It consists of a straight-edge ruler, made of a metal or wood, with one of the edges is bevelled and graduated known as fiducial edge. It consists of two vanes which are perpendicular to its ends, fitted with hinges at their bases, known as sight vanes. These are kept folded down on the ruler when not in use. One of the sight vanes is provided with a narrow slit having three holes. This is used as eye vane. The other, used as object vane, is open and carries a hair or thin wire at its centre. Thus, the line passing through the slit of the eye vane joining the thin wire of the object vane and passing beyond is known as the line of sight of a plane alidade. A string is fitted at the top of the sight vanes and is used for inclined sight. In some alidade, a compass needle as well as a spirit level gets fitted in a box engraved at its base. However, the plain alidade is not very accurate. (Figure 33.2) show a plain alidade.

Figure 33.2 Plain Alidade

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