Types of Surveys
Once a decision has been taken
during preliminary investigations about the general feasibility and
desirability of a railway line, surveys are undertaken before the construction
of the new line. The following types of surveys are normally conducted:
details of these surveys are discussed in the following sections.
This survey consists of a rapid and rough investigation of the
area with a view to determine the technical feasibility of the proposal as well
as the rough cost of one or more alternatives to the new line. The
reconnaissance survey (RECCE) is normally based on contoured survey maps and
other data already available without carrying out detailed investigations in
the field. With the help of the maps, different alternative alignments of the
new line are studied.
general topography of the country is studied by the survey team and then field
data are collected.
1 Survey Instruments
The reconnaissance survey is
mostly conducted using survey instruments that rapidly measure approximate
distances and heights. The survey instruments used are the following:
To get magnetic bearings of the proposed alignment.
Aneroid barometer To ensure
relative heights of various points.
Abney level or hand level or
clinometer To measure the gradients or angles of slopes.
Binocular To view the physical features.
Pedometer To get an idea of the total
length traversed while walking.
2 Modern Surveying Instruments and
Modern surveying instruments make
extensive use of infrared beams, laser beams, as well as computers. Using these
instruments, it is possible to carry out fairly accurate surveying efficiently
at all times, eliminating human error.
Electromagnetic Distance Measurement
EDM instruments rapidly and
automatically measure both horizontal and vertical distances. The readings can
be displayed on built-in computer screens. Examples of such instruments are the
geodimeter and the tellurimeter, which have been used in the past for
electronic distance measurement of up to 80 km during day or night. Modern EDM
instruments are much more advance and versatile.
Use of Computers
The results of the field survey
are recorded in the form of angles and distances in the normal field book or
electronic notebook. Using computers, it is possible to do all calculations as
well as plot accurately. Thus, output from the EDM can be fed into the
computer, which in turn can plot plans and sections.
Use of Laser in Surveying
Laser is an acronym for light
amplification by stimulated emission of radiation. Its property of low
diversion is used for alignment purposes. The invisible line of sight in
ordinary survey instruments is replaced by the bright red beam of the Laser.
This beam is intercepted by the target composed of light-sensitive cells
connected to the display panel. Its most important aspect is that the beam is
in a perfect straight line. Distances up to 70 km can be measured using laser.
For short distances infrared beams are used.
3 Field Data
The following field data are collected during the
topography of the country
heights of the different points falling on the alignment
of rivers, streams, and some hydrological details of the same
of roads and highways
(e) Nature of
soil at different places
location of various station sites
points on the alignment, through which the railway line must pass
4 Project Report
for Reconnaissance Survey
Based on the above data, a report should be prepared by the
engineer in charge of the project bringing out clearly from the financial point
of view whether or not the prospects of the line surveyed are such as to make
it worthwhile to undertake further investigations to construct the line. The
project report should be accompanied by an abstract estimate of the cost of the
The report and estimate should be accompanied by a
map of the area on a scale of 20 km to 1 cm and an index map of 2.5 km to 1 cm.