Trigonometry, surveying and navigation by Wentworth G. A. (George Albert) 1835-1906

Author:Wentworth, G. A. (George Albert), 1835-1906
Language: eng
Format: epub
Tags: Trigonometry, Surveying, Navigation
Publisher: Boston, Ginn & company
Published: 1903-03-25T05:00:00+00:00

Fig. 5.

Fig. 6.

Loeal SiftiulNUieei. Before a bearing is recorded^ care should 1)6 exercised that the chain, pins, and other instruments that w(Hild affect the direction of the needle, are removed from the vicinity of the compass. Even after the greatest care in this respect is exercised, the direction of the needle is often affected by iron ore, ferruginoos rocks, etc.

'ELevent Bearings. When the bearing of a line has been taken, the instrument shoul(i be removed to the other end of the line and the reverse bearing taken. The number of degrees should be the same as before, but the letters should be reversed.

To take the bearing of a line one end of which cannot be seen from the other. Bun a random line (§ 4, 3); then (Fig. 4),

whence the angle CAB may be found. This angle combined with the bearing of the random line will give the bearing required.

Another method will be given in § 19.

To measure a horizontal angle by means of the needle. Place the compass over the vertex of the angle, take the bearing of each side separately, and combine these bearings.

To measure angles of elevation. Bring the south end of the compass towards you, place the eye at the lower eye-piece, and with the hand hold a card on the front side of the north sight, so that its top edge will be at right angles to the divided edge and coincide with the zero mark; then, sighting over the top of tlie card, note upon a flag-staff the height cut by the line of sight; move the staff up the elevation, and carry the card along the edge of the sight until the line of sight again cuts tlie same height on the staff; read off the degrees of the tangent scale passed over by the card.

To measure angles of depression. Proceed in the same manner as above, using the eye-piece and tangent scale on the opposite sides of the sights, and reading from the top of the sight.

INSTRUMENTS AND THEIR USES.

.203

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.