vertical circle images into a single field of view.In the DEGREE-GRADUATED SCALES (fig.11-16), the images of both circles are shown asthey would appear through the microscope of the1-min theodolite. Both circles are graduated from0° to 360° with an index graduation for eachdegree on the main scales. This scale’s graduationappears to be superimposed over an auxiliary thatis graduated in minutes to cover a span of 60 min(1°). The position of the degree mark on theauxiliary scale is used as an index to get a directreading in degrees and minutes. If necessary, thesescales can be interpolated to the nearest 0.2 minof arc.The vertical circle reads 0° when thetheodolite’s telescope is pointed at the zenith, and180° when it is pointed straight down. A level linereads 90° in the direct position and 2700 in thereverse. The values read from the vertical circleare referred to as ZENITH DISTANCES and notvertical angles. Figure 11-17 shows how thesezenith distances can be converted into verticalangles.Figure 11-17.-Converting zenith distances into verticalangles (degrees).In the MIL-GRADUATED SCALES (fig.11-18), the images of both circles are shown asthey would appear through the reading micro-scope of the 0.2-mil theodolite. Both circles aregraduated from 0 to 6,400 mils. The main scalesare marked and numbered every 10 mils, with theFigure 11-16.-Degree-graduated scales.Figure 11-18.-Mil-graduated scales.11-23
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