THE RE-TRIANGULATION OF GREAT BRITAIN-II

ISSN： 1752-2706

Source： Survey Review, Vol.4, Iss.26, 1937-10, pp. : 194-206

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Abstract

Reconnaissance.—It has been possible to draw up a “paper scheme” for most of the primary triangulation by examination of large-scale topographic maps for possible obstructions to the proposed rays (after due allowance for curvature and refraction along the ray); the fact that certain of the proposed lines had been definitely observed in the existing primary or secondary triangulation was of course of material assistance. Since, however, all observations were to be to luminous beacons, requiring a close organization, it would have been unsound to draw up an observing programme on the strength of this paper scheme alone. The omission of a few key rays, subsequently found to be obstructed by timber, whose height cannot be appreciated from maps, or by local features which had grown up since the last triangulation, would have entailed some confusion in the observing programme, the establishment of additional stations for occupation during a later season, and the reoccupation of stations surrounding such additional points; in short, the probability of a year's delay in obtaining sufficient material for adjustment, and a considerable loss of economy. Arrangements were accordingly made for the paper scheme of the English main chain to be verified and amended on the ground by special reconnaissance parties in 1935, when in fact no instruments or beacons were available to commence observing in any case. In the same way, field reconnaissance of the Scottish main chain and of the Western figure (Wales and the S.W. peninsula) was completed in 1936 and a start made on the Eastern figure (East Anglia and S.E. England), so as to get the reconnaissance and station preparation well ahead of observing. Experimental reconnnaissance of a few secondary blocks—for which, as also for the primary reconnaissance of the flat enclosed East Anglian country, paper schemes are practically useless—was also commenced in 1936 and is being pushed ahead rapidly in 1937.