EVALUATION OF THE COEFFICIENT OF TERRESTRIAL REFRACTION

Abstract

In two previous articles (E.S.R., vol. iv, nos. 23 and 25) it was shown that, at the time of maximum diurnal temperature in the tropics, a definite relationship exists in the lower layers of the atmosphere between the magnitude of the coefficient of terrestrial refraction at a point and the height of that point above plain level, provided the weather is fine and clear. In fact the coefficient K increases with the height h, within certain limits which are probably defined by the condensation layer.     

EVALUATION OF THE COEFFICIENT OF TERRESTRIAL REFRACTION

Abstract

In a previous article on this subject (Empire Survey Review, January 1937) the writer sought to show that for trigonometrical observations of vertical angles made near noon in the Tropics the coefficient of refraction depends chiefly on height above ground level in the case of stations sited within a few hundred feet above the general level of the ground surface. Indeed, the computed values of the coefficient K show a definite and appreciable increase with “h”, the height of the observing station above ground level; it is usually assumed that K decreases with increase in height above the Mean-Sea-Level surface. From analysis of the results obtained by varying h but holding the heights above Mean Sea Level fixed the writer came to the conclusion that the variations in K could only be due to abnormal values of dt/dh and d²t/dh², “t” denoting the air temperature. Now it is generally recognized by meteorologists that abnormal lapse-rates of temperature do frequently occur in the lower air layers in the Tropics; but up to the present time no temperature soundings in Nigeria are available. Recently, however, the writer came across the results of the aerological soundings made by an expedition in East Africa during the year 1908. The results of many of the soundings were of no use for the purpose of this paper; many of the observations were not taken at or near noon, and in others counterlapses of temperature in the lower layers indicated that conditions were not normal. A set of observations taken at Mombasa between 10 and 11 a.m. were eventually chosen as offering an example of what might reasonably occur in the lower layers of the atmosphere.     

THE CALCULATION OF THE HEIGHT OF KILIMANJARO

Abstract

The observations to height Kilimanjaro were made from two ground stations, Domberg (5,081·6 ft.) and Lelatema (5,323.1 ft.) and from a point called Kibo near Kaiser Wilhelm Spitze which is regarded as the highest point on the crater rim. It was originally intended to include a third ground station, Kifaru, but it was discovered that the ice cap obstructed observations between this point and the top.     

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.     

THE PENGGAWAS' ROADS OF KELANTAN

Abstract

The survey of roads in the densely settled areas of the Kelantan plain is a problem the solution of which is interesting, more because it illustrates one of the facets of the surveyor's life than because it is likely to have any general application.     

THE ACCURACY OF TOPOGRAPHICAL MAPS

Abstract

“Accuracy” in relation to maps is a term very frequently and widely used. Nevertheless it is hard to obtain any comprehensive precise definition of the word in this connection.     

NOTES ON THE USE OF ANEROID BAROMETERS IN THE GOLD COAST

Abstract

For several years it had been realized that aneroids in the Gold Coast showed a distinct lag in the readings when subject to fairly large changes of height. The range of height in the Colony, however, being relatively small and control heights fairly numerous, little interest was taken in the cause, which was generally thought to be due to hysteresis. * All aneroids in use on the Gold Coast are graduated on Airy's scale which is based on latitude 45° and temperature 50° F. In 1921 Mr. C. L. T. Griffith, at that time Chief Instructor of the Survey School, carried out various tests with a number of aneroids, and from these tests concluded that the main source of error arose from inappropriate graduation of the height-scale relative to pressure; using as constants latitude 15°, temperature 86° F., and mean humidity 67 per cent., he worked out a proposed general scale for the Tropics. Ten years later the purchase for test purposes of new aneroids graduated to this scale was considered but was eventually postponed when it was learnt that the question of a special scale for use in the Tropics was under consideration at home by a special Committee consisting of representatives of the Admiralty, War Office, Air Ministry, and National Physical Laboratory.     

THE CENTRE OF AFRICA

Abstract

It is not clear th at a knowledge of the position of the “centre” of any area, be it a county or a continent, serves any useful purpose, though it may, perhaps, help to define our ideas as to the shape of the area in question. Such information may take its place amongst those contents of the mind which are rather of the nature of art than of science, and minister harmlessly to our curiosity.     

THE ORDNANCE SURVEY AND THE FINAL REPORT OF THE DEPARTMENTAL COMMITTEE OF 1935

Abstract

The conclusions of this Committee have aroused small interest so far, because war was upon us almost as soon as the publication (1938). They may, however, greatly affect our surveys and maps of the future, and it is time to examine them. First of all, however, it is desirable to see the background against which the Committee's recommendations appear. For that reason the following chronological summary is given. It is divided into three periods, those of Growth, of Hesitation and of War, and it is given to supplement the chronology of the Final Report in matters which are discussed therein.     

THE STEREOGRAPHIC SOLUTION OF THE SPHERICAL TRIANGLE

Abstract

The Stereographic Projection, owing to the ease and accuracy with which it can be drawn on a small scale, offers natural attractiveness for the treatment of spherical geometry upon a plane surface. It would therefore be rash for a present-day writer to claim as novel what may well be an infringement of patent rights morally belonging to Hipparchus, who possibly knew most of what is worth knowing about the matter 2,000 years ago. However, since a fairly extensive delving into writings upon the subject has not brought to light anything quite on the lines here put forward, it may be worth while to systematize in this paper some processes which the present writer has found practically useful for some time past.     

A NOVEL METHOD OF MEASURING THE COEFFICIENT OF THERMAL EXPANSION OF INVAR

Abstract

In a letter published in a recent issue of Nature, Prof. L. F. Bates and Mr J. C. Wilson, of University College, Nottingham, have described a new and novel method of determining the coefficient of thermal expansion of invar. Although this method is hardly likely to be applied to the measurement of the coefficient of expansion of long invar tapes, such as are used by surveyors, yet it is so novel and ingenious in itself that a short reference to it may not be out of place in this Review. One extremely interesting thing about it is that no measurements of a length, or of changes of length, are involved.     

THE LIFE OF A SURVEYOR IN JAMAICA

Abstract

Legend has it that when Don Christopher Columbus was describing Jamaica to his patrons Ferdinand and Isabella, he crumpled a piece of parchment in his hand and placed it upon the table before their Spanish Majesties, saying “That is Jamaica”.     

THE APPLICATION OF THE GAUSS METHOD OF COLLIMATION TO THE ADJUSTMENT OF SURVEY INSTRUMENTS

Abstract

Gauss's method of collimation is usually given scant attention in survey text-books. The following notes are based on experiments carried out at Accra in making a collimation base, the collimator being a Watts 2I-inch self-adjusting level fitted with a parallelplate micrometer.     

DEVIATION OF THE VERTICAL

Abstract

This paper endeavours to put forward good reasons, practical as well as academic, for needing a knowledge of the deviation of the vertical in all countries, and it then describes the instruments and methods used by the writer in his first season on the observation of a geoidal section through Great Britain. Unfortunately there has been no time as yet for the computation of the latter, so this cannot be a full report on it.     

AZIMUTH COMPUTATION IN THE OLD PRINCIPAL TRIANGULATION OF THE BRITISH ISLES

Abstract

The old Principal Triangulation of Great Britain and Ireland is a classic, both in execution and publication. The names of many of those who planned it, made it possible, carried it out or computed it have become legendary in the Ordnance Survey. Such are Roy, Dalby, Ramsden, Mudge, Colby, Drummond, Steele, O'Farrell and, greatest of them all, Clarke.     

THE LEVELLING OF A THEODOLITE

Abstract

A Fully equipped theodolite is provided with plate levels, an alidade level, and a striding level. An instrument not so equipped has no title to be considered a “Universal Instrument”, that is to say, an instrument designed for every kind of both terrestrial and celestial measurement. Without a striding level, for example, nothing beyond relatively rough astronomical measures can be expected in general. Modern instruments, capable of giving considerable refinement in terrestrial measures, are frequently not furnished with a striding level; and it is sometimes assumed, with the tacit approval of the makers, that such instruments are equally capable of giving refined astronomical results. On the older type of instrument a striding level—rarely not supplied—could have been, and sometimes was, extemporized; it seems as if ignorance of astronomy of position has led, at least in part, to the construction of theodolites in such a manner as actually to render such extemporization difficult.     

ADJUSTMENT OF THE SECONDARY TRIANGULATION OF SOUTH AFRICA

Abstract

The Secondary Triangulation of South Africa consists of a uniform network of triangles of from 5- to 10-mile sides, enmeshed in the Geodetic and Primary Triangulations. As a rule the Primary Triangulation is rigorously adjusted by least squares, and the Secondary made to conform to it by an approximately rigorous method which was introduced into the Trigonometrical Survey in 1920 by the late Dr van der Sterr.     

THE ADJUSTMENT OF A FIRST-ORDER LEVELLING NETWORK

Abstract

In the E.S.R. January and April numbers of 1955, Vol. xiii, Nos. 95 and 96, Mr. Hsuan-Loh Su described the “Adjustment of a Level Net by Successive Approximations and by Electrical Analogy”. It does not seem to be as generally known as it should be that the rigid least square solution can be greatly simplified by utilizing the electrical analogy and solving by Kirchhoff's method. The method as detailed below has been in use for over 40 years.     

SIR DAVID GILL AND THE GEODETIC SURVEY OF SOUTH AFRICA AND THE ARC OF THE 30TH MERIDIAN

Abstract

In the memoir of the late Capt. G. T. McCaw which appeared in the January number of this Review (vii, 47,2), reference was made to the part which the late Sir David Gill played in the origin of the work on the survey of the Arc of the 30th Meridian in Africa. This year is the centenary of Gill's birth, as he was born in June 1843, and it is therefore timely to give some account of his work during his long term of office as Her Majesty's Astronomer at the Cape which resulted inthe inception and completion of the Geodetic Survey of South Africa and the survey of the Arc to the southern shores of Lake Tanganyika. He died on 24th January 1914.     

SOME NOTES ON THE ADJUSTMENT OF TRAVERSES

Abstract

It has now become a truism that angles in traverses are observed with a far greater accuracy than the sides, and this applies to all categories of traverse work: precise, transit-and-tape, tacheometric traverses, etc. The result of this is that the effect of angular errors on misclosures in co-ordinates is sometimes so small when compared with that of linear errors that it may be considered as negligible.