Knowledge of Different Types of Scales, Determination of R.F and Uses of Scales
In surveying and technical drawing, it is not always possible to represent actual dimensions of objects on paper due to size limitations. Therefore, scales are used to reduce or enlarge the size of objects proportionally. Understanding different types of scales, determination of Representative Fraction (R.F.), and their uses is essential for students in the Surveyor trade.
Concept of Scale
A scale is the ratio between the dimensions of an object on a drawing and its actual dimensions on the ground. It helps in representing large areas or objects in a smaller and manageable form while maintaining accuracy.
Scales are used in maps, plans, and engineering drawings to ensure that measurements are proportional and can be easily interpreted.
Representative Fraction (R.F.)
Representative Fraction (R.F.) is defined as the ratio of the length on the drawing to the actual length on the ground, both expressed in the same units.
R.F. = (Length on Drawing) / (Actual Length on Ground)
For example, if 1 cm on the drawing represents 1 meter on the ground:
R.F. = 1 cm / 100 cm = 1/100
R.F. is always expressed as a fraction with numerator equal to one. It is unitless and provides a clear idea of the scale used in a drawing.
Methods of Expressing Scale
Scales can be expressed in three common ways:
- Statement Scale: Example: 1 cm = 10 m
- Representative Fraction (R.F.): Example: 1/1000
- Graphical Scale: A line divided into equal parts representing actual distances
Types of Scales
There are different types of scales used in surveying and drawing, depending on the requirement and level of accuracy.
1. Plain Scale
A plain scale is used to measure two units or a unit and its subunits. For example, meters and decimeters. It is simple to construct and commonly used in basic drawings.
2. Diagonal Scale
A diagonal scale is used to measure three units or to obtain more precise measurements. It allows readings up to two decimal places and is used where higher accuracy is required.
3. Vernier Scale
A vernier scale provides very high precision and is used in advanced instruments. It helps in measuring small distances accurately.
4. Comparative Scale
A comparative scale is used to compare two different units, such as kilometers and miles. It is useful in maps and geographical studies.
5. Scale of Chords
This type of scale is used in angular measurements, especially in drawing angles without a protractor.
6. Engineer’s Scale
An engineer’s scale is a triangular scale with multiple graduations used for various scales in engineering drawings.
Determination of R.F.
The determination of R.F. is an important step in scale construction. It involves converting both the drawing length and actual length into the same unit and then forming a ratio.
Steps to determine R.F.:
- Convert actual length into the same unit as drawing length.
- Form the ratio of drawing length to actual length.
- Simplify the ratio to get the R.F.
Example: If 2 cm on drawing represents 20 meters on ground:
20 meters = 2000 cm
R.F. = 2 / 2000 = 1/1000
Uses of Scales
Scales are widely used in surveying and engineering for various purposes:
- To represent large distances in a reduced size
- To prepare maps and plans accurately
- To measure distances on drawings
- To convert drawing measurements into actual ground distances
- To maintain proportion and accuracy
Importance of Scales in Surveying
In surveying, scales are essential for plotting field data on drawing sheets. Accurate scaling ensures that maps and plans correctly represent the surveyed area.
Without proper scales, it would be difficult to interpret drawings or use them for construction purposes. Scales help in maintaining consistency and avoiding errors.
Precautions While Using Scales
- Always use the correct scale for the drawing
- Ensure accurate measurement while drawing
- Avoid using damaged or worn-out scales
- Maintain proper units throughout the drawing
- Check calculations of R.F. carefully
Application in Surveyor Trade
In the Surveyor trade, students frequently use scales for plotting maps, preparing layouts, and interpreting survey data. Knowledge of different scales and R.F. helps in producing accurate and professional drawings.
Students are trained to construct scales, determine R.F., and apply them in practical situations. This skill is essential for fieldwork and office work in surveying.
Conclusion
Scales are an important tool in surveying and technical drawing. Understanding different types of scales, determination of Representative Fraction, and their uses is essential for accurate representation of data.
For ITI Surveyor trainees, mastering the concept of scales helps in developing precision and professionalism. With proper knowledge and practice, students can effectively use scales in their work and ensure high-quality results.