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:
(a) Traffic survey
(b) Reconnaissance survey
(c) Preliminary survey
(d) Final location survey
The 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.
The 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:
Prismatic compass 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 Techniques
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
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 reconnaissance survey.
(a) General topography of the country
Approximate heights of the different points falling on the alignment
(c) Positions of rivers, streams, and some hydrological details of the same
(d) Positions of roads and highways
(e) Nature of soil at different places
(f) Rough location of various station sites
(g) Controlling points on the alignment, through which the railway line must pass
(h) Facilities for construction
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 line.
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.