camera. The glass pressure plate is, however, replaced by a metal mask which divides up the exposed portion of the film in such a manner that the seven separate pictures project after rectification into an exact square, the rectified picture representing a photograph taken with a camera having a total angular field of 120 °. The rectification is done in a special rectifier in which the six oblique pictures are rectified simultaneously, during" printing, to the scale of the central picture which is magnified about 1¼ times. It is in the rectifier that the British apparatus differs most from continental and American models, for in other multi- lens cameras the rectification is done by rotating each picture in succession into position in the rectifier. The design of the Bar r&St roud rectifier is exceedingly skilful and compact, and although simultaneous rectification necessarily involves a little complication in the lighting

P l a t e 2a. S e v e n - l e n s C a m e r a , Rec t i f i ed p r i n t

149

P l a t e 2b. S e v e n - l e n s C a m e r a , C o n t a c t p r i n t

arrangements, since the amount of light required to pr in t the side pictures is many times that required for the projection of the central picture, it saves so much time that it is considered to be worth while. The enormous saving in photo- graphy and flying time which can be effected by the use of multi- lens cameras can be seen from the following table. . . . .

P l a t e 3. C a m e r a c a l i b r a t i o n a p p a r a t u s

Flying height 15000 ft. t F. 24 camera "Xpres" lens

Side of each picture Area of each picture Number of photographs per thousand square miles Miles of flying per thousand square miles

2,9 miles 8,4 square miles

410 460

7-lens camera

9,8 miles 96 square miles

32 140

Calibration Apparatus for Vertical Aircraft Camera. (Plate 3.)

It consists of three principal parts, the mirror shown on the lower part of the photo- graph, and the microscope shown on the top and a tripod stand. The mirror is placed below the stand, and levelled, the level being checked by means of the small spirit level shown in the front of the photograph, which rests on three toes on the surface of the glass mirror. The camera is then fixed above the mirror with the lens pointing vert ically

150

downwards, and the camera is focussed to infinity. The microscope is then placed over the cross lines on the glass pressure plate of the camera. In the middle of the body tube of the microscope is a clear glass reflector tilted to an angle of 45% which enables the cross lines to be i l luminated by a beam of horizontal light entering an aperture in the tube, shown in the photograph. If the camera is correctly focussed to infinity, the cross lines in the horizontal mirror will be seen s imultaneously with their image, the former will only be on the true vertical axis of the camera when the two coincide. The apparatus, therefore, enables the cross lines to be easily adjusted to their correct position.

Having now dealt briefly with the equipment required for taking an air photo- graph, I can pass on to methods of plotting and the ins t ruments used for this purpose. I have already explained that in Great Britain the normal problem is that of adding detail to a map already complete. The 1:2500 map (to which I refer) does not show contours: consequently it is unnecessary to use equipnlent designed for contouring and height determination. In the methods actually employed, the photographs are takeu as near ly vertical as possible with a lens of about twenty-inch focus at a height calculated to give a contact or negative scale of 1:5000. In pr in t ing the negatives are enlarged to 1:2500, the scale determinat ion being made by project ing the image of the enlarged photograph on to a print of the last edition of the map and securing the best general fit possible by simple enlargement without rectification. The prints thus obtained are examined in a stereoscope and all new detail is inked up in vermilion. The pr in t is then compared with a copy of the latest edition of the six-inch map, on which contours are shown, and the area is divided up for plott ing purposes into a number of small triangles the vertices of which are points of old detail, these points being so selected that each tr iangle lies sensibly in one plane (not necessarily horizontal). The control points thus selected are traced off from the existing map and the photograph adjusted to them in a simple epidiaseope.

The epidiascope u sed consists of a drawing table to the legs of which are fixed, near the floor, a board carried on gimbal axes and i l luminated from one side by a powerful lamp which can be made to throw a beam of light on to any part of the picture. The beam of light is passed through a glass tank of water to intercept heat rays before they can reach and affect the size of the print. Placed vert ical ly above the centre of the photograph is a lens of which the focus can be varied by separation of the components. This lens enables an image of the pr int to be brought to a focus on a glass plate let into the drawing table on which a piece of tracing paper can be placed. The epidiaseope does not provide for more t]lan a limited range of enlargement or reduction, bu t . s i nce the photograph is already approximately on the scale of 1:2500, the range of movement is enough to enable the adjustment to the control to be made. The new detail is then traced off and penned, in, after which the trace is sent to the field for checking on the ground. The procedure is not theoretically exact, but has been found sufficiently accurate for the purpose in view. Simple though it is, however, it has been found that when the cost of photography is added to the cost of plotting, it is only cheaper than the "normal" methods of revision done on the ground in limited areas where much recent bui lding development has taken place.

There is, however, another possible use for air photography in Great Britain which has recently come up for consideration. The t : 2500 map of Great Britain is not, strictly speaking, a national map, but a collection of county maps, that is to say, when the 1 : 2500 survey was started in 1853 it was projected as a number of completely separate series each on its own meridian. There are in :[act 39 such separate series each comprising either a single county, or a group of small counties. For some time past this ar rangement has been found inconvenient , and it has recently been decided to re-arrange the 1 : 2500 survey as a single series, continuous over the whole country, on a single projection. The problem of assimilating the different series, and of getting rid of the discrepancies

151

which exist at some of the junct ions of one series with another, has to be dealt with. The first step, obviously, is to assimilate the tr iangulations, but this has proved very difficult owing to the impossibility of f inding many of the original tr igonometrical stations. It has, in fact, been necessary to make a completely new tr iangulat ion over the whole country with stations, four or five miles apart. This work is now in hand. though it will not be completed for several years.

The next step is to interpolate between these tr igonometrical ly fixed points a large n , m b e r of additional points, one to two miles apart, on which to assimilate the detail of the existing maps, or to plot new detail if required. These additional points would normal ly be fixed by a fourth order tr iangulation, and their accuracy must therefot'e

P l a t e 4.

T h e B a r r & S t r o u d Z. D. 16 S t e r e o s c o p e

be decidedly greater than the accuracy of ordinary detail surveying, even on the 1:2500 scale. It will be observed that the problem here is one of plotting points and not one of drawing detail. It seemed that it was one which might be solved by the "radial line" method of plott ing (known in Great Bri tain as the "Arundel" method), the first stage of whid~ is the fixation, by aerial t r iangulat ion, of selected points along a photographic strip. The ordinary graphical methods of radial line plotting (whidl have been developed in Great Britain with the use of the Barr & Strond Z.D. 15 Stereoscope) could not be expected to give the required accuracy, however careful the drawing, but :it was believed that the radial line method should be susceptible of considerably greater accuracy ' than is obtainable by grapMc methods. The principal theoretical source of error in this method, which is admittedly only an approximation, is the use of the principal point instead of the plumb point as the origin of the intersecting rays for fixing other points. In graphic plotting, however, it is probable that, when the tilts are small, errors in drawing and measuring the basal angles due to inexact identification ot' points in passing from one stereoscopic pair to the next, and errors caused by i r regular expansion and contraction of the fihn base, when taken together, exceed the theoretical errors due to the incorrectness of the under ly ing assumption. Experience in photo-- graphing with the assistance of automatic pilots has encouraged the hope that, with suitable equipment and skilled crews, it should be possible to take strips of photographs with uniform overlaps and with tilts never exceeding one quar ter of a degree, and it

152

Pla te 5a, ~b.

The T h o m p s o n C o m p a r a t o r op t i ca l m a r k e r s

was thought that if errors clue to the other causes can be eliminated, tile accuracy obtainable by air t r iangulat ion need not be greatly inferior to that of third and fourth order ground work.

The Thompson Comparator. (Plates 5 and 6.)

With this object in view a special eomparator, designed by Captain Thompson of the Royal Engineers, has been constructed for the Ordnance Survey by the Cambridge. Ins t rument Company. The ins t rument is a combination of a measuring stereoscope and an accurate coordinate measuring machine. The photographs are carried on two rotating turn- tables each m.ounted separately on a carriage - - called the X carriage - - movable

153

in a direction paral lel to the stereoscope eye-base (this is called the X direction). The stereoscope, which allows for 6 magnification, is mounted on another carriage, called the Y carriage, moving in a direction exact ly perpendicular to the former. The two X carriages call be locked together, but ead~ can be given an independent movement so that a differential or paral lact ic movement of the photographs in the X direction can be given. The instrument does not provide for direct measurement of the basal angles of the air tr iangulation. In order to overcome errors arising from i r regular distortion of the film base, or of prints, the angles are calculated from rectangular co-ordinates measured in the machine with the aid of an accurate non-distort ing r6seau which is inscribed on the pressure plate of the air camera and recorded photographical ly on each negative at exposure. This use of a r6seau is based on the practice of precise astronomical photography and is a characteristic feature of this instrument. The other characteristic features (apart from the mechanical arrangements which are in some respects the most interest ing points in the design) are the method of ensuring exact identification of the points, and the method of changing over from one stereoscopic pair to the next. These are best explained by describing the method of use.

Plate 6.

The T h o m p s o n Comparator ,

s h o w i n g opt ica l m a r k e r s in pos i t ion

154

A pair of photographs having been placed on the turn- tables and centred the tables are rotated to bring the base lines on each photograph into the X direction. The separation of the photographs is then varied unti l the floating mark in the stereoscope appears as though resting on the ground when centred over each principal point in turn. The floating mark is formed by the junct ion of two dissimilar T shaped marks, one in each eye-piece. This type of mark is used to facilitate correspondence setting'. When ihe setting is exact, the two marks fuse and appear as a single cross resting on the ground. Any lack of correspondence in the photographs, or any error in ident ifying the same point on the two photographs of a pair, will immediately be revealed by the mark dividing and one half appearing out of l ine with, or separated from, the other.

The base lines having been set in correspondence the next step is to select the intersected points. It is unnecessary to select a conspicuous point of detail. If is sufficient to place the floating mark in a suitable position and bring it down on to the ground in the view. The fact that the mark appears as a cross in contact mith the ground is evidence that the two consti tuent parts of the mark are correctly aligned on the same point of detail. Where one or other of the photographs is slightly tilted there may be some lack of correspondence at the edges of the photographs even when the base setting is correct. This will be revealed by the splitt ing of the floating mark and has to be el iminated without changing the base setting. The stereoscope is therefore provided with a paral lel plate-micrometer device for enabl ing the images of points seen in the photograph to be moved in the Y direction without :moving the photographs. The

PLAN

Fig . 1

S E C T i 0

point having been identified in this manner its rectangular co-ordinates and those of selected corners of the r6seau with the reading of the paral lel plate micrometer are read off and booked.

The next step is to mark the points in sud~ a manne[' that when one photograph of the pair is removed and the next photograph in the strip is inserted the same points will be exactly identified on the new photograph. This is done by centring over the points special "optical" markers which superimpose on the point the vir tual image of a graticule or cross-hair . The optical marker consists of a small tube carrying a glass plate engraved with a fine cross. Approximately central in the tube is a lens and at

155

the other end of the tube a 45 ° prism. The optical system forms a real image of tile cross at the point M (see f igure 1) and a v i r tual image at M'. The tube is ca r r ied on a short spindle which can be c lamped to the turn- table , and the posit ion of the v i r tua l image can be opt ical ly ad jus ted so that the cross coincides exact ly , in depth as well as direction, with the fused image of the f loating mark, and therefore of th6 point to be identified. When the marke r s have been p laced in posit ion on one photograph, the other can be removed and the next photograph in the s t r ip inserted. The stereoscope is made symmet r ica l to r ight and left about the centra l axis so that af ter removing one photo-

i

1

F i g . 2

g'laph it is only necessary to rotate tile one remaining in the stereoscope through 180 ° and place the new photo- g'~aph on the vacant turn- table . The next stereoscopic pa i r can then be measured upside down. When the new photo- graph has been inserted, the stereoscope is moved unti l the f loating mark coincides with the cross of the marker . The stereoscope is then c lamped and the marke r removed. If the photographs arc exac t ly in correspondence the fused m a r k should appear to rest on the ground. If there is lad< of correspondence it is e l iminated as before by means of the pa ra l l e l plate micrometer . As expla ined above, the fact that the f loating mark appears to rest on the ground, au tomat ica l ly ensures that the correct point has been ident i f ied on the third photograph. This optical method of mark ing avoids any necessi ty of damaging the photographs and is be l ieved to be cons iderab ly more precise than any graphic method.

The optical unit is housed in a separa te casting bol ted to the top of the Y cat'~'iage. The optical t ra in can be seen in f igure 2. Light from the photograph (indicated by the le t ter L) passes first through the para l le l sided pla te dl which is a block of glass 50 mm thick mounted in a f rame enabl ing it to be t i l ted by a micrometer M~. Di rec t ly above the micrometer is the objec t ive line d2 whidl br ings the image of the photographs to a focus on the m a r k at d6. Prisms d3, da, d~, deflect the rays into the eyepiece of the stereoscope and form an erect image.

The Thompson Automat ic Plot ter . (Plates 7 and 8.)

Before descr ibing the at tempts to produce automat ic plot t ing machines in Grea t Britain, it may be permiss ible to recal l that the idea of automat ic plot t ing from photo- graphs was or ig inated by the late Br igad ie r -Genera l F. V. Thompson who, in i907, when a Major in the Royal Engineers, devised an appara tus for automat ic plot t ing from stereoscopic pai rs of photographs taken from ground stations.

Major Thompson was not, of course, the p ioneer of stereoscopic methods of measuring. The credi t of this idea seems to belong to Dr. H. G. Fourcade, a South Africat~, who read a paper to the South Afr ican Philosophical Society in 1901 in which he descr ibed

156

a stereocoinparator of his own invent ion for measuring the distance of a point by means of two photographs taken at the end of a short measured base. "Fourcade's work in this connection has not, 1 think, received fhe recognition it deserves, for he anticipated Professor Pulfrich in this design by more than a year, though it was, of course, Pulfrich who, with the assistance of yon Orel, produced the Stereoantograph and greatly improved upon F.V. Thompson's methods. The first attempts to use air photographs for automatic plotting were influenced by their work and consequently envisaged the use of photo- graphs exposed in an approximately horizontal direction at right angles to the line of flight of the aircraft. The designs of the first air photographic plotters were based on that of the stereoautograph, in which the setting of each photograph was done independent ly from measurements made at the field station. At first the settings had to be made after the computation of a re-section in space from independent ly fixed ground control points visible in the foreground of the photog!'aphs; but, after a time, methods of adjust ing the photographs to the control points by trial were evolved, and unti l quite recently this has been the normal method of working. [n 192~ Fourcade produced a design for a stereogoniometer in which the setting of two photographs in exact

P l a t e 7.

T h e T h o m p s o n P l o t t e r ,

v i e ~ f r o m t h e b a c k

correspondence could be done without any knowledge of the positions of the air cameras and without knowledge of the position of ground points, l ie showed that i[ the gonio- meters were mounted together on a single axis represent ing the air base, tile correspondence setting could be effected by a relat ively simple systematic trial procedure, and that when five suitably placed common points had been set in c(~rr'espondenee, the relative positions of the two plhotographs with respect to one another and to the air base had been found. By relating each photograph of a strip in tu rn to its neighbour and to the in tervening air base, Foureade worked out a complete system of aerial t r iangulat ion which enabled the relative positions, orientations etc., of all the

157

photographs in a suitably flown strip to be accurately determined without any reference to the ground at all. The absolute scale and orientation of the whole system could then be determined By adjust ing the strip, as a whole, to a few ground points appearing anywhere thereon. I have already indicated that the question of aerial t r iangulat ion was one in which British surveyors had been specially interested. Fourcade's ins t rument held out Such possibilities for improvement in this technique that the Air Survey Committce (an official committee on which the different services interested are represented), arranged for two experimental instruments to be made by Messrs.

P l a t e 8

Barr&Stroud f o r ' t h e War Office. Subsequently, D r . Fourcade worked out a design for a plotting attachment to his instrument, and more recently a fully automatic plotting machine, Based on the Fourcade principle, has been designed by Captain E. t7I. Thompson, R. E., in collaboration with Messrs. Barr & Stroud, and has Been constructed By the latter for tile War Office.

The Thompson Plotter consists of three distinct parts: (1) tile goniometers, .represent- ing the air camera at the two ends of the air base; (2) the stereoscopic viewing system, and (3) the plotting mechanism.

The goniometers make use of the Porro-Koppe principle and are mounted on the ends of a rigid Bar representing the air base with five degrees of :freedom, that is to say, each plate can be rotated in its own plane about its centre. This is called the "position- angle" setting (see fig. a) and represents two degrees of freedom. Ea& goniometer can be revolved also in the plane of the Bar about axes perpendicular to this plane and passing through the front node of the camera lens. These are called the "declination movements" and represent a further two degrees of free- dora. Final ly, one goniometer can be rotated relat ively to the other round the bar. This is called the "differential polar rotation" and represents the fifth degree of freedom. These five movements are sufficient for the correspondence or in ternal setting. In addition the whole Bar ear ry ing the two goniometers can Be rotated so that Both gonio- meters revolve together round the bar without al teration of their relative positions. This movement is required for the external setting or ad jus tment of the g r o u n d control.

~ 1 5 ~ S e t t i n g m o v e m e n t s

, ~a o ' , .-, o'

~ ' , : ; Q-' = ' c : = ~ a. ~ ~, ,o

~ i : 8 :_~ . - " -:'8

L~ i L. z : '~.~ i E..o..g p.,~

| ~L....._l~__a[~___~ ~ /~x,~-~-~-~-~-~-~t; Mark ~

" , . , /..-" ! # l "elescopes'" ', ,'

.......... Piotlmg rods ........ i

Base ¢ a e r , a g e . . . . . ~. . . •

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These setting movements are quite independent of the plot t ing me&an i sm and are left und i s tu rbed once the sett ing has been completed.

The stereoscopic viewing System comprises two short telescopes carr ied on gimbal joints which enable them to pivot in all directions round the front node of the camera lens. A short distanee from the lens the col l imating axis of each telescope is deflected

~ ' l ~ GONIOMETER

F CAMERA LENS

GRAIIICULE ~ ~: 3

HEIGHT OF IMAGE

~ S T I N G GLASS ~ R E F L E C T I N ( ~ PRISM

F i g . 4

into the stereoscope by a system of mir rors so a r r anged that in any posit ion of the telescope the visual r ay is eventua l ly brought into prolongat ion of the polar axis, and is then ref lected to the front into the stereoscopic eyepiece by a system of prisms.

The layout of the optical system is shown in f igure~. It provides for two types of plot t ing mark, one s imilar to that used in the Thompson compara tor and in tended for the correspondence setting, and the other a simple dot in tended for plot t ing after the

setting has been completed. The eyepiece also provides for t ransposa l of tile two pictures so as to give a pseudoseopie view, that is to say, a reversa l of the rel ief of the picture. The mir rors M1, Ms, Ma form, with F1, a pa ra l l e log ram in all positions of the plot t ing rods, while the image is kept upr ight by the pr ism P= rota ted by a system of half-speed gear ing dr iven off the polar axis. The marks a r e engraved on glass plates f ixed in the telescope at Fs, and are brought to a focus by the optical system in the focal p lane of the eyepiece.

The plot t ing system consists of a base car r iage which controls tile movement of the plot t ing penci l and on which the base length is set out. The car r iage is movable on rai ls in the X and ]1 directions and of being raised or lowered in the Z direction. To the base carr iage are attached, by slides, the ends of guide rods which represen t the rays from any given point in the view to its two const i tuent images ou the photographs. The guide rods are a lways in prolongat ion of the col l imating axis of the two telescopes and are eoplanar with the air base in al l positions. The movements of the base car r iage are t ransmi t ted to a plot t ing table by sui table gear ing in the usual manner .

The Thompson plot ter is not in tended to be a universa l instrument . It is def ini te ly in tended for photographs taken nea r ly vert ical , a l though in pr inc ip le it could be appl ied

Fig. 5

to any type of picture. The goniometers provide for photographs taken within ten degrees of the ver t ica l only, while the ba r represent ing the air base al lows for a range of horizontal sett ing of six degrees. These may seem ra ther na r row limits but I am incl ined to th ink that for the sys temat ic mapping of a whole country , as opposed to the mapping of small detached propert ies , it wilt a lways be more economical to t ry and control the photographic condit ions and to design plot t ing mechanisms accordingly,

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rather than to design a plotter to allow and correct for random tilts of almost any magnitude. Again it is a well reeognised principle of surveying that the fixing of control should be separated from the plotting detail. For control purposes it is only points that have to be fixed and the accuracy of their fixing must be considerably greater than that required for detail plotting'. Hence, a machine that is accurate enough for "control" work is almost sure to be needlessly accurate and therefore needlessly expensive for detail plotting. It is for this reason that British designers have tried to keep plotting mechanisms as simple as possible. Every redundant movement necessarily adds to the complication and cost of lhe whole, and though occasions may be found when such movements can be .sed, I th ink that it will be found best eventual ly to omit them.

As the Thompson plotter has only recently been completed, I cannot say anything about its actual working but it may be useful if I conclude by indicating some of the dmraeteristie features of its design and their advantages.

The theoretical advantage of the Fourcade principle, on which the design is based, lies, of course, ill the fact that internal and external settings are separated entirely from one another and from the plotting mechanism. I have no doubt at all that this will lead to more rapid and more accurate settings. It leads also to the great me&anical advantage that the observing telescopes can be constrained to lie in the basal plane in all positions. Thanks to this constraint exact stereoscopic correspondence will not depend on the precision of the settings, or movements, of the base carriage. It is not unreason- able to expect that this will ensure high precision of operation ,~.ithout extraordinary care (and therefore high cost) in me&anical construction.

Geodittische A n w e n d u n g der l_ne]~biIdmessung Appl ica t ions g3oddsiques de la Photogramm~tr te adrienne Geodettv(~[ Appl ica t ions o] the a ir yhotogram~metry

Syslemalisthe Fehler bei der Aerolriangulalion ~ Von W. Schermerhorn nnd K. Neumaier.

Mitteilung aus dem Geod~tischen Inst i tut in Delft.

I, Einleitung.

In einer I{eihe yon Publ ikat ionen der letzten Jahre, so in Dr. v. Grubers grund- legender Abhandlung ,,Beitr~ige zur Theorie und Praxis yon Aeropolygonierung und Aeronivellement"L in dem yon W. Schermerhorn verfallten holl~ndischen Landesbericht zum 5. In terna t ionalen Photogrammetrischeu Kongrefl in Prom 1938 ~ und auch in der kiirz- lich erschienenen Dissertat ionsarbeit 3 yon Dr.-Ing. C. C. Wang ,,Der Einflu{~ systematischer Mal]stab- und Konvergenzfehler bet Aerotr iangulat ionen mittels Mehrbildger~ten", wer- den die bet Aerotr iangulat ionen auf t re tenden systematischen Eehler, and da wieder be- sonders die systemaiischen L~ingsneigungsfehler, eingehend behandelt . Es wurde nahezu durchweg die Annahme gemacht, dal] diese Fehler mehr oder weniger in ihrer Wirkung

* A r6sum6 in English will be published in no. 1, 1940~ Un r6sum6 en frangais sera publi6 dans le numbro t0 1940.

1 v. O r u b e r : Aeropolygonierung und Aeronivellement; B.u.L., 1935, S. 127 ft., 167 ft. 5. I n t e r n . K o n g r e l t ; a) B.u.L., 1938, S. t45f[., b) Photogrammetria, 1939, H . l . u . 2.

3 C h i h - C h o W a u g : Der Einfluf~ sysfematischer Mal~stabs- und Konvergenzfehler bet Aerotriaugulationen mittels Mehrbild-Kartierger'~iten. Dissertation der T.H. Berlin.