FIRST-ORDER TRAVERSES IN CEYLON

Abstract

In the last three years about 250 miles of “precise” traverse have been surveyed in this country to provide control for detail surveys. A brief account of the results may be of general interest. The traverses are situated where trig. points are far apart, and the cost of subsidiary triangulation would have been excessive on account of the flat nature of the country.     

RECONNAISSANCE MAPPING FROM TRIMETROGON AIR PHOTOGRAPHS IN THE ANGLO-EGYPTIAN SUDAN

Abstract

Trimetrogon air surveys have been used extensively in the United States and Canada but seem to have had less favour on this side of the Atlantic in spite of the fact that large areas over here were photographed in this way during the last war. The Sudan is a very large country, but not a rich one; and up to the present it has not been able to undertake its own aerial photography. The gift of a very large number of American trimetrogon photographs covering half the country was therefore very welcome, and it seemed worth our while to see what could be done with them in spite of the known disadvantages of this type of photography for anything but small scale mapping. The methods of planimetric mapping from these photographs developed in the United States were expressly designed to be as simple as possible and to be capable of division into a number of easy tasks which could be performed by any educated person after a short course of training. Nevertheless we felt that they could be simplified a great deal more if the problem was approached from a different angle, particularly in view of the very high standard of flying which was apparent in the photographs covering the Sudan.     

BASE MEASUREMENT IN THE ANGLO-EGYPTIAN SUDAN

Abstract

Since 1899 cadastral mapping in the Sudan has been concentrated along the banks of the Nile from the Egyptian frontier to latitude 13°N., in the towns, and in the area of the Gezira, south of Khartoum, where cotton has been developed. These surveys were controlled by theodolite and steel tape either in the form of traverses or rectangulation. The early triangulation was used mainly to control topographical surveys, for it was essential to cover the whole country as rapidly as possible with a series of maps on scale 1: 250,000. As a consequence much of this early triangulation is of a relatively low order of accuracy, indifferently marked on the ground, and unfit for inclusion in a framework for medium and large scale mapping.     

SOME FACTORS IN DETERMINING HEIGHTS FROM AIR PHOTOGRAPHS

Abstract

The graphical methods developed at the War Officeof mapping from strips of vertical photographs are generally widely known. It may be fairly said that they frequently present a reasonably economical method of mapping the planimetry of a country on medium topographical scales when there is a normal third-ordertrigonometrical control, but that so far it has not been possible to provide economically for the accurate depiction of the ground relief. The reason, for this failure is due to the fact that the control required fot contouring is about one fixed height per square inch of map, and the cost of providing this control will normally be of the same order as the cost of mapping by ground methods.     

NOTES ON THE BASE LINES OF THE CEYLON TRIANGULATION

Abstract

The triangulation of Ceylon depends for its scale upon two bases, each about 5½ miles long, situated at Negombo on the West Coast (latitude 7° 10′) and at Batticaloa on the East Coast (latitude 7° 40′). Both bases are in low, flat country; brick towers up to 70 feet high had to be built over the terminals to enable observations to be taken to surrounding points. These lines have recently been re-measured.     

THE ACCURAOY OF ASTRONOMIOAL OBSERVATIONS FOR POSITION

Abstract

The following paper describes some of the results obtained on it, tour of the Katsina and Kano Provinces of Northern Nigeria between November 1953 and February 1954. The purpose of the tour was to obtain latitudes and longitudes by astronomical observations for air photo mapping control in the northern parts of these provinces where triangulation is not available. The area is mostly flat and sandy.     

THE RADIAL LINE ASSUMPTION

Abstract

It is very many years since the Radial Line method was first used in America and England, and, so far from going out of favour, as was predicted by some European surveyors, it is now thriving in the Slotted Template form. The history of the method is an example of the Anglo-Saxon genius for persevering in a practical compromise. Had we been tempted by the voices of the photogrammetric Rhine Maidens to adopt theoretically exact methods to solve all problems, virtually no new mapping would have been carried out during World War II and many thousands of square miles of sparsely populated territory in the U.S.A. and British Commonwealth would still remain unmapped. A much greater mapped area is associated directly or indirectly with such names as Bagley and Hotine than with others more familiar at international conferences.     

Achieving the Impossible: The 1∶500 000 ‘Irrigation Map’ of South Africa, 1935–1937

Abstract

Since 1906, South Africa boasted an excellent geodetic framework, and in 1920 an official Trigonometrical Survey Office (TSO) was established. In spite of these achievements, the country, by the 1930s, still lacked reliable topographical maps. One reason for this was that the secondary and tertiary triangulations of the country were still incomplete; another was that the decision-making process as regards surveys and mapping rested with a variety of statutory bodies instead of just one. In 1934, A. D. Lewis, Director of the Department of Irrigation, committed his department to execute a general topographical survey of the country and produce a topographical map on a scale of 1∶500?000. Lewis’ decision met with much resistance from within the country and abroad, but the project was completed in a record period of 4 years. Published just before the Second World War, the map was of inestimable value to the South African Defense Force. It not only became the standard topographical map of large parts of South Africa for many years, but its contents also figured in the first map series of the TSO.     

THE ORTHOMORPHIC PROJECTION OF THE SPHEROID—IV

Abstract

We now turn to a question which has received much attention of recent years; the possibility of transforming angular and linear field measures to an orthomorphic projection so that the results of a survey may be computed directly in plane Co-ordinates without having to go through the spheroid at all. Initially, orthomorphic projections were introduced into surveying practice for this very object. Over short lines they import so little distortion of angles that minor surveys, whose error of angular measurement is comparable with such distortion, may be reduced in the rectangular co-ordinate system of an orthomorphic projection just as if the earth were flat. But the present application goes far beyond that. We no longer ignore distortions of angles and lengths, but systematically introduce them into the field measures so that work of higher precision and of considerable extent may also be computed and adjusted in plane co-ordinates.     

ON THE LAMBERT CONFORMAL PROJECTION AS APPLIED IN CHINA

Abstract

There are no proper projections for use in geodetic work in a country which has great extensions both in latitude and longitude. For, if a single projection of any kind be applied in such a case, the linear and angular distortions would be so great at the boundary that it is very difficult or even impossible to apply the corrections to them. In order to render it possible for any projection to be applied, the area in question should be divided either into strips bounded by meridians or into zones bounded by parallels. In the former case the Transverse Mercator or Gauss’ projection may be used, while in the latter, the Lambert conformal projection is the most suitable. China is such a country as that mentioned above. It covers an area extending from 16°N. to 53°N. in latitude and of no less than sixty-five degrees in longitude. The problem of choosing a projection for geodetic work depends only on how the area is to be divided. It has been decided by the Central Land Survey of China to adopt the Lambert conformal projection as the basis for the co-ordinate system, and, in order to meet the requirements of geodetic work, the whole country is subdivided into eleven zones bounded by parallels including a spacing of 3½ degrees in latitude-difference. To each of these zones is applied a Lambert projection, properly chosen so as to fit it best. The two standard parallels of the projection are situated at one-seventh of the latitude-difference of the zone from the top and bottom. Thus, the spacing between the standard parallels is 2½ degrees. This gives a maximum value of the scale factor of less than one part in four thousand, thus reducing the distortions of any kind to a reasonable amount. The area between these parallels belongs to the zone proper, while those outside are the overlapping regions with the adjacent ones. All the zones can be extended indefinitely both eastwards and westwards to include the boundaries of the country.     

AIR SURVEY AND THE PHOTOGRAPH

Abstract

As air-photographs are being more and more used for survey purposes, Empire surveyors who have not yet made a thorough study of aerial survey may be interested in a little elementary photographic geometry and its application to map-making. A map may be described as an orthogonal proj ection of the ground upon a horizontal plane, reduced to some convenient scale, and a photograph as a conical projection of the ground upon the focal plane of the camera. If the focal plane is horizontal at the instant of exposure and the ground being photographed is perfectly level, the two projections are exactly similar and the photograph is indeed a map. Unfortunately these conditions which are illustrated in fig. 1 are extremely rarely encountered, and photographs usually need correcting for various distortions.     

LOCAL ATTRACTIONS AND THEIR INFLUENCE ON THE DETERMINATION OF HEIGHTS ABOVE MEAN SEA LEVEL

Abstract

Preliminary Note.—The substance of this article was written in 1921 at the request of Lieut-Col. Wolff, who was then in charge of the Levelling Division of the Ordnance Survey and with whom the author collaborated in writing “The Second Geodetic Levelling of England and Wales, 1912–21” under the direction of Sir Charles Close. It was not intended for publication and was not again considered until 1928, when a discussion by correspondence was started by the Surveyor-General of Ceylon on the subject of hill circuits in levelling. In this discussion the survey authorities in Great Britain, Canada, India, and South Africa took part, but the main theme was the accumulation of error due to the large number of sightings necessary in hilly country and the question whether a common formula for such country and for flat country was justifiable. In his contribution Dr. van der Sterr made a brief allusion to the subject of the present paper and Dr. de Graaff Hunter went into details. His contribution and the following remarks therefore have some arguments in common.     

CADASTRAL AIR SURVEY

Abstract

I Hope Dr Wolff's interesting article will stimulate to action some of those who have not given sufficient attention to air survey as a method of mapping or planning those areas which lend themselves to that method. At the same time, whether so many of us are as conservative as the author appears to think is a matter for doubt, and moreover we do not all look upon air survey as an “innovation”. As Dr Wolff writes from Palestine he might be interested to know that as the second British Officer to take up field survey work in the War in 1915—Major (now Brigadier) Winterbotham being the first—I was one of the earliest in my profession to study, and to assist in the development of, air survey. In 1915 our unit in the 3rd Army was the first to make use of air photographs for the production of a regular series of trench maps. There are consequently few surveyors more interested in the method. I have watched the progress of the science and the work of the Air Survey Committee with continued interest and have used air methods whenever these have been possible or suitable. On my last visit to Paris I tried my hand at plotting with a new instrument at the Service Géographique and was much impressed by the work that was being done.     

POINT IDENTIFICATION ON AIR PHOTOGRAPHS

Abstract

Perhaps the most important requirement in any air survey is that the ground surveyed points used as a basis for controlling the photography should be correctly identified on the photographs. In the writer's experience more difficulties and delays have been caused by misidentifications of these points in the field than perhaps from any other cause, and yet very little has been written on this important subject, and there are no generally recognised methods of point identification. In this article a system of point identification is described which the writer has used with success in certain types of country, and it is hoped that the article may lead to more contributions on the subject.     

THE TRIANGULATION OF CULTIVATION OVERLOOKED BY HIGH GROUND

Abstract

1. In the Sudan, as in Egypt, the greater part of the detailed survey control required for the cadastral mapping of the cultivated land beside the Nile has been in the form of traverse. This land is very flat and partly covered by crops at all times of the year, so that normal triangulation presents many difficulties, while there are many roads and canal banks which form natural routes for traverse. Nevertheless, the traversing is often slow because of the difficulty of carrying accurate measurements and lines of sight across ground made uneven by subsidiary irrigation canals, and in many cases encumbered by houses and date palms or other trees. Moreover a certain amount of triangulation is necessary to tie the surveys of the two banks together and to connect islands to the banks.     

Mapping for Cycle Touring in Britain; Past,Present, and a Possible Future

Abstract

The Lack hitherto of studies of cycle-touring mapping in Britain is noted, and some basic attributes of cycle touring, including speed, daily distance, gradients and wind, are described. Essential and desirable qualities for a cycle-touring map are enumerated, including the need to depict all rural routes available to cyclists; it is concluded that the optimum scale is in the range 1:100 000–1:126 720, and that relief should by contours at an interval not greater than 30 metres. National mapping in the optimum scale range which were published after 1945 are then reviewed, and reasons suggested as to why none has been kept in print. The mapping available for cycle touring in Britain in 2001 is Ordnance Survey (OS) 1:50 000 Landranger series contains all the necessary information, it is too bulky to be suitable; and that most of the other maps published in the optimum scale range have inadequate relief information and all offer very limited geographical cover. The potential competition for the OS 1:50 000 from a national map series in the optimum scale range is acknowledged, and an outline specification for a 1:125 000 series is described, which seeks to minimize that competiton whilst providing all the necessary information for cycle touring.     

PRACTICAL SAG CORRECTION IN CATENARY TAPING

Abstract

When a line is being measured with a steel tape suspended in catenary but standardized on the flat, the correction for sag is usually computed from the weight of the tape. It can, of course, be determined by actual measurements on the ground, if these are made both on the flat and in catenary, but this is not usually done as it is a tedious process in the absence of a proper standardizing base.     

TRAVERSING IN ANTHILL COUNTRY

Abstract

It is laid down in the Survey Rules of Northern Rhodesia that all areas to be alienated under freehold title or long lease must be surveyed by triangulation and that traverse methods must not be employed without the permission of the Commissioner for Surveys. This of course is all very well under normal text-book conditions, but there comes a time when triangulation is neither safe nor economical. Such conditions prevail in the Copperbelt districts of N. Rhodesia, where the bush is thick, the terrain more or less flat and the haze usually considerable. To make things a little more difficult the countryside is littered with anthills, both small and large; some as much as 30 feet high and 100 feet in circumference at the base. Traversing under these conditions can be very interesting and at times definitely backbreaking.     

THE ORTHOMORPHIC PROJECTION OF THE SPHEROID

Abstract

Chesterton did not, of course, intend this gibe to be taken literally. But the more we consider what he would doubtless have called the “Higher Geodetics”, the more we must conclude that there is some literal justification for it. Not only are straight lines straight. A sufficiently short part of a curved line may also be considered straight, provided that it is continuous (i.e. does not contain a sudden break or sharp corner), and provided we are not concerned with a measure of its curvature. Similarly a square mile or so on the curved surface of the conventionally spheroidal earth is to all intents and purposes flat. We shall achieve a considerable simplification, without any approximation, in the treatment of the present subject by getting back to these fundamental glimpses of the obvious, whether the formalists and conformalists accept them or not.     

A SIMPLE FORM OF CATENARY APPARATUS FOR MINOR CONTROL TRAVERSES IN ROUGH COUNTRY

Abstract

In 1945 I was sent down to Yambio in the southern Sudan to carry out a detailed large scale survey of an area scheduled for the headquarters of an extensive development programme known as the Zande Scheme. The area is not far from the Nile-Congo watershed and the ground is very uneven, covered by ant-hills, and clothed in tall grass and thick bush which includes trees nearly a hundred feet high. Surveys of this kind had not been executed, in the southern Sudan previously except in towns. Nearly all the available experience of cadastral survey work in the Sudan,was therefore based on the very open and flat conditions of the north, where chaining along the ground is the natural and best way of measuring distances within the accuracy required. Except for the geodetic base measurement gear there was therefore nothing in the way of catenary apparatus available, though it was obvious that this would probably be the best equipment for the conditions to be expected in the south. However I had been engaged for most of my survey career up to this time on surveys of a more exploratory nature on much smaller scales such as 1/250,000, and I was unwilling to start experimenting until I had gained some experience in large scale work.