--- title: "Tacheometry without staff, procedure" subject: "Surveying II" chapter: 2 key: "semester_4/surveying_ii:staffless-tacheometry-procedure" source: https://www.bctengineer.com/q/semester_4/surveying_ii/staffless-tacheometry-procedure site: BCT Engineering --- # Tacheometry without staff, procedure Surveying II · Chapter 2: Tacheometry - Asked in: 2079 Chaitra, 6 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/surveying_ii/staffless-tacheometry-procedure - Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-2 ## Question What is tacheometry surveying? You are preparing a topographic map with using method of tacheometry. In the field you missed the staff anywhere now there are is no staff. Is this possible to survey by using tacheometry? If so, please mention the procedures completely with neat sketch. ## Short answer Yes, tacheometry can be done without a staff using the tangential method with a marked ranging rod (two targets known distance apart), subtense method with a horizontal bar, or reflectorless EDM. Procedure: set up theodolite, hold marked rod vertical, read horizontal bearing and vertical angles to both marks, compute horizontal distance and RL from the angle difference and known target spacing. ## Answer **Tacheometry** is a method of surveying in which horizontal distances and elevations are found from optical measurements made with a tacheometer (a theodolite with stadia hairs) and a graduated staff, without chaining. It is used for contouring, topographic and detail surveys in rough ground. #### Can the survey be done without a staff? Yes The staff only provides a known length to be subtended at the instrument. Any known length, or a reflectorless instrument, can replace it: 1. **Tangential method with a ranging rod:** paint (or tie bands on) two targets on a ranging rod or pole a known distance $s$ apart (for example 1.5 m), and hold the rod vertical on the point. Measure the vertical angles $\theta_1$ (upper mark) and $\theta_2$ (lower mark): $$ D=\frac{s}{\tan\theta_1-\tan\theta_2},\qquad V=D\tan\theta_2 $$ 2. **Subtense method:** hold a ranging rod or tape of known length $l$ horizontal and perpendicular to the line, and measure the horizontal angle $\beta$ it subtends: $D=\frac{l}{2}\cot\frac{\beta}{2}$. 3. **Reflectorless total station:** the distance to the ground is measured directly by EDM, and the angles are read on the circles. ```text Target 2 ● -- | s | known length (ranging rod) Target 1 ● -- O ------------------- D \ theta2 theta1 ``` #### Procedure (tangential method) 1. Set up, centre and level the theodolite on the control station, and measure the height of the instrument. 2. Hold the marked ranging rod vertical at the first detail point, with the lower mark at a measured height $h$ above the ground. 3. Read the horizontal circle (bearing) and the vertical angles to the two marks. 4. Compute $D$ and $V$, and the RL: $\text{RL}=\text{RL of axis}+V-h$. 5. Repeat for all details, check against a known point, and plot each point by its bearing and distance. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Can tacheometry be done without a staff? A. No, the staff is essential B. Yes, any known length such as a ranging rod with two marks can replace it C. Only with a level D. Only if the bearings are known Answer: B. The staff only supplies a known length; a marked ranging rod (tangential) or reflectorless total station works. (Key point: Staff substitute) 2. With a ranging rod marked s apart and vertical angles θ1, θ2 to the marks: A. D = s/(tanθ1 - tanθ2), V = D tanθ2 B. D = s/(tanθ1 + tanθ2), V = D tanθ1 C. D = Ks cos^2θ D. D = s tanθ Answer: A. The two vertical angles give D, and V = D tanθ to the lower mark. (Key point: Tangential with ranging rod) 3. RL of the detail point is: A. RL of axis + V - h (height of lower mark above ground) B. RL of axis - V + h C. RL of axis + h D. RL of station - V Answer: A. Add V to axis level and subtract the height of the mark above the ground. 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