Walkover survey is carried out on these routes. Walk over survey means going over the area associated with the routes and collecting features observed other than those existing on the map. In addition the indications on following features are also checked:
- Communication lines
- Power lines
- Expanding villages and towns
- Rich gardens and plantations
- Reserved forests and high tree areas
- National Parks & Wild life sanctuaries
- Archaeological monuments
- Aerodromes, radar centers etc.
- Steep sloping terrain, Areas prone to land slides, soil instability etc.
- Prohibited areas declared under statutory regulations
Preliminary survey: On completion of walkover survey proposal of the most suited route is further studied before taking preliminary survey. The main objective of preliminary survey is to understand the route to the ground with such deviations as may be necessary as per field constraints. It involves generally fixing of important points of the route, identification of major crossings, general classifications of soils, measurement of route length etc. The preliminary survey will include detailed investigations for locations along the route.
Project Execution Stage:
On completion of preliminary survey and approval of Feasibility report, the detailed survey of the route is carried out. The detailed survey consists of accurately determining the number and types of roads along with extensions, special road required, number and types of important crossings, special road required etc. It generally involves the following actions:
- Leveling
- Plotting and profiling on a scale of 1:25,000
- Final and pegging of locations
- Trial pit excavations
- Detailed soil investigations wherever required
The final output of the detailed survey is in the form of tower schedule showing the type of tower with associated angle of deviation, span length, wind and weight spans, associated foundations along with its classifications, geological and geotechnical, data and any crossing involved in that span.
Limitations of Conventional methods of map study/ surveying apart from the considerable time involved, the following limitations are generally associated with conventional methods of surveying:
- The topographical maps used for walkover survey and preliminary survey can be very old and do not contain recent changes in inhabitation pattern, vegetation coverage and water bodies etc.
- The process of elimination is done at the ground itself, which increases the number of angle towers and length of line.
- The surveying staffs do not have bird’s eye view of the present ground condition this may particularly result in large inaccuracies in estimation of civil works (benching and revetment) quantities in terrain involving large undulations.
- There is every possibility of error in recording the ground data and subsequent transfer on the route map
Route Planning and Fixing of MIS can be Achieved Through the Following Procedure:
- Data Input: Survey of India maps, Satellite data, railway maps, land use maps, settlement maps etc are used as input for data base preparation
- Above inputs through GIS are used to update the maps. Other specific maps may also be created.
- Using computer run algorithms or linear programming techniques optimal transmission line route may be finalized.
- On this output from GIS domain post fieldwork is done and location latitude and longitude of the deviation points are identified on ground using Geographical position system (GPS).
Further in terrain having large undulations, stereoscopic satellite imageries may be used selectively to interpolate the contours and digital terrain model can be used to select the route in hilly regions.
Detailed Survey:
Satellite images can be used as a reliable tool to record the precise elevations and plan measurements of the selected route. This may involve taking following actions
- Transferring of Images to studios for processing and detailed measurements
- Authentication through ground verification
It is said that camera never speaks lie. That is precisely the greatest advantage of satellite images in detailed survey application i.e. traditional methods. Further once the ground profile in digital form is obtained the estimation of various types of earthworks involved viz. Benching, revetments etc. can be made using software.
Appropriate techniques for obtaining soil conditions, sub soil conditions, water tables, treacherous terrain conditions etc for accurate estimation of civil works need to be explored.
There may be some error involved in using satellite imagery for preliminary survey due to the “shadow effect”. In the above case it was found sometimes difficult to differentiate between forests and other greenery. Thus exact boundaries of forests could not be demarcated
- Resolution plays an important part in interpreting satellite images. Higher resolution may improve the visual content may reveal ground realities more closely.
- Ground profile and subsoil data cannot be accessed through satellite imageries.
- Digitization of complete zonal maps rather select features would provide a complete replica in integrated environment.
- Satellite data of resolution of 1 or 5.8 meter will be preferable. However the overall cost economics shall have to be worked out since this data will be more costly but will completely avoid field works.
- Symbols of physical features in satellite imageries are not easily recognizable by transmission line engineers.