3. Give a short summary of the text.

UNIT 6

Text B. Trigonometric levelling

/. Read aloud and memorize the following, words;

average(v) - виводити середнє число

coefficient - коефіцієнт

combine (v) - поєднувати(сь)

consist in - полягати в

eliminate - ліквідувати

erect (v) — підніматись, встановлювати

height - висота

//. Expressions for the text comprehension:

atmospheric pressure - атмосферний тиск chief difficulty - головна трудність compute the difference - обчислити рІЗНИ ЩО

horizontal angle - горизонтальний кут

horizontal distance - горизонтальна

відстань

independent reading - незалежний

показник

on the basis of- на основі

ordinary theodolite - звичайний теодоліт

refraction of air - рефракція повітря

simultaneous observations - одночасні

спостереження

instrument - інструмент locality - місцевість record (v) - реєструвати signal (n) - сигнал station - станція uncertainty - невпевненість vary - змінюватись

single reading - одиночний показник

special vertical circle - спеціальне

вертикальне коло

telescope direct - труба з прямим

зображенням

telescope inverted - труба теодоліту з

зворотнім зображенням

the only way - єдиний шлях

triangulation work - тріангуляційна робота

trigonometric levelling - тригонометричне

нівелювання

vertical angle - вертикальний кут

Trigonometric levelling

Trigonometric levelling consists in computing the difference in elevation of two points on the basis of horizontal distance and the vertical angle between them. It is usually combined with the triangulation work, the vertical angles being measured at the same time as the horizontal angles, a vertical angle is measured to some definite point on the signal the height of which above the station was determined when the signal was erected. The height of the instrument above its own station should also be measured and recorded. In a very precise work angles arc measured with a special vertical circle instrument. In a less precise work an ordinary theodolite the vertical arc of which reads to 50 sec. or to 20 sec. may be used but with such instruments only single readings can be made. In this case the best results are obtained by averaging several independent readings half of which are taken with the telescope direct and other half with the telescope inverted.

The chief difficulty in obtaining accurate results by trigonometric levelling is the uncertainty in the coefficient of refraction of the air. This varies with locality, temperature and atmospheric pressure and the only way its effect can be eliminated is by taking simultaneous observations between two stations.

Assignments

1. Comprehension questions;

l.What does trigonometric levelling consist in? 2. With what is trigonometric levelling combined? 3. When are vertical angles measured in trigonometric levelling? 4. How are angles measured in a very precise work? 5. What is the disadvantage of a less precise work in trigonometric levelling? 6. How does the coefficient of air refraction vary? 7. What telescopes

are used in trigonometric levelling?

2. Speak on the procedure of trigonometric levelling.