Surveying is a fundamental aspect of civil engineering and land development, providing the essential data required for designing infrastructure and understanding terrain features. Among the various techniques employed in surveying, contouring stands out as a crucial process for mapping elevations and creating detailed representations of the land surface. This article delves into the concept of contouring in surveying, exploring its methods, procedures, and practical applications.
I. Introduction
A. Definition of Contouring in Surveying
Contouring in surveying involves determining the elevation of various points on the ground and accurately representing these points on a contour map. A contour map is a two-dimensional representation where lines connect points of equal elevation, thereby illustrating the three-dimensional shape of the terrain.
B. Importance of Contouring
Understanding the topography of an area is essential for numerous engineering and planning projects. Contouring provides vital information about the slope, elevation changes, and natural features of the land, which are critical for designing roads, buildings, dams, and other infrastructure. Additionally, contour maps aid in environmental management, urban planning, and agricultural development by offering a clear picture of the land’s physical characteristics.
II. Methods of Contour Surveying
Contour surveying can be broadly classified into two primary methods: the Direct Method and the Indirect Method. Each method has its own set of procedures, instruments, and suitability depending on the project’s requirements and the terrain’s complexity.
A. Direct Method of Contouring
1. Procedure
The Direct Method involves establishing both vertical and horizontal controls for points lying on the desired contour line. This process begins with vertical control through leveling instruments. A level is positioned at a commanding location within the survey area, typically after conducting fly levels from a nearby benchmark. The surveyor determines the plane of collimation (the horizontal line of sight) and calculates the required staff reading for the specific contour line.
The instrument operator instructs the staff person to adjust their position vertically until the desired staff reading is achieved. Concurrently, the surveyor establishes the horizontal control for that point using precise instruments. This process is repeated for multiple points along one or two contour lines, often utilizing a plane table survey for optimal efficiency. Although the Direct Method is labor-intensive and time-consuming, it offers high accuracy, making it ideal for small survey areas.
2. Instruments Used
- Leveling Instruments: Essential for establishing vertical control.
- Plane Table: Facilitates simultaneous horizontal and vertical measurements.
- Hand Levels and Abney Levels: Occasionally used to expedite leveling tasks.
3. Characteristics
- Accuracy: High precision in determining elevation points.
- Efficiency: Slow and tedious, suitable for smaller areas.
- Instrument Shifting: The leveling and surveying instruments may need to be repositioned to cover larger areas, ensuring both instruments remain in proximity for effective communication.
B. Indirect Method of Contouring
1. Procedure
The Indirect Method focuses on establishing horizontal control before addressing vertical measurements. Initially, horizontal control points are established through chain surveys, compass surveys, or plane table surveys. Once these points are set, leveling is performed at selected locations to determine their elevations. These reduced levels are then plotted on a plan, and contour lines are interpolated between the established points.
2. Methods for Selecting Points
Several techniques can be employed to select points for leveling, each suited to different types of terrain and project requirements:
- a. Method of Squares This method involves dividing the survey area into a grid of squares, typically ranging from 5m x 5m to 20m x 20m. Elevations at all grid points are determined through leveling, and the contour lines are interpolated based on these measurements.
- b. Method of Cross-Section Cross-sectional points are taken at regular intervals along a designated line, often aligned with the direction of a road or railway project. This method is particularly effective for projects that require detailed elevation profiles along specific routes.
- c. Radial Line Method Originating from a central point, several radial lines are extended outward. Elevations are measured at selected distances along these lines, making this method ideal for hilly or mountainous areas. Theodolites with tacheometry facilities are commonly used to enhance accuracy in such terrains.
3. Interpolation Methods
Once elevation points are established, contour lines are interpolated using one of the following methods:
- a. Estimation Involves visually estimating the position of contour lines between known points.
- b. Arithmetic Calculation Uses mathematical formulas to determine the exact position of contour lines.
- c. Mechanical or Graphical Method Utilizes tracing sheets with parallel lines to linearly interpolate contour points, maintaining arithmetic accuracy while speeding up the process.
III. Drawing Contours
A. Procedure
After establishing the contour points through either the Direct or Indirect Method, the next step is to plot these points on a plan. Smooth contour lines are then drawn to connect points of equal elevation, typically using tools like French curves to ensure accuracy and smoothness.
B. Best Practices
- Thicker Contour Lines: Every fifth contour line is often made thicker to enhance readability and provide a clear visual reference.
- Labeling Elevations: Each contour line is labeled with its corresponding elevation, and for larger maps, labels are also placed at the ends of the lines.
- Maintaining Terrain Features: Surveyors ensure that the drawn contours accurately reflect the natural characteristics of the terrain, preserving features such as ridges and valleys.
IV. Contour Maps and Their Uses
A. Characteristics of Contour Maps
Contour maps are invaluable tools that convey the three-dimensional shape of the land in a two-dimensional format. They possess several key characteristics:
1. Contour Lines Must Close
Contour lines are continuous and must form closed loops, although the loops may extend beyond the boundaries of the plan.
2. Spacing of Contour Lines
- Widely Spaced Contours: Indicate a flat or gently sloping surface.
- Closely Spaced Contours: Signify steep terrain with rapid elevation changes.
- Equally Spaced Contours: Reflect a uniform slope across the terrain.
- Irregular Spacing: Suggest uneven or varied topography.
3. Specific Terrain Features
- a. Concentric Closed Contours
- Depressions or Ponds: Contours that are concentric with decreasing elevation values toward the center indicate a depression or pond.
- Hills: Concentric contours with increasing elevation values toward the center denote a hill.
- b. U-Shaped Contours These indicate ridges when the U-shape is convex toward the lower ground.
- c. V-Shaped Contours V-shaped contours with convexity toward higher ground represent valleys.
4. Intersection of Contour Lines
Contour lines typically do not intersect. However, when they do:
- Meeting Lines: Suggest the presence of a vertical cliff.
- Crossing Lines: Indicate overhanging cliffs or caves.
B. Uses of Contour Maps
Contour maps serve a wide array of applications in engineering and environmental management:
1. Site Selection
Civil engineers analyze contour maps to identify suitable locations for construction projects, ensuring stability and feasibility based on the terrain.
2. Profile Generation
By drawing cross-sections from the contour map, engineers can create ground profiles that aid in determining the depth of cutting and filling required for projects like roads and railways.
3. Visibility Analysis
Contour maps help assess the visibility between two points, which is crucial for projects like telecommunications towers or surveillance installations.
4. Route Planning
Designing routes for infrastructure such as roads, railways, canals, or sewer lines involves minimizing and balancing earthworks, which is facilitated by contour maps.
5. Hydrological Studies
Understanding water flow, catchment areas, and flood levels is essential for the construction of dams, bunds, and reservoirs. Contour maps provide the necessary elevation data to model water behavior accurately.
6. Volume Calculations
Contour maps enable the calculation of volumes for earthmoving projects and the determination of reservoir capacities, ensuring efficient resource management.
V. Conclusion
A. Recap of Contouring Importance
Contouring is a pivotal process in surveying that provides a detailed and accurate representation of the land’s topography. It forms the foundation for successful engineering projects, urban planning, and environmental management by offering critical elevation data and terrain insights.
B. Future Applications
Advancements in surveying technology, such as drone-based photogrammetry and LiDAR, promise to enhance the precision and efficiency of contouring. These innovations will enable more comprehensive and rapid data collection, further expanding the applications of contour maps in various fields.