A contour is defined as an imaginary line of constant elevation on the ground surface. It can also be described as the line formed by the intersection of a level surface with the ground.
The process of identifying these contour lines on the earth’s surface is known as contouring. Understanding soil type and land contours is essential for developing a landscape plan that optimizes land use in farming or gardening.
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Methods of Contouring for Accurate Agricultural Land Planning
The method of establishing or plotting contours on a plan or map is known as contouring. It requires the planimetric positioning of points and drawing contours based on the elevations of these points.
Contouring involves vertical control to locate points on contours and horizontal control for plotting points accurately. The instruments used determine both horizontal and vertical positions.
Generally, field contouring methods are divided into two categories:
1. Direct methods
2. Indirect methods
Direct Method of Contouring for Precise Land Elevation Mapping
In the direct method, the contour is physically traced on the ground. Only points falling on a desired contour are surveyed, plotted, and joined to form the contour line.
This method is slow and tedious but used for large-scale maps, small contour intervals, and high precision. It involves:
1. Vertical control: A benchmark is established in the project area. The leveling instrument is set on a commanding position, and back sight readings are taken on the benchmark.
For example, if the back sight reading is 1.485 m and the benchmark elevation is 100 m, the instrument height becomes 101.485 m. To locate the 100.5 m contour, the staff reading should be 0.985 m, and pegs mark these positions.
For higher contour values, the instrument is moved forward, and foresight readings taken from a new known elevation point until the entire area is contoured.
2. Horizontal control: Horizontal positions are controlled through plane table surveying or accurate location of points.
Indirect Method of Contouring Using Guide Points and Interpolation
The indirect method involves taking spot levels of selected guide points using a level. The horizontal positions of these points are measured or computed and plotted on a plan. Contours are drawn by interpolation between these points. Common indirect methods include:
1. Square or Grid method: The survey area is divided into a grid, with sizes varying between 5 m x 5 m to 25 m x 25 m depending on terrain and required precision.
Spot levels are taken at grid corners and salient ground features such as hilltops and depressions to ensure accurate contouring by interpolation.
2. Cross Section method: A baseline or profile line is established, with cross sections taken perpendicular at regular intervals. Staff readings along these lines are recorded and used to calculate reduced levels.
The points are plotted, and contour lines are drawn by interpolation. This method is suitable for route surveys.
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Methods of Interpolating Contours for Accurate Land Elevation Estimation
Interpolation is the process of estimating contour lines proportionately between plotted points. Two common methods are:
1. Arithmetical calculation: For example, between points A and B with elevations of 98.75 m and 100.75 m respectively, separated by 10 m, the location of a 99.00 m contour can be calculated proportionally by difference in levels and distance.
2. Graphical method: Using tracing paper and radial lines, the contour positions are graphically interpolated between two known elevations. This method visually assists in drawing intermediate contour lines.
Characteristics of Contour Lines Relevant to Farming Land Analysis
Contour lines have distinct properties useful in land analysis:
- Vertical distance between contours is uniform.
- Horizontal spacing inversely indicates slope steepness; closer lines mean steeper slopes.
- Contours are perpendicular to ridge and valley lines.
- Contour lines never cross, merge, or pass through permanent structures.
- Contour lines must close on themselves, representing hills or depressions.
- Contours deflect uphill at valleys and downhill at ridges.
- Contours do not have sharp turns, maintaining smooth curvature.
Contour Interval and Its Significance in Agricultural Land Survey
The contour interval is the vertical distance between consecutive contours. For example, contours at 100 m, 98 m, and 96 m have a 2 m interval. The interval depends on:
- Terrain nature.
- Map scale.
- Survey purpose.
Flat lands have small intervals (e.g., 0.25 m to 0.75 m), while hilly areas have larger intervals (e.g., 5 m to 15 m). Small-scale maps use larger intervals, whereas large-scale maps use smaller intervals.
Types of Contour Lines Found on Topographical Maps for Land Use Planning
Topographical maps use contour lines to represent the shape and elevation of land surfaces. Contour types include:
1. Index lines: Thickest and labeled lines showing elevation above sea level.
2. Intermediate lines: Thinner lines between index lines, typically five between each index line.
3. Supplementary lines: Dotted lines indicating flatter surfaces.
A contour map provides valuable information about land elevation differences.
The direct method traces contours on the ground by surveying points on each contour line, while the indirect method uses spot levels at selected points and interpolation to locate contours.
Both methods are essential tools in agricultural land management, helping to plan for soil conservation, irrigation, and crop placement based on terrain features.
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