In difficult terrains, particularly in hills, modern techniques and survey aids can be utilized for the preliminary survey of railway lines. Some of these survey aids are the following
(a) Satellite imagery (remote sensing data)
(b) Aerial photographs
(c) Topographic maps/contour maps
(d) Digital terrain modelling (DTM)
(e) Photogrammetric plotted sheets
Modern techniques were utilized e.g., for carrying out the survey of a proposed railway line in Kashmir valley from Udhampur to Qazigund. This area consists of difficult terrain, full of mountains and valleys and sensitive to terrorist activities, making fieldwork very difficult.
Satellite imagery provides a bird's eye view of large areas. Such maps are available from the Indian Space Research Organisation and are updated about once a month. Ground conditions can be well appreciated with a combination of satellite images and topographic maps. Using these, two or three promising alternative corridors can be marked. The corridor that satisfies the survey objectives best is then chosen for further analysis. The use of satellite data in the initial stages of planning has been greatest in those areas for which the existing map coverage and support data are inadequate, and where field evaluation is extremely difficult.
Aerial photographs of the entire country are taken once every 3 to 5 years and these are available with the Survey of India. These photographs are used to gather further details of the chosen corridor. A critical examination of the corridor using the photographs helps in finalizing river crossings, tunnel locations, station sites, etc.
Digital Terrain Modelling
Further details of the railway line are worked out with the help of digital terrain modelling and contour maps. Computer aids are used for defining the most economical alignment. Ground stations are then fixed in the form of two mutually visible points about a kilometre apart in a 10-km stretch. All other details for the preliminary survey can be worked out with the help of contour maps, photogrammetric plotted sheets, and other computer aids.