Première page Page précédente
Page suivante Dernière page Réduire l’image 120% Agrandir l’image Revenir à la taille normale de l’image Rotation antihoraire 90° Rotation horaire 90° Imprimer la page
Basculer à gauche  Basculer à droite

686

UNITED STATES COAST AND GEODETIC SURVEY.

Now the identification of the doubtful points may be proceeded with. Having seleeted a point on one of tfie photographs, tfie corresponding silk thread is moved to bisect that point, noting the position of the thread with reference to the scale in this position. The other thread is moved to bisect the corresponding graduation mark on the second scale, when this thread will also bisect the corresponding point on the second photograpli.

(/) Platting the intersections of horizontal directions to pictured points.—After enongh pictnres hâve been seleeted to control the cartographie development of a certain area and the identification and marking of corresponding points hâve been completed, projections of ail these points on the horizon lines of the pictures are marked (their abscissæ are measured) and transferred to the straight edge of a strip of paper, including the marking on the paper strip of the principal point of every photograph. Each paper strip bears the same désignation (in red ink) as the picture to

which it belongs.

These strips are now placed up-on the platting sheet on the picture traces to which they belong in such a manner that the principal points of paper strips and picture traces coin eide, and in this position they are securely held to the working sheet by means of small thumb tacks or paper weights.

To plat the horizontal projections of a point, shown and marked on two prints, two fine needles are inserted into the jilatted station points I and II, fig. 62 (of the two prints) and a fine silk thread at-tached to a small paper weight w (by fine rubber band b) is secured to each needle by a loop.

The thread attached to station needle I is now moved over the weighted paper strip (indicating the picture trace on the plan) until it bisects the horizontal projection a' of the picture point a. The weight is now placed upon the working plan, holding this thread (under slight tension) in this position. The second thread, attached to the needle in station II, is placed over the projection a" of the image of the point A, also under tension of the rubber band. The point of intersection of the two threads will be the position on the plan of the point to be platted (= A). After this position of A upon the plan lias been checked, in the same manner, by means of another photograph (thread and paper strip) taken from a third station, III, and containing the image a"f of the point A, its platted position is marked by a dot in red ink and its désignation corresponding with that given on the prints is also affixed.

After a suflicient number of points hâve been platted in this manner by intersections, and after they hâve ail been supplied with the letters or nuinerals given them on the prints, their élévations are determined and also added in red ink. Frequently the désignation of the points by letters or symbols are only added in pencil on the working sheet, to be erased after the élévations of the points hâve been affixed to them in red ink. f

When the strips of paper should overlap each other, as shown in fig. 63, the part CD of the picture trace PQ is marked off on the strip MM lying under PQ, the paper strip PQ is placed in proper position, and the marks on its edge are transferred to the line GP. The strip PQ is now placed uuder MM, the marks on the latter along GP serving the same purpose as those of PQ.

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 96,87 %.

La langue de reconnaissance de l'OCR est le Français.