Preparation and Scope
Definition of Reconnaissance Survey: The reconnaissance survey is an extensive study of an entire area that might be used for a road or airfield. Its purpose is to eliminate those routes or sites which are impractical or unfeasible and to identify the more promising routes or sites. So lets go and read about “What is Reconnaissance Survey in Civil Engineering”
Existing maps and aerial photographs may be of great help. Contour maps show the terrain features and the relief of an area. Aerial photographs show up-to-date planimetric details.
The reconnaissance survey must include all possible routes and sites. The reconnaissance survey report should summarize all the collected information, including a description of each route or site, a conclusion on the economy of its use, and, where possible, appropriate maps and aerial photographs.
Design of Reconnaissance Survey
- Design and military characteristics should be considered during the reconnaissance survey. Keep in mind that future operations may require an expanded road net. A study of the route plans and specifications is necessary. If these are unavailable, use the following as guides.
- Locate portions of the new road along or over existing roads, railroads, or trails, whenever possible.
- Locate the road along ridges and streamlines, keeping drainage structures to a minimum. Keep the grade well above the high waterline when following a stream.
- Select a route as near to sources of material as practical, and locate the road along contour lines to avoid unnecessary earthwork.
- Locate the road on the sunny side of hills and canyons and on that side of the canyon wall where the inclination of the strata tends to support the road rather than cause the road to slide into the canyon.
- Locate roads in forwarding combat zones so that they are concealed and protected from enemy fire. This may at times conflict with engineering considerations.
- Select locations which conserve engineer assets, avoiding rock work and excessive clearing.
- Avoid sharp curves and locations which involve bridging.
Collection of Data
Upon completion, the reconnaissance survey should support the routes surveyed and provide a basis of a study showing the advantages and disadvantages of all routes reconnoitered. Typical data collected in a reconnaissance survey are Following
- Sketches of all routes reconnoitered
- Reports of feasible routes. Data on clearing and grubbing
- The number of stream crossings involving bridge spans exceeding 20 feet or 6 meters.
- The approximate number of culverts and spans less than 20 feet or 6 meters.
- Descriptions and sizes of marsh areas and other natural obstacles.
- Unusual grade and alignment problems encountered.
- Anticipated effects of landslides, melting snow, and rainfall.
- Soil conditions and stream and substrate conditions at proposed bridge sites.
- Discrepancies noted in maps or aerial photographs.
- Availability of local materials, equipment, transportation facilities, and labor.
- Photographs or sketches of reference points, control points, structure sites, terrain obstacles, and any unusual conditions.
Use of Maps
After the Collected Data next step is Use of Maps in the reconnaissance survey. The procurement of maps is a very important phase of the reconnaissance. The surveyor should locate and use all existing maps, including up-to-date aerial photographs of the area to be reconnoitered. Large scale topographic maps are desirable because they depict the terrain in the greatest detail. The maps, with overlays, serve as worksheets for plotting trial alignments and approximate grades and distances.
The surveyor begins a map study by marking the limiting boundaries and specified terminals directly on the map. Between boundaries and specified terminals, the surveyor observes the existing routes, ridge lines, water courses, mountain gaps, and similar control features. The surveyor must also look for terrain which will allow moderate grades, the simplicity of alignment, and a balance between cut and fills.
After closer inspection, the routes that appear to fit the situation are classified. As a further study shows the disadvantages of each route, the surveyor lowers the classification. The routes to be further reconnoitered in the field are marked using pencils of different colors to denote priority or preference. Taking advantage of the existing terrain conditions to keep excavation to a minimum, the surveyor determines grades, estimates the amount of clearing to be done on each route, and marks stream crossings and marsh areas for possible fords, bridges, or culvert crossings.