THE DOUBLE IMAGE TELEMETER

Abstract

With the double image telemeter distances are measured optically by placing a prism in front of a portion of the object glass, so that two overlapping images are obtained. The image seen through the prism has moved through a fixed angle determined by the prism, and if a graduated staff is sighted the linear shift of image can be measured by reading the position of the displaced zero against the staffimage as seen through the uncovered portion of the objective. The distance from staff to instrument can then be calculated from the linear image shift and the deflection angle produced by the prism by the usual formulae of tacheometry.     

THE GEODIMETER: AN INSTRUMENT FOR THE ACCURATE MEASUREMENT OF DISTANCES BY HIGH-FREQUENCY LIGHT VARIATIONS

Abstract

The constant K in equation (12) represents distance expended through time lags in the instrument itself, and, although the value of K can be calculated from electrical data, this would not be very satisfactory and it would be better to determine it directly by means of observations over a line of known length. In addition, the point from which K would be reckoned is not a convenient one for actual field measurements. Instead of this, it is more convenient to choose an index mark on the instrument itself and referall measurements to this and thence to the mark over which the instrument is set up.     

INFLUENCE OF GEOIDAL TILT ON AZIMUTH

Abstract

If the horizontal circle of a theodolite is tilted, the azimuth of a point in the field will be affected. The tilt may be a mere question of dislevelment but it may be also due to a deviation of the vertical which the level on the instrument cannot measure. In either case the effect on azimuth or bearing is of the same nature, which is worth consideration for more than one reason. That it has previously been investigated elsewhere is no valid reason against renewed consideration here, particularly as the consequences are probably unknown to some readers.     

The use of atmospheric dispersion in optical distance measurement

The development of lasers, new electro-optic light modulation methods, and improved electronic techniques have made possible significant improvements in the range and accuracy of optical distance measurements, thus providing not only improved geodetic tools but also useful techniques for the study of other geophysical, meteorological, and astronomical problems. One of the main limitations, at present, to the accuracy of geodetic measurements is the uncertainty in the average propagation velocity of the radiation due to inhomogeneity of the atmosphere. Accuracies of a few parts in ten million or even better now appear feasible, however, through the use of the dispersion method, in which simultaneous measurements of optical path length at two widely separated wavelengths are used to determine the average refractive index over the path and hence the true geodetic distance. The design of a new instrument based on this method, which utilizes wavelengths of6328 ? and3681 ? and3 GHz polarization modulation of the light, is summarized. Preliminary measurements over a5.3 km path with this instrument have demonstrated a sensitivity of3×10 ⁻⁹ in detecting changes in optical path length for either wavelength using1-second averaging, and a standard deviation of3×10 ⁻⁷ in corrected length. The principal remaining sources of error are summarized, as is progress in other laboratories using the dispersion method or other approaches to the problem of refractivity correction.     

COMPUTATION OF DISTANCES OF LONG ARCS FOR RADIO PURPOSES

Abstract

When we require the distance between two points on a spheroid, there are at least six different lines between the points which might be taken.     

THE ANTI-SHIMMER LEVEL AND LEVELLING STAFF

Abstract

The object of the instrument and special staves is to enable levelling observations to be carried on in conditions of heat-shimmer which would hinder or prevent work by the ordinary methods. The instrument can, however, be used as an ordinary level in normal conditions. The staves for use with the instrument are marked with main bull's-eyes at intervals of 10 cm. and smaller intermediate bull's-eyes at the half-intervals (5cm. from each main bull's-eye). These intervals can be replaced by English units, if desired, and the reverse faces can also be marked with ordinary graduations for use in clear and steady conditions.     

THE YEARS 1935–6 AND 1936–7 IN THE GOLD COAST

Abstract

THE Gold Coast can now be said to have recovered from the slump which caused such disastrous consequences in 1931–2 and resulted in a heavy reduction in the establishment of the Survey Department. So far there has been no increase in staff, but more badly needed money for buying stores and increasing the labour staff has been available, and authority has now been given for an increase of 12 in the establishment of African Surveyors; this will result in the re-opening of the Survey School in 1937.     

THE EAST AFRICAN ARC: IV. BASE MEASUREMENT

Abstract

The gear used for the measurement of the Kate Base of the East African Arc was specially designed by McCaw in conjunction with Messrs. Cooke, Troughton, & Simms, as a result of considerable experience in the measurement of geodetic bases in Fiji, Sweden, and in various parts of Africa; it is now known by the trade name of the “Macca” Base Measurement Gear. It has since been used in Southern Rhodesia, Tanganyika, Mauritius, and South Africa, and if the central pool of instruments recommended at the recent Empire Surveyors Conference is approved, it seems likely that its beat will soon be still more extended. It must be about the only first model of a surveying instrument which has not occasioned widespread controversy and complaint, and for this desirable state of affairs it is not unreasonable to suppose that the rare co-operation between the practical surveyor and the instrument-maker in its design is responsible.     

The determination of potential difference by the joint application of measured and synthetical gravity data: a case study in Hungary

In an elementary approach every geometrical height difference between the staff points of a levelling line should have a corresponding average g value for the determination of potential difference in the Earth’s gravity field. In practice this condition requires as many gravity data as the number of staff points if linear variation of g is assumed between them. Because of the expensive fieldwork, the necessary data should be supplied from different sources. This study proposes an alternative solution, which is proved at a test bed located in the Mecsek Mountains, Southwest Hungary, where a detailed gravity survey, as dense as the staff point density (~1 point/34 m), is available along a 4.3-km-long levelling line. In the first part of the paper the effect of point density of gravity data on the accuracy of potential difference is investigated. The average g value is simply derived from two neighbouring g measurements along the levelling line, which are incrementally decimated in the consecutive turns of processing. The results show that the error of the potential difference between the endpoints of the line exceeds 0.1 mm in terms of length unit if the sampling distance is greater than 2 km. Thereafter, a suitable method for the densification of the decimated g measurements is provided. It is based on forward gravity modelling utilising a high-resolution digital terrain model, the normal gravity and the complete Bouguer anomalies. The test shows that the error is only in the order of 10⁻³mm even if the sampling distance of g measurements is 4 km. As a component of the error sources of levelling, the ambiguity of the levelled height difference which is the Euclidean distance between the inclined equipotential surfaces is also investigated. Although its effect accumulated along the test line is almost zero, it reaches 0.15 mm in a 1-km-long intermediate section of the line.     

High tilt susceptibility of the Scintrex CG-5 relative gravimeters

We report on the susceptibility of the Scintrex CG-5 relative gravimeters to tilting, that is the tendency of the instrument of providing incorrect readings after being tilted (even by small angles) for a moderate period of time. Tilting of the instrument can occur when in transit between sites usually on the backseat of a car even using the specially designed transport case. Based on a series of experiments with different instruments, we demonstrate that the readings may be offset by tens of $\upmu $ Gal. In addition, it may take hours before the first reliable readings can be taken, with the actual time depending on how long the instrument had been tilted. This sensitivity to tilt in combination with the long time required for the instrument to provide reliable readings has not yet been reported in the literature and is not addressed adequately in the Scintrex CG-5 user manual. In particular, the inadequate instrument state cannot easily be detected by checking the readings during the observation or by reviewing the final data before leaving a site, precautions suggested by Scintrex Ltd. In regional surveys with car transportation over periods of tens of minutes to hours, the gravity measurements can be degraded by some 10  $\upmu $ Gal. To obtain high-quality results in line with the CG-5 specifications, the gravimeters must remain in upright position to within a few degrees during transits. This requirement may often be unrealistic during field observations, particularly when observing in hilly terrain or when walking with the instrument in a backpack.     

User Desires and Graphics Capability in the Academic Environment

Abstract

In this paper the cartographic grids engraved on two antique instruments from Iran for finding the sacred direction and distance to Mecca are discussed. It appears that these grids can be well explained in terms of the Mecca-centred retro-azimuthal orthographic projection described in 1968 by J. E. Jackson. In this projection the lines of constant latitude reduce to a set of ellipses with their major axes parallel to the equator and the lines of the constant longitudes reduce to a set of non-equidistant straight lines parallel to the north-south direction. It is shown that the curves actually engraved on the instrument conform to this projection and can be fairly easily constructed. This interpretation of the grid on the Iranian instruments stands in contrast with another explanation, recently proposed by King (1999), which is based on medieval Arabic concepts such as the so-called 'methods of the zijes'. Insufficiently accurate workmanship makes it impossible to distinguish between the two explanations through the study of the instruments themselves. The newly gained insight into the projection itself, however, shows that a direct relation between the Iranian maps and Islamic mappings insight knows from the ninth century, as suggested by King, does not exit. Thus, it is concluded that it is as yet completely unknown when and where the very idea behind the Iranian cartographic grid was first conceived, and that the quest for their historical background is still open.     

A PROJECTION METHOD OF MAPPING FROM AIR PHOTOGRAPHS

Abstract

An instrument was previously described in the Transactions of the Royal Society of S. Africa for the setting of pairs of photographs, taken from air stations, in correspondence; that is, in the same relative positions that the camera occupied in space at the moments of exposure. The principle of the instrument may be recalled.     

THE EAST AFRICAN ARC

Abstract

It was suggested some time ago in the Review (E.S.R., vol. ii, no. 9, p. 182) that observing procedure in a ruling triangulation should be made the subject of a discussion at the forthcoming Empire Survey Conference. I hope it will be. We shall perhaps learn why India finds thirty measures necessary, as no doubt they are necessary in India, whereas South Africa and Southern Rhodesia are able to secure much the same degree of accuracy from the same instrument with only eight; why Canada, again with the same instrument, prefers the golden mean of sixteen; why some of us still prefer the measurement of angles to directions vvhile others would insist entirely on the measurement of directions from a “close” R.O. It is only by pooling the experiences gained in diverse circumstances that we can avoid being overborne by our own successes or failures, encountered possibly in very exceptional circumstances which may not recur.     

AERIAL CAMERA CALIBRATION

Abstract

Accidents will happen and the surveyor is therefore sometimes required to do the best he can with photographs taken with an uncalibrated camera which is not available for subsequent calibration. This note outlines a method by which an approximate principal distance can be obtained for high oblique air photographs from points appearing in the photographs.     

THE RANGEFINDER IN TROPICAL TOPOGRAPHICAL SURVEYING

Abstract

In text-books on surveying the rangefinder is usually dismissed in one sentence, which merely remarks on its inaccuracies and places in the category of scientific curios an instrument which should be most valuable to the plane-tabler in certain types of country. It is to combat this effect and to help topographers in their arduous work that these notes have been written. They are written from experience gained on topographical survey, on a scale of 1/125,000, in a tropical dependency where reduction of cost is of primary importance.     

THE TRIANGULATION OF CULTIVATION OVERLOOKED BY HIGH GROUND

Abstract

14. General. Except for one or two located by an auxiliary triangle or by ray and distance, every point is fixed by a fully observed triangle of which the base is a pair of pillars. To justify the larger and more expensive party required for this method a high rate of observation must be maintained. The two observing periods available each day in the northern Sudan are the 3–4 hours starting just before dawn, and the 2–3 hours which end with sunset. Since moveluent in the cultivation is generally slow and on foot or by donkey, the longer morning period is best used for observing the single angles at each of a series of points there. The shorter and hotter afternoon period may then be used for observing the rounds of angles at each of a pair of pillars, which can normally be reached or at least approached by car. Asfar as possible the points observed to from these pillars will be those occupied on either the preceding or following morning, so that the triangles can be closed as soon as possible. Up to nine triangles have been closed in a day.     

ALMUCANTAR POSITION LINES

Abstract

This article discusses the observation equations which may be solved graphically by plotting position lines using the method of zenith distance intercepts, or solved analytically by the method of least squares.

The general observation equation is modified for the particular case in which zenith distance is made equal to assumed co-latitude, thus simplifying the reduction of the observations.

Adaptation of the theory to use with a theodolite is discussed together with the effects of sources of error and the methods which are proposed for their elimination.

A routine of reduction is proposed and an example is given.     

THE LAY-OUT OF THE EAST AFRICAN ARC

Abstract

The only essential difference between geodetic triangulation and any other of the fifteen “orders” of triangulation—which were once proposed, and happily rejected, at an International Conference—is that steps are taken to secure the high degree of accuracy necessary over the large areas to be covered. Some of the steps taken to secure increased accuracy may well be used to insure economy in secondary work, as for instance the use of fewer readings of a large instrument, or the use of luminous signals in conditions of poor visibility; while any surveyor may at any time have to connect his work to a geodetic triangulation, using much the same methods.