Surveying and Levelling Instruments, Theoretically and Practically Described. / For construction, qualities, selection, preservation, adjustments, and uses; with other apparatus and appliances used by civil engineers and surveyors in the field.

by William Ford Stanley

Fig. 122.—Vernier reading centrally to 3′.

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Fig. 123.

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In Fig. 122 the 45 of the vernier is coincident with a line of the limb, this must, therefore be 45′; and as the index arrow is past 44°, it is 44° 45′. If the vernier had read the division next past the 45, the division being to 3′, this reading would have been 44° + 45′ + 3′ = 44° 48′. The same principles may be applied to any subdivision. Circles are commonly divided by the vernier in various ways to give readings from 5′ to 5″.

Theodolites reading to 30 seconds are usually divided degrees and thirds of degrees on the circle and minutes and halves on the vernier, as illustrated (Fig. 123), the reading in this case being 153 degrees 40 minutes on the circle and 8 minutes 30 seconds on the vernier, giving a total reading of 153° 48′ 30″.

A 20 second reading usually has divisions of 20 minutes on the circles and these are subdivided into minutes and thirds by means of the vernier.