Reconnaissance survey is done to examine the general character of the area for the purpose of determining the most feasible route or routes for further more detailed investigations.
Data collected from reconnaissance survey is used for feasibility study of all different routes, preparation of approximate estimates of quantities and costs. This helps in selection of most suitable alternatives. This survey also helps in determining any deviations required in the basic geometric standards to be adopted for the highway facility.
Reconnaissance survey is generally not required for the work consisting of improvements to existing roads unless bypass roads are involved.
Reconnaissance Survey Methods:
The reconnaissance survey methods may be conducted in the following sequence:
a) Study of topographical survey sheets, agricultural, soil, geological and meteorological maps and aerial photography.
b) Aerial photography wherever necessary and feasible, and
c) Ground reconnaissance including another round of serial reconnaissance for inaccessible and difficult stretches, where required.
Study of Survey Sheets and Maps:
Reconnaissance survey starts with a study of the all available maps. After study of the topographical features on the maps, a number of economical alignments feasible in a general way are selected keeping in view the following points:
(i) The alignment to take into account all the control points and to be shortened and more economical compatible with requirements of gradients and curvature.
(ii) Shape of the alignment.
(iii) Avoidance as far as possible of marshy ground, steep terrains, unstable hill features and areas subject to severe climatic conditions, flooding and inundation.
(iv) Need of connecting important villages and towns.
(v) Bridging problems, and
(vi) Need to preserve environment and maintain ecological balance.
The aerial photographs, if taken may be to a scale of 1:20000 to 1:50000 to supplement the information from topographic maps. If stereoscopic techniques are applied, aerial photographs can yield quantitative data, and if studied by a skilled photo interpreter, significant soil and subsoil information.
Aerial Reconnaissance Survey:
An aerial reconnaissance will provide a bird’s eye view of the alignments under consideration along with the surrounding area. It will help to identify factors which call for rejection or modification of any of the alignments. Final decision about the alignments to be studied in detail on the ground could be taken on the basis of the aerial reconnaissance.
Ground Reconnaissance Survey:
Ground reconnaissance consists of general examination of ground by walking or riding along the probable routes and collecting all available information necessary for evaluating the same. In the case of hill sections, it may be advantageous sometime to start the reconnaissance from an obligatory point situated close to the top. If an area is inaccessible for the purpose of ground reconnaissance, aerial method should be used to clear the doubts.
While reconnoitering on the ground, it is advisable to leave reference pegs to facilitate further survey operations.
Instruments generally used for ground reconnaissance include compass, Abney level / alti-meter, pedometer, aneroid barometer, clinometers, ghat tracer etc. Walkie-talkie sets are useful for communication, particularly in difficult terrain.
Use of instruments mentioned above to obtain ground slopes, maximum gradients, elevation of critical summits or stream crossings and location of obligatory points, serves as a check on the maps being used.
Reconnaissance Survey Reports:
Based on information collected during the reconnaissance survey, a report should be prepared. The report should include all relevant information collected during the survey and a plan to the scale of 1:50000 showing the alternative alignments studied along with their general profile and rough cost estimate.
It should also discuss the merits and demerits of the different alternatives to help the selection of one or more alignments for detailed survey and investigation.