Page 156 - 4670

however, is that it is easily subjected to the influence of topographic
conditions along the survey lines. To obtain distance measurement
with higher accuracy, one has to invest large amount of human and
material resources, to choose and arrange the survey routes, which can

be complicated and costly. Moreover, such distance measurement can
not be carried out if there are rivers, lakes, hillocks or ravines along
the survey lines. With the progress of science and technology, in the

1940s a new type of distance measuring instrument came into being –
the optical-electro distance measuring instrument, which was also the
earliest type of electromagnetic distance measuring (EDM) device.
Later microwave, laser and infrared EDM instruments emerged one

after another.
For very rough reconnaissance surveys or approximate estimates
pacing may be suitable.

For distances over 5 km, GPS satellite methods, which can
measure the vectors between two points accurate to 1 ppm, are usually
more suitable. GPS measurements provide inter-point distances over

any distance and without the need for inter-visibility, but the
procedure is somewhat more complex.

Task 3. Answer the questions to the text.

1. Why do geodesists require to measure distance?
2. What are the four methods for measuring distances?
3. What is the major flaw of graduated tapes and subtense bar

measurements?
4. Can a surveyor use graduated tapes to measure distances in
places where there are rivers, hillocks?

5. What was the earliest type of EDM instruments?
6. When do geodesists use pacing?
7. When are the GPS satellite methods used?

Task 4. Define whether the following statements are true or
false and translate them. Correct the false ones, according to

the text.

1. Distance measurement is required to set out the positon of a
point in construction work.
2. Distances can’t be measured by GPS.
3. Graduated tapes include measuring ropes, tape measures and

steel tapes.

156