---
title: "Vertical/horizontal control, stream velocity"
subject: "Surveying II"
chapter: 1
key: "semester_4/surveying_ii:vertical-horizontal-control-stream-velocity"
source: https://www.bctengineer.com/q/semester_4/surveying_ii/vertical-horizontal-control-stream-velocity
site: BCT Engineering
---

# Vertical/horizontal control, stream velocity

Surveying II · Chapter 1: Traversing

- Asked in: 2066 Magh (old course), 8 marks
- Video: [Horizontal and Vertical Control - Setting out Works - Surveying-II](https://www.youtube.com/watch?v=OcaMZ845-Ug) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/surveying_ii/vertical-horizontal-control-stream-velocity
- Whole chapter: https://www.bctengineer.com/md/semester_4/surveying_ii/chapter-1

## Question

Define vertical and horizontal control. Describe the process of measurement of velocity and flow of stream.

## Short answer

Horizontal control fixes plan positions by triangulation, traversing or GPS; vertical control fixes elevations above datum by levelling. For stream flow, measure cross-section area from depth intervals, obtain velocity by float or current meter, then discharge Q = sum of strip area times velocity.

## Answer

#### Vertical and horizontal control
- **Horizontal control:** a network of points (stations) whose plan positions, i.e. coordinates, are established accurately by triangulation, traversing or GPS. Details are located from them.
- **Vertical control:** a set of benchmarks and points whose elevations above a datum (mean sea level) are fixed accurately by levelling. Contours, sections and levels are referred to them.

#### Measurement of velocity and flow of a stream
**1. Select a straight reach** of uniform section, and set out a cross-section line across the stream.

**2. Cross-sectional area:** measure the width with a tape or by ranging, take depths (by sounding rod, echo sounder or levelling) at regular intervals $b$ across the section, and find the area by the trapezoidal rule:

$$
A=\sum \frac{(d_i+d_{i+1})}{2}\,b
$$

**3. Velocity** by one of these methods:
- **Float method:** use a surface float (a bottle or an orange). Mark a length $L$ of the reach (about 30 m) and time $t$ the float takes to cover it. Surface velocity $v_s=L/t$ (mean of several trials). The mean velocity is $v_m=k\,v_s$ with $k\approx0.8$ to $0.9$.
- **Current meter:** hold the meter at 0.6 of the depth below the surface (which gives the mean velocity), or take readings at 0.2 and 0.8 of the depth and average them. The velocity comes from the meter's rating equation $v=a\,N+b$, where $N$ is the revolutions per second.

**4. Discharge:** divide the whole section into strips and add up:

$$
Q=\sum A_i\,v_i
$$

or, for a single section, $Q=A\times v_m$.

Check the work by repeating the observations, and repeat at different times or stages to obtain the stage-discharge relation.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Vertical control means:
   A. Points whose coordinates are fixed by traversing
   B. Benchmarks and points whose elevations above a datum are fixed by levelling
   C. A network of triangles
   D. Points used only for setting out angles
   Answer: B. Vertical control fixes elevations (levelling to benchmarks); horizontal control fixes plan positions by triangulation, traversing or GPS. (Key point: Horizontal vs vertical control)

2. A surface float covers 30 m in 20 s. The mean stream velocity is taken as:
   A. Exactly 1.5 m/s
   B. About 0.8 to 0.9 times 1.5 m/s
   C. 1.5 x 1.2 m/s
   D. 30 x 20 m/s
   Answer: B. Surface velocity = L/t = 1.5 m/s; mean velocity = k x surface velocity with k about 0.8 to 0.9. (Key point: Float method)

3. With a current meter, mean velocity is obtained by holding it at:
   A. The surface only
   B. The bed
   C. 0.6 depth, or averaging readings at 0.2 and 0.8 depth
   D. 0.5 of width
   Answer: C. Velocity at 0.6 depth (from the surface) or the average of 0.2 and 0.8 depths approximates the mean velocity in the vertical. (Key point: Current meter depth)

4. Discharge of the stream is computed as:
   A. Q = width x depth only
   B. Q = velocity / area
   C. Q = sum of depths
   D. Q = area x mean velocity (summed over strips)
   Answer: D. Q = A x v_m, or sum of A_i v_i when the section is divided into strips. (Key point: Discharge)

## Related in Surveying II

- [Horizontal control specification for major traverse](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atraverse-horizontal-control-specification) — Surveying II, chapter 1
- [Optical distance principle, tangential horizontal distance](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aoptical-principle-tangential-horizontal-distance) — Surveying II, chapter 2
- [Methods of tacheometry, tangential horizontal distance](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Atacheometry-methods-tangential-horizontal-distance) — Surveying II, chapter 2

## Related in other subjects

- [Methods of stream flow measurement](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aflow-measurement-methods-micro-hydro) — Energy Resources, chapter 5
- [Stream flow from 0.2d and 0.8d measurements](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Astream-flow-measurement-data) — Hydro Power, chapter 2
- [Stream function and velocity potential problem](https://www.bctengineer.com/question.md?key=semester_4%2Ffluid_mechanics_mechanical%3Astream-function-potential-check) — Fluid Mechanics, chapter 3
- [Rod and spring, angular velocity at vertical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_mechanics_dynamics%3Arod-spring-pivot-15kg-explain-impulse) — Applied Mechanics (Dynamics), chapter 6