1.16.5 - Methods of contouring
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Understanding Contouring Methods
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Today, we're discussing two methods of contouring in surveying: the direct and indirect methods. Who can tell me what they think contouring is?
Is it how we represent elevation on maps?
Exactly! Now, the direct method finds the elevation of points directly on the contour line. Does anyone know how this method works?
I think it involves using a level instrument, right?
That's right! The level instrument helps to determine the elevation by computing staff readings. This method, while accurate, is also quite slow. Let's remember that it's good for small areas.
Exploring the Indirect Method
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Now, let's talk about the indirect method. Can anybody describe what this method involves?
It's about taking known reference levels to interpolate contour lines, right?
Exactly! This method uses reference points known as spot levels. Specifically, we apply approaches like the Method of Squares. Who can explain how this works?
I think it involves dividing the area into squares and marking grid points.
Correct! And then we determine the elevations at these grid points. Interpolating helps us create contour lines quickly on larger terrains.
Choosing the Right Method
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When would you choose one method over the other? Let's brainstorm.
If the area is small, like for a building site, I’d use the direct method.
And for larger areas, like mapping a city, the indirect method would be better because it's faster.
Great insights! Remember, selecting a method depends on the size of the area, required accuracy, and the time available. Who can summarize what we learned today?
We explored the direct method for accuracy and the indirect methods for efficiency.
Introduction & Overview
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Quick Overview
Standard
The section covers the direct method of contouring, where surveyors determine elevations of points directly on a contour line, and the indirect method, which interpolates contour lines using reference points. Emphasis is placed on accuracy and suitability based on the terrain and area coverage.
Detailed
Detailed Summary
In surveying, contouring involves mapping the elevation levels of the terrain to represent three-dimensional topography on two-dimensional surfaces. Two principal methods of contouring are discussed:
Direct Method
The direct method involves finding the elevation of specific points along a selected contour line. A level instrument is set up at a vantage point to determine the elevation of the collimation line, from which staff readings for the intended contour are computed. Surveyors adjust the levelling staff until the desired reading is recorded, and the elevation is marked on the plotted map.
While this approach yields high accuracy, it is tedious and slow, thus primarily suited for smaller areas.
Indirect Method
In contrast, the indirect method leverages known reference levels (spot levels) to interpolate contour lines. This method can include several strategic approaches:
1. Method of Squares: The area is segmented into squares, and grid points are established. Elevations are collected, and interpolated values are used to plot contour lines.
2. Method of Cross-section: Useful in varying topographies by taking cross-sectional readings.
3. Radial Line Method: Employs radial points expanding from a central point.
This method, while potentially less accurate than the direct method, allows for more extensive coverage and quicker data collection, particularly with larger areas.
Audio Book
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Direct Method of Contouring
Chapter 1 of 2
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Chapter Content
There are two main methods of surveying:
(a) Direct method- This method finds the elevation of points in the area which lie on the selected contour line. The level instrument is set at a commanding position in the area and the elevation of line of collimation of instrument is determined. The required staff reading for a given contour line is then computed. The levelling staff is moved up and down in the area till the required staff reading is observed. The elevation of that point is marked on the plotted map. In this manner, all the points with same staff readings in the area are established, and in the process the instrument is also shifted to another station to cover more area. The same process is repeated for the next value of contour. This method is accurate but slow and tedious, hence suitable for small areas.
Detailed Explanation
The direct method for contouring involves identifying the elevation of specific points along a contour line. First, a leveling instrument is placed at a location with a clear view of the area. The height of the instrument is established, which aids in determining the required staff readings. The leveling staff is adjusted to find the point that matches the desired elevation (the contour line). This involves moving the staff up or down until the correct reading is obtained. Recorded points at the same elevation are marked on the map, illustrating the contour line. This method is noted for its accuracy, but it is slow and time-consuming, making it best suited for smaller areas where detail is critical.
Examples & Analogies
Imagine you're trying to find the height of a water level in a swimming pool. You have a measuring stick (the leveling staff) that tells you how deep the water is. You move the stick up and down until you find the point that matches a certain water height (contour line). Once you find that point, you make a note of it. You repeat this process at other locations in the pool, marking all the points where the water reaches the same height. Just like this, the direct method helps map out ground elevations accurately.
Indirect Methods of Contouring
Chapter 2 of 2
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Chapter Content
(b) Indirect methods- Here, RLs of some selected points (also known as spot levels) are taken. The contour lines are interpolated using the RLs of these selected points. For selecting the points, several approaches may be used: (i) Method of squares, (ii) Method of cross-section, and (iii) Radial line method. Out of the three approaches, first approach is most commonly used. In the method of square, the area is divided into a number of squares and all grid points are marked on the ground at regular interval. The grid interval is to be decided by the surveyor, depending on the terrain and scale of mapping. The RLs of all grid points are determined by levelling. The square grid is plotted on the drawing sheet, and RLs of grid points marked. Points of specific elevation value are drawn by linear interpolation method. These points are joined together with smooth contour line and its elevation value is written along the contour line. French curves may be used for drawing the smooth lines. To provide, more readability to contour values, every fifth contour line is made slightly thicker.
Detailed Explanation
The indirect method of contouring utilizes selected reference points, known as spot levels, to infer elevations across a broader area. This method doesn't require measuring every point directly. Instead, surveyors take elevation readings at strategically chosen locations. The area is typically divided into squares, creating a grid where points are systematically sampled. Once these points are established, their elevations are recorded. Using linear interpolation, surveyors estimate the elevations between these points to draw contour lines. This method is generally faster than direct contouring and allows for the mapping of larger areas without losing significant detail.
Examples & Analogies
Think of baking a cake. Instead of measuring the temperature of every part of the cake with a thermometer, you check the temperature in selected spots – the edges and the center. Then you determine how well-cooked the entire cake is based on these checks. Similarly, in indirect contouring, surveyors collect elevation data at chosen spots, enabling them to draw a detailed map without examining every inch of land.
Key Concepts
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Direct Method: A technique in contouring that maps elevations directly by measuring points on the contour line.
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Indirect Method: A technique in contouring that uses known reference points for interpolation.
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Spot Levels: Reference points used for determining elevation and further interpolating contour lines.
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Interpolation: The calculation of unknown values between two known data points.
Examples & Applications
In a small construction site, using the direct method allows for accurate elevation measurements for laying foundations.
For a large park, using spot levels and the indirect method enables quicker contour representation over vast areas.
Memory Aids
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Rhymes
To map the heights with great delight, use direct for small, for large let indirect take flight.
Stories
Imagine a surveyor named Sam who had to measure a tiny park’s elevation accurately, so he used the direct method. But when tasked with a large mountain range, he wisely chose the indirect method, saving time while still creating a detailed map.
Memory Tools
D = Direct method, I = Indirect method: Develops accuracy with direct, Interpolates rapidity with indirect.
Acronyms
DINE
Direct Is Neat & Exact; Indirect is Faster & Helpful.
Flash Cards
Glossary
- Direct Method
A contouring method that directly measures elevations along a contour line using level instruments.
- Indirect Method
A contouring method that utilizes reference point elevations to interpolate and estimate contour lines.
- Spot Levels
Selected points in a survey with known elevations, used for interpolation in indirect contouring.
- Interpolation
The process of estimating unknown values between two known values.
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