Understanding Contour Plans for Route Surveys

A contour plan is a topographical map that shows the shape of the ground through contour lines. Contour lines connect points of equal elevation. For route surveys, such as those for highways, railways, or pipelines, a contour plan is essential for selecting the most suitable alignment, estimating earthwork quantities, and designing gradients.

Methods of Contouring

There are two main categories of contouring methods:

• Direct Contouring: Involves locating and marking points of equal elevation directly on the ground and then plotting them on the map. This is typically done using instruments like theAbney level or hand level and is suitable for smaller areas or when high accuracy is needed for specific features.
• Indirect Contouring: This method involves establishing a framework of surveyed points with known horizontal positions and elevations (spot heights). Contours are then drawn on the map by interpolation between these points. This is more practical for larger areas and route surveys where the terrain needs to be mapped efficiently.

Route Survey Contour Plan Preparation

The preparation of a contour plan for a route survey typically requires mapping the terrain over a corridor along the proposed route. This involves:

1. Establishing control points along the proposed alignment.
2. Taking spot levels at various points within the survey corridor, including points perpendicular to the alignment (cross-sections) and points off the alignment to capture the general topography.
3. Plotting these points with their elevations on a plan.
4. Drawing contour lines by interpolating between the plotted spot heights based on their elevations.

Evaluating Contouring Methods for Route Surveys

Let's consider why the Indirect method of contouring is the most suitable for preparing a contour plan for a route survey:

• Suitability: The indirect method allows surveyors to efficiently collect spot heights over a wider area relevant to the proposed route. By taking levels at grid points or along radial lines from control stations, a comprehensive dataset is obtained, enabling the interpolation of accurate contour lines that represent the ground's undulation along the potential path.
• Comparison with other methods:
  • The Method of cross profile is a technique used within route surveys to understand the terrain perpendicular to the centerline, aiding in detailed design and earthwork calculations, but it's not the primary method for generating the overall contour plan of the route corridor.
  • The Method of squares is primarily used for calculating areas from maps or plans, often involving grid systems, rather than generating contour lines themselves.
  • The Method of trace contour usually implies drawing contours directly from observed points or following existing contour lines, which is less efficient for initial contour plan preparation in route surveys compared to the interpolation techniques used in indirect contouring.

Therefore, the indirect method is the standard and most practical approach for creating the necessary contour plan during the preliminary stages of a route survey.