Ebook Topographic Stadia Surveying

Telescope Cross-hairs, Adjustable or Fixed. The telescope to be used in stadia work is equipped with three horizontal cross-hairs. The spacing of these cross-hairs may be either adjustable or fixed. For general use the fixed cross-hairs are preferred by most engineers and surveyors, though they are not without their disadvantages.

When the cross-hairs are adjustable frequent testing of the instrument rating may be necessary.

When the cross-hairs are fixed (this term being here use in the sense of permanent in their relative positions, i.e., non-adjustable), the instrument's rating as reported by its maker should be carefully tested before any surveys requiring the limit of attainable accuracy are undertaken.

Tachymetry. Tachymetry is that branch of surveying which deals with the rapid measurement of distances, as, for instance, the determination of distance from an instrument by sighting with its telescope to a rod.

Telemeter. The term "telemeter" may be applied to any telescope equipped with cross-hairs for measuring distance, or equipped with micrometer screw or other device for accurate determination of the length of rod subtended by an angle of known amplitude.