Fieldwork for contouring can be performed in different ways, depending on the instruments used. The methods for identifying contours are generally categorized into two primary types:
- Direct Method
- Indirect Method
Direct Method
In this approach, the contours are mapped directly in the field by identifying and marking several key points along each contour line. These points are then transferred to a plane table, where the corresponding contour lines are drawn. While this technique is slower and not ideal for large-scale surveys, it is preferred when high accuracy is required, particularly in smaller areas where precision is crucial.
The fieldwork for this method can be broken down into two main phases:
- (a) Vertical Control: Determining the exact elevation of the points along the contours.
- (b) Horizontal Control: Plotting the marked points on the plane table.
In some cases, these two tasks can be carried out simultaneously, with one surveyor responsible for measuring elevations (vertical control) and another for plotting the positions (horizontal control) using a plane table.
Direct Method No. 1 for Contouring
(a) Vertical Control
Instruments Required:
A leveling instrument, leveling staff, and a plane table with necessary accessories.
Procedure:
- Start by establishing a permanent benchmark in the project area with a known elevation above mean sea level, or choose a reference point with an arbitrary elevation (e.g., 500 meters).
- Position the leveling instrument at a convenient, elevated spot (Point A), ensuring a clear view of the benchmark to take accurate readings.
- Take a reading from the leveling staff placed on the benchmark. Let’s assume the reading on the middle crosshair is 1.523 meters.
- Calculate the height of the instrument’s line of sight (line of collimation) by adding the benchmark’s elevation to the staff reading (500 + 1.523 = 501.523 meters).
- To mark the 500-meter contour, direct the staff holder to points where the staff reading is 1.523 meters, and mark those points with wooden pegs. Similarly, for the 501-meter contour, identify positions where the staff reading is 0.523 meters (501.523 – 501 = 0.523 m). Note that at this setting, contours above 501 meters cannot be measured, but contours below this value (e.g., 499 m, 498 m) can be located depending on the staff’s length (4 m, 4.5 m, or 5 m). Ensure that the line of collimation intersects the staff to properly locate the contour.
- Establish a forward station (Point F) and record the staff reading. Assume the reading is 3.426 meters. The reduced level of Point F will be 501.523 – 3.426 = 498.097 meters.
- Move the level to a new location (Point B) and take a reading on the staff at Point F. Suppose the back staff reading is 0.852 meters.
- Compute the new line of collimation at Point B by adding the reduced level of Point F to the back staff reading (498.097 + 0.852 = 498.949 meters).
- Use this new setup to continue locating contours of lower values, such as 497, 496, and 495 meters.
- Repeat this process until the entire area is contoured.
2. Horizontal Control
Instruments Required:
A plane table with accessories, chains, and other necessary tools.
Procedure:
- For small areas, contour points can be surveyed directly using the radiation method on a plane table. This method allows for a straightforward plotting of contours from a single station.
- In larger areas where contours cannot be fully plotted from a single station, the plane table needs to be shifted to a new location. To facilitate this, a long radial line is drawn from the initial station to the new one.
- Upon moving the plane table to the second station, it is reoriented using the Back Ray Method, ensuring alignment with the previously plotted radial line. The distance between the two stations is then measured and accurately plotted on the scale of the plane table.
- Contour points on various elevations are surveyed using the radial line method from each station. This process is repeated, moving the plane table as needed, until all the contour points for the area are plotted.
- It is essential to plot a sufficient number of points at close intervals for precise contouring. The more frequent the points, the more accurate the contour lines will be.
Important Considerations:
- When plotting the contours, the consecutive points on a given contour should be connected by straight lines on the plane table. However, care must be taken to also include points at locations where there are changes in slope.
- For example, if contour points are on both sides of a ridge (points
aandbon one side, and pointsc,d,e, andfon the other), it’s crucial to plot a point on the ridge itself. Simply connecting points across the ridge without accounting for this may lead to significant inaccuracies in the contour line. A similar approach is necessary when contouring stream beds, valleys, or other irregular terrain features.
Direct Method No. 2 for Contouring
Instruments Required:
Indian tangent clinometer, ranging rod, plane table, and a chain.
This method involves tracing contours by identifying and marking multiple points on each contour. Unlike the previous approach, a clinometer is used for vertical control instead of a level. Distances between the plane table station and the identified contour points are measured using a chain.
Procedure:
- Begin by establishing several benchmarks within the survey area through standard leveling techniques.
- Set up the plane table at a prominent location and level it carefully. If known control points are available, plot them on the plane table. If not, choose a suitable reference point on the plane table corresponding to your station location, ensuring it aligns with the layout of the area.
- Place the Indian tangent clinometer on top of the plane table, adjusting its leveling screw to achieve accurate leveling. Aim the clinometer’s sight at a ranging rod positioned vertically on a nearby benchmark.
- The person holding the ranging rod should attach a 5 cm wide white cloth around the rod and adjust it until the clinometer’s reading is zero at the top edge of the cloth strip. Measure the exact height of the cloth strip above the benchmark using a tape.
- If the ground level of the benchmark is 405.62 meters and the height of the cloth is 1.85 meters above ground, the height of the sighting point (eye hole) is 407.47 meters (405.62 + 1.85).
- To locate points on the 405-meter contour, adjust the cloth strip to a height of 2.47 meters above the benchmark (407.47 – 405.00 = 2.47). The rod holder can then move to various points where the clinometer reads zero, marking the contour points. The distances to these points are measured using a chain and plotted on the plane table based on lines radiating from the station.
- For locating other contours, such as 404 meters or 403 meters, adjust the cloth strip to heights of 3.47 meters and 4.47 meters, respectively. Repeat the process to identify sufficient points for accurate contouring.
- This method is particularly useful when a level is unavailable during plane table operations, or when a single surveyor is responsible for both vertical and horizontal control. In this approach, both types of control are achieved simultaneously from the same station. It is important to measure distances from the point directly below the sighting eye vane, as illustrated in Fig. 7.10.
Direct Method No. 3 for Contouring
Instruments Required
Photogrammetric plotting machine and a pair of vertical aerial photographs.
This method utilizes aerial photographs for direct contouring and represents the latest advancement in surveying technology. In this approach, vertical aerial photographs are captured with a 60% overlap, ensuring the camera axis remains vertical throughout. These photographs, either as diapositives or negatives, are then mounted on the picture carriers of photogrammetric machines such as the Wild A7, A8, or B8.
The process begins with establishing the relative orientation of the 3D model. This involves systematically adjusting the photographs through rotation and translation, ensuring that corresponding images from the two exposures align perfectly across the model area. This procedure is referred to as ‘Relative Orientation’ or the ‘Removal of Y-Parallax.’
Once relative orientation is achieved, the model’s correct scale is determined by adjusting the distance between the perspective centers of the photographs. The entire model is then adjusted along the X, Y, and Z axes—rotating and shifting it—until the heights and distances of known reference points in the model match their real-world coordinates.
For absolute orientation, at least four points with known plan and height coordinates are needed, one at each corner of the model. These points ensure the model is positioned correctly in 3D space, with the fourth point serving as a check.
To plot the desired contour, a floating mark is set at the required elevation. The mark is then moved horizontally in the X and Y directions while keeping the Z-axis fixed. A coordinatograph attached to the photogrammetric machine records the contour on the plan. By adjusting the Z-axis settings, additional contours at different elevations can be plotted.
This method ensures that every point along a contour is accurately plotted, making it the most precise approach to contouring. It is typically used in cases where the highest degree of accuracy is essential.
Indirect Method of Contouring
In the indirect method, spot levels are determined for a sufficient number of points. These points can be easily plotted on a plane table, as they often form the corners of regular geometrical shapes, such as squares, rectangles, or triangles. However, it is rare to find a point with an exact elevation matching a specific contour line. Therefore, spot levels are also taken at critical locations, including hilltops, ridge lines, stream beds, and the lowest points in depressions, to accurately represent these features when drawing contour lines.
The contours between the spot levels are then interpolated and drawn. This approach is sometimes referred to as contouring by spot levels.
The indirect method is commonly used for small-scale surveys covering large areas. Compared to the direct method, it is more cost-effective, faster, and less labor-intensive.
The indirect method of contouring can be carried out in three main ways:
- Square Method
- Cross Sections Method
- Tacheometric Method
1. Square Method
In the square method, the area to be surveyed is divided into a grid of squares, with side lengths typically ranging from 5 meters to 25 meters. The size of these squares can vary based on factors such as the terrain, the contour interval, and the map scale. It is not necessary for all squares to be the same size; adjustments can be made as required by the map.
The corners of these squares are marked on the ground, and spot levels are determined for each corner using standard leveling techniques. Additionally, spot levels are taken for key features like hilltops and depressions, with their distances from the nearest square corners noted.
The squares are then plotted on the map at the desired scale, and the reduced levels of both the corners and significant ground features are recorded. From there, contours of the required elevations are interpolated.
Suitability: This method is most appropriate for areas with gentle slopes and minimal vegetation, where the terrain is relatively uniform.
2. Cross Section Method
In the cross-section method, cross sections are established perpendicular to the central line of the area being surveyed. The spacing between these cross sections depends on factors such as the contour interval, the scale of the plan, and the nature of the terrain. Typically, cross sections are spaced 20 meters apart in hilly areas and 100 meters apart in flat areas.
Salient features along the central line, as well as on the cross sections, are also identified and recorded. It’s important to note that the cross sections do not have to be exactly perpendicular to the center line; they can be inclined at suitable angles if necessary.
Once the central line and the cross sections have been plotted on the desired scale, the reduced levels are recorded. Contours are then interpolated based on these reduced levels.
Suitability: This method is particularly suited for creating contour plans for linear features such as roads, railways, or canal alignments.
3. Tacheometric Method
In the tacheometric method, a series of radial lines are drawn at known angular intervals from a central point on the ground. Points at equal distances along each radial line are marked, and important features or changes in the terrain are identified by measuring vertical angles and staff readings (bottom, middle, and top wires). These measurements are used to calculate the reduced levels (elevations) and horizontal distances from the instrument station using tacheometric formulas. For further details, refer to the chapter on Tacheometric Surveying.
The radial lines and the positions of the points on each line are then plotted to scale on a map, with the corresponding spot levels entered. Once this data is recorded, the required contours can be interpolated based on the spot levels.
To cover the entire area, the instrument is set up at additional commanding tacheometric stations such as B, C, D, etc., and the same process is repeated.
Suitability: This method is ideal for contouring areas with long strips of terrain, particularly in mountainous or undulating regions where direct chaining would be difficult.
Differences between Direct and Indirect Method
| Basis of Difference | Direct Method | Indirect Method |
|---|---|---|
| Definition | Points on contour lines are directly located and plotted. | Spot levels are measured, and contours are interpolated between these points. |
| Accuracy | The most accurate method but slow and troublesome. | Less accurate, but quicker and less troublesome. |
| Time and Effort | Time-consuming and labor-intensive due to detailed fieldwork. | Quicker and less labor-intensive, as fewer field measurements are taken. |
| Cost | Very expensive. | Cheaper and more cost-effective, especially for large areas. |
| Suitability | Used for small projects where greater accuracy is needed, such as factory layouts or structural foundations. | Used for large projects where greater accuracy is not needed, such as road, canal, or railway layouts. |
| Field Requirements | Requires instruments like levels, plane tables, and clinometers to locate each contour point. | Fewer instruments are required, and it relies on methods like squares, cross-sections, or tacheometry. |
| Terrain | Unsuitable for hilly terrain. | Suitable for hilly terrain; can be used with the tacheometric method for road or canal surveys. |
| Applications | Used for detailed surveys like construction or small-scale engineering works. | Suitable for large-scale surveys such as environmental studies, route planning for roads, canals, or railways. |
| Calculation of Reduced Levels | Calculations are done in the field and cannot be checked later. | Reduced levels are calculated later, allowing for checking as needed. |
| Contour Interpolation | No interpolation is required since contour points are directly measured. | Contours are interpolated between spot levels. |
