AERIAL RECONNAISSANCE FOR TRIANGULATION

Abstract

Since 1929 much of the primary triangulation carried out by the Geodetic Service of Canada has been preceded by an aerial reconnaissance of the areas: during this reconnaissance a tentative selection is made of station sites, and likely lines of sight are indicated. Varied types of topography have been covered—mountainous, rolling, flat wooded, mountain valleys. In most cases there were three common factors: the areas were well watered with lakes and rivers which permitted low flying in safety with pontoonor skii-equipped planes, ground transportation was difficult, and no contour maps existed. In some of the areas existing maps were very incomplete; of a few, reconnaissance aerial maps were available in which the planimetry was good; the better the map the easier the aerial reconnaissance. In all cases it was considered necessary that the air operations be checked by ground visits to the tentatively selected stations.     

GPS精密单点定位在空中三角测量中的应用

结合一例带有试验性质的GPS辅助空中三角测量的工程实践,从不同的角度和数据处理方案,分析了GPS精密单点定位结果,结果表明采用GPS精密单点定位辅助空中三角测量的方案不仅可行,而且不需要地面基准站支持,提高了摄影测量效率并可降低测量成本.     

AERIAL TRIANGULATION FOR CONTROL PROVISION

The author has been involved in aerial triangulation, in various capacities, for some 27 years. This period has seen some very significant changes worthy of remark. This essay is a general review of that period; it does not go into detail and no formulae are quoted. The idea has been to look at the more notable developments, skipping the earlier and well documented period in favour of more recent times, particularly as regards the references. The objective will have been achieved if it provides a starting point, indicating further reading, for those who wish to study the subject in greater depth.     

THE YELL AERIAL TRIANGULATION EXPERIMENT

The paper considers the experimental comparison of a method of aerial triangulation in previous experimental use by Eden and the standard Ordnance Survey method using stereocomparators. The accuracy of plan fixations was high by both methods, with Eden's method providing the better result. A comparison is made of the times taken under experimental conditions for both methods.     

LOCAL ERRORS IN RELATION TO ANALYTICAL METHODS OF AIR TRIANGULATION

Abstract

In view of the limitations of the use of plotting machines in air triangulation a detailed investigation of the practical possibilities of the analytical methods has become necessary, especially now that inexpensive means of reducing the labour of calculation is in course of realisation.

In analytical methods it is practically possible to correct the measured photographic coordinates for determinable errors. Also, the use of a reseau as developed by the Ordnance Survey facilitates the reduction of several distortions. It is however argued and demonstrated that some errors of a local character remain without effective checking.

A discussion of the nature of local errors shows that the number of points usually employed for relative setting is too small to be effective in disclosing them. An increase of the number of observed points is therefore suggested, and should be expected to improve the accuracy in the meanwhile.

An assessment of the value of doubling the number of minor control points and of the improvement resulting from the use of reseau is made from a statistical analysis of data kindly provided by the Ordnance Survey.

Comparison of 89 overlaps carrying one centimetre reseau and 100 overlaps without the reseau shows that the use of reseau reduced the root mean square error of evaluation of want of correspondence by 13 per cent., which just passes the 10 per cent. point of significance. The use of double minor control on the reseau overlaps gave an improvement which is about equal to that due to the use of the reseau. Doubling the number of minor control points is shown to increase the effective number of degrees of freedom from one to an average of four, which should reduce effectively the chances of large errors passing unnoticed through the stages of relative setting.

Work on the frequency of local errors of various sources is in hand, and will probably be published in later issues of the Review.     

ANALYTICAL AERIAL TRIANGULATION WITH ANGULAR MEASUREMENTS

In recent years interest has been increasing in the use of measurements from single photographs for analytical work. A monocular instrument based on the theodolite is described, together with preliminary test results.