Three-wire stadia field procedure for topo map

In three-wire stadia tacheometry, the top, middle (axial) and bottom hairs are read on a vertical staff. The method gives the distance from the intercept and a check on the readings (middle reading $\approx$ mean of top and bottom).

Field procedure

1. Reconnaissance and control: establish control stations (traverse) over the area; know the RL of a benchmark or station.
2. Set up the tacheometer on a station, centre and level it. Measure the height of the instrument (HI) with a tape.
3. Orient: with the horizontal circle at zero (or a known bearing) sight a known reference station or magnetic north.
4. Back-sight to a benchmark or known point with the staff to find the RL of the instrument axis.
5. Detail points: the staff-man holds the staff vertical at each ground feature (corners, break lines, spot heights). For each point:
   • bisect the staff and read the horizontal circle (bearing),
   • read the top, middle and bottom hair (staff readings),
   • read the vertical angle, with the middle hair at the same height as the HI where possible (this simplifies the calculation) or at any convenient reading,
   • book the readings with a point description.
6. Check that the mean of top and bottom hair equals the middle reading (within 0.005 m), and close on a known point at the end of the day.
7. Computation: $s=$ top $-$ bottom, $D=Ks\cos^2\theta$, $V=\frac{1}{2}Ks\sin2\theta$, RL $=$ RL of axis $\pm V-h$.
8. Plotting: each point is plotted by its bearing and horizontal distance (protractor and scale) or by coordinates, with its RL, and contours are interpolated for the topographic map.

Field book (typical)