--- title: "Tangential tacheometry, total station procedure" subject: "Surveying II" chapter: 2 key: "semester_4/surveying_ii:tangential-tacheometry-total-station-field-procedure" source: https://www.bctengineer.com/q/semester_4/surveying_ii/tangential-tacheometry-total-station-field-procedure site: BCT Engineering --- # Tangential tacheometry, total station procedure Surveying II · Chapter 2: Tacheometry - Asked in: 2073 Magh, 4 marks - Video: [S3 CE- SurveyLab - Tangential Tacheometry](https://www.youtube.com/watch?v=WzMp5DTMM2o) — Shalu Thomas - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/tangential-tacheometry-total-station-field-procedure - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-2 ## Question Describe tangential tachometry. Explain the field procedure of tachometric survey by total station for preparing topo map. ## Short answer Tangential tacheometry finds distance from vertical angles to two targets s apart: D=s/(tanθ1−tanθ2), V=D tanθ2. For a total station topo survey, set up over control, orient to a backsight, measure prism points with feature codes and prism height, then download and process into a DTM and contour map. ## Answer #### Tangential tacheometry In tangential tacheometry the horizontal distance is found from vertical angles only. Two targets (vanes) are fixed on a staff a known distance $s$ apart, and the vertical angles $\theta_1$ (upper vane) and $\theta_2$ (lower vane) to them are measured with a theodolite. For two angles of elevation: $$ D=\frac{s}{\tan\theta_1-\tan\theta_2},\qquad V=D\tan\theta_2 $$ The RL of the staff point is the RL of the axis $+V-h$, where $h$ is the height of the lower vane above the staff point. (For two depressions, $D=s/(\tan\alpha_1-\tan\alpha_2)$; for one elevation and one depression, $D=s/(\tan\theta+\tan\alpha)$.) It is useful when the theodolite has no stadia hairs, or when the distance is too large for the stadia intercept to be read accurately. #### Field procedure of a tacheometric survey by total station for a topographic map 1. **Control:** establish control stations by traverse, with known coordinates and RL (from GPS or an existing control network). 2. **Set up the total station** over a control station, centre and level it, and measure the HI. Enter the job name, station coordinates (N, E, RL), HI and corrections (temperature, pressure, prism constant). 3. **Orient the instrument:** sight a known back-sight station and enter its coordinates (or azimuth), so that the horizontal circle is oriented to grid north. Check by measuring another known point. 4. **Measure details:** a reflector (prism) on a pole of height $h_r$ is held on each point, and $h_r$ is entered in the instrument. Aim the telescope at the prism and press measure; the total station records the horizontal angle, the vertical angle and the slope distance, and computes $N$, $E$ and RL automatically. 5. **Code the points** with feature codes (road, building, tree, stream, spot level), and take all break lines, boundaries and enough spot heights, radiating from the station; check by re-sighting the back-sight before moving. 6. **Move to the next control station** and repeat, tying the surveys together with common points. 7. **Download the data** to a computer, process it in software to make a Digital Terrain Model, interpolate the contours and draw the topographic map with a scale, north arrow and legend. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In tangential tacheometry, distance is obtained from what? A. Vertical angles to two targets a known distance apart on the staff B. The stadia intercept between two hairs C. The angle subtended by a horizontal bar D. A slope distance read from tape Answer: A. Two vanes a known distance s apart are sighted and angles θ1, θ2 measured; D = s/(tanθ1 - tanθ2). (Key point: Principle of tangential tacheometry) 2. Which formula gives the horizontal distance in tangential tacheometry (both angles of elevation)? A. D = s(tanθ1 - tanθ2) B. D = s/(tanθ1 - tanθ2) C. D = s/(tanθ1 + tanθ2) D. D = Ks cos^2θ Answer: B. Heights of the two targets above the axis are D tanθ1 and D tanθ2, whose difference is s. (Key point: Tangential formula) 3. Why is a known back-sight station entered when setting up the total station? A. To set the prism constant B. To measure the height of instrument C. To orient the horizontal circle to grid north so coordinates are correct D. To level the instrument Answer: C. Entering the back-sight coordinates/azimuth fixes the circle orientation, so computed N, E are in the grid system. (Key point: Orientation of total station) 4. What does the total station record/compute for each prism sighting? A. Only the staff intercept B. Only the bearing C. Only the slope distance, N, E and RL are computed later by hand D. Horizontal angle, vertical angle, slope distance, and N, E, RL automatically Answer: D. With HI and pole height entered, the instrument computes coordinates and RL directly. 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