Victorian Institute of Engineers
ESTABLISHED 188 3.)
1937, DECEMBER 15TEi
PROCEEDINGS
A General Meeting of the Institute was held in the Reception Room, Scots Church Hall, Russell Street, Melbourne, on the evening of 15th December, 1937. The President, Mr. F. C. Hall, occupied the chair. Minutes of the meeting held on 17th November were read and confirmed.
MR. ERIC PYKE, A.F.R.Aë.S., A.M.I.E.Aust., delivered a lecture entitled "Photogrammetry," which he amplified with forty illustrations. The lecture aroused much interesting com- ment, and the author was accorded a hearty vote of thanks.
Discussions were adjourned on this paper and upon the paper by Mr. Roy J. Bennie on the Layout of Industrial Buildings and Equipment.
At the conclusion of the meeting the President extended to all members the Best Wishes of the Season.
LECTURE
PHOTOGRAMMETRY.
Abstract of Lecture by Eric Pyke, A.F.R.Aë.S., A.M.I.É.Aust., Member.
Photogrammetry may be defined as the art and practice of making scale drawings and plans from photographs.
The subject is a very intricate one, and, to quote a world- famous American firm of aeronautical surveyors, "modern aerial mapping requires a highly specialised organisation, with experts in the following subjects :-
Flying. General photography.
Aerial photography. Map assembly.
Ground surveying. Mosaic making.
Geodetic computing. Blending.
Photogrammetric plotting Map reproduction & finishing.
machine operation. Topographical draughting.
Multiple lens transformations. Optics."
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It is of course obvious that in one lecture given by one man it would be quite impossible to do justice to such a field of inter- related subjects, and I shall endeavour, by skimming lightly over the range of field and office instruments involved, to convey to you just a suggestion of the immense possibilities of photo- graphic methods of producing accurate maps, plans, and draw- ings, and an idea of their value in the assessment and develop- ment of the resources of one's country.
Photogrammetry is almost entirely unpractised in Australia, and, with the exception of the stereoscope, it is probable that not a single example of the field or office instruments has reached our shores, notwithstanding that photogrammetry has been practised abroad, principally in Europe, for upwards of a quarter of a century.
Before going farther, I should like to express my indebtedness to the following firms (and their representatives) who have placed at my disposal much information, and a stock of illus- trations, from which I have not hesitated to draw for the purposes of this lecture. These firms are :-
Messrs. Zeiss-Aerotopograph, G.m.b.H., Jena, Germany.
Messrs. Henry Wild, A.G., Heerbrugg, Switzerland.
Messrs. Fairchild Aerial Corporation, of U.S.A.,
Messrs. Societâ Anonima Rilevamenti Aerofotogrammetrce, Roma, Italy.
and to the work " Stereoscopic Photography," by A. W. Judge, London.
As said at the beginning, photogrammetry is the making of scale drawings from photographs.
Most of the appliances used in the photogranmietric produc- tion of accurate maps, complete with contour lines and other altitude data, depend on the human power of seeing objects in relief when they are viewed with two eyes. The method which makes use of this principle is called stereophotogrammetry, or, usually, photogrammetry.
Valuable and interesting as simple photography is as a record or as a mnemonic, a little thought will show that a flat photo- graph does not depict a landscape (or a structure) as the eyes perceive it—it depicts the subject as seen by one eye only.
The reason for this failure to record a truthful impression is that the components of the subject possess solidity and depth, and are at different distances from the camera. The physical features have three dimensions and volume, whereas the photo- print has only two dimensions and area.
The stereoscopic power, i.e., the "solid-seeing" ability possessed by human beings, arises from the fact that, when looking at the same object with both eyes, two slightly different images are formed, the left eye seeing a little more of the left side of the object, and the right eye a little more of the right side.
If we take two photographs of the same subject from stations, one of which is displaced laterally from the other, and then, by means of suitable appliances, view the left picture with the left eye, and, simultaneously, the right picture with the right eye, then the two pictures will appear to fuse themselves into a single solid one.
The simplest arrangement, when the photographs are to be
"viewed" only, and not measured, is a camera with two lenses set apart a distance equal to the separation of the human eyes (22 inches or 6 mm. about). This outfit is equivalent to two cameras at the ends of a baseline of 21 in., the operation of the shutters being synchronised. Simple stereograms produced in this way are (or were) viewed in a stereoscope.
But when measurements are to be taken or deduced from the photographs, the lens-separation is increased. In this type of stereoscopic camera the twin cameras are separated by a tubular support, making an appliance which vaguely resembles a small range-finder. The increased separation of the lenses increases the sense of solidity and depth in the resulting pair of photo- graphs.
Te7jr!estrial Photogrammetry, or Camera-surveying on the ground.—Relatively simple camera arrangements, like those just referred to, are limited to short-distance work, up to say 100 métres for a one-métre base, and the appliances are used for such work as photographing buildings, small works, street acci- dents, the faces of criminals, and so on.
For photographic land-surveying on the ground, the procedure is less simple, and the apparatus is correspondingly more refined. From two standpoints separated laterally, we photo- graph the same landscape, moving the camera to the other end of the base for the second exposure. In this case the camera has a single lens, and the base length is obtained by direct measure- ment. The length of the base depends in general on the distance to the farthest point of the terrain to be mapped, and is usually between one fourth and one fifteenth of that distance.
The two ends of the base need not be at the same level, but their difference in height should not exceed say one quarter the base. The optical axis of the camera when at the left-hand
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station is parallel to that at the right-hand one, but these parallel axes may be tilted upwards or downwards, and they may be square-on to the base or swung to right or left, according to the requirements of the ground being photographed, or to the limitations of the mechanical apparatus to be used subsequently in the plotting from the photographs. The important thing to note is that, in all cases, the length of the base and the exact positions of the camera-axes in relation thereto are accurately known.
Herein terrestrial photogrammetry differs essentially from air photogrammetry, in which the camera-positions are only approxi- mately known.
A type of photo-theodolite (combination of very high grade camera and theodolite) was shown on the screen.
Air Photogrammetry.--It will be apparent that, in the case of more or less horizontal photographs taken on the ground, quite unimportant features of the foreground may often loom big enough to cut off entirely the view of distant features. From the air, however, comparatively few features of the landscape remain concealed from view.
When the country is absolutely flat, a vertical photograph from an aeroplane is virtually a map of the district, but in rolling or hilly country a simple photograph contains many distortions. The areas of high level (hilltops, and so on) being closer to the camera, come out at a larger scale, much as do the feet of the gentleman reclining in a steamer chair when photo- graphed by the inexperienced amateur. There are other misleading distortions. From stereoscopic pairs of photographs, however, one can obtain maps which are accurate, not only as to horizontal projection and scale, but also in regard to levels and contours. The principal difficulty is that the position of the camera at each exposure—and therefore the base length—
is only approximately known, and it is necessary to have control- points on the ground, the position and altitude of which are known, and which can be identified in the photographs.
In air surveys the base length is frequently made about one third of the height above the ground being flown over. With vertical photographs under these conditions each photo. overlaps the preceding one by 60%, which permits each plate to be combined with the one ahead (or astern) to make a stereoscopic pair
In covering appreciable areas of ground, the usual practice is to fly along parallel lines so as to yield strips of photos. in which each exposure overlaps about 60%,
while the adjacent strips overlap about 30%. The photo- interval (i.e., the base length) is influenced by the focal length of the lens, the flying height, the speed of the aeroplane, and the desired overlap of the successive exposures. The exposures follow one another automatically at time intervals determined by computation from the above factors. The camera is usually -electrically driven, but, due to slight differences in speed, altitude, and course of the aeroplane, due to atmospheric or other causes, there must be expected some slight unevennesses in the base length from pair to pair
a
photographs, even under good conditions.The lecturer showed illustrations of some types of air cameras.
For some purposes the assembly of the resultant photos. into one grand photograph of the landscape is sufficient, but if any scaling off is to be done, each photo. should be "rectified" before assembling into the "mosaic," which is the name given to the composite assembly.
"Rectification" is carried out in a machine or instrument, such as the Wild-Odencrants rectifier. It will convert a photo- graph of a landscape taken with a camera, of which the optical axis is inclined (though it might be intended to be vertical), into the photograph which would have been obtained had the camera-axis been truly vertical, and had the landscape been its own vertical projection on to a horizontal plane. The rectified photographs can be used for scaling, horizontal distances, and they also fit together much more accurately, forming what is known as a "rectified mosaic."
To extract the maximum amount of information from a stereo- scopic pair of photographs, a stereoscopic (universal) plotting machine offers the most valuable aid. The two best-known makers of such equipment are Zeiss, of Germany, and Wild, of Switzerland. Machines for the same purpose have been made in Italy, France, and Great Britain. Relative merits will not be discussed here.
With a good modern instrument, fitted with all possible attach- ments, it is possible, from one pair, of photographs, to produce, at the same time, one plan, or two, or three plans at different scales, and simultaneously one or more sections of the country with the usual distorted vertical scale if necessary. "Simul- taneously" is perhaps not strictly accurate, but means with the same setting of the photographs. Naturally more than one operator is required in such cases for economy of time.
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The lecturer showed pictures of two leading makes of stereo- plotting machines. One of these is less accurate, and is recom- mended by the makers for producing relatively small-scale maps, of the order of 1 : 25,000 and smaller (say 22 in. per mile).
A relief model, or plastic picture, of the landscape (or struc- ture, or face of the criminal, or whatever it may be) is built up before the eyes of the observer, and by suitable arrangements and attachments it becomes possible mechanically to draw a contoured plan representing to scale the subject matter of the pair of photographs in the machine.
A slide was shown which conveyed a general impression of the .r•
use of a stereoscopic pair of photographs in the plotting machine
for obtaining altitude data. _,.., 3
Accuracy of Measurements.—The following will serve to give 0 an idea of the obtainable accuracy. One should always bear in
mind the purpose for which the job is being undertaken, and C
the scale of the finished drawings. Z
For land maps drawn to a scale of .1 : 10,000 (about 800 feet rr J = 1 inch) the tolerance laid down by the Swiss Government is r
Errors of position (roads, buildings, streams, etc.) , plus or minus three metres. At the scale of the map, this = 0.3 milli.
' métres on the paper, or about one-hundredth of an inch.
Errors of Height—For individual points, plus or minus 1 métre. For contours, métres one plus 3tan A (where A is angle of slope) .
In most cases the results achieved by photogrammetry are well within the tolerance, and the horizontal accuracy is greater than can be drawn at the specified scale.
The lecturer at this stage projected on to the screen a number of "Anaglyphs," prepared from stereoscopic air photos., which were viewed through the red-green spectacles provided.
From these illustrations, although they lose a little in projec- tion, one could probably be able to appreciate how much is revealed in stereoscopic viewing.
Some plotting machines, such as the Zeiss Multiplex, actually use anaglyphic re-creation of the landscape in the plotting process, while other machines, such as the Wild "Autograph"
and the Zeiss "Stereoplanigraph," project in monochrome only.
: t as follows :
Photogrammtietry in Stockbreeding.—A recent application of photogrammetry is in stock improvement. Judges, with their differences of opinion, temperament, and short memories, may come and go, but it is now possible to make accurate contour plans and volume measurements of any kind of beast from photographs. The potential value of this in a nation-wide endeavour to improve its herds will, I think, be obvious.
The beast is photographed with a stereo-camera, and after the negatives are projected and plotted in the stereoplanigraph, we get contour plans of the animal. In some parts of Europe criminals are put through the same process, but probably for different reasons.
. It has also been reported that the Swiss police make stereo photographs of street accidents, to provide accurate material for the subsequent legal processes. Such photographs take but a few moments to make, and the mess can then be cleared away.
The information-content of a number of these photos. is infinitely greater than that of the most carefully prepared chain survey of the scene ; the pictures themselves can be understood by anybody, and there is no room for argument.
Photogrammetry in Forestry.—In addition to plans and visual records, it is now possible actually to measure the timber content of a stand of trees from a pair of photos., and with an accuracy that is scarcely to be believed. In the ease of certain European hardwoods, measurements taken with the Stereoplanigraph, from a pair of photos., taken at 10,000 feet, have given the volume of lumber correct to within 4% of the volume checked on subsequent milling. It is not likely that any estimation made on the ground could approach this accuracy.
The lecture was illustrated by forty projections.
