A plain scale is a scale used to measure two units or a unit and its subdivision.
It is commonly used in engineering drawings to represent dimensions accurately.
Plain scales help convert actual measurements into scaled drawing measurements.

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Plane Scale in Engineering Drawing

Introduction

A Plane Scale is a type of engineering scale used to measure and represent two units or a unit and its subdivisions accurately on engineering drawings, maps, plans, and layouts.

It is one of the most important scales studied in Engineering Graphics and Engineering Drawing because it allows measurements to be read in two dimensions, such as:

• Meters and decimeters
• Kilometers and meters
• Feet and inches
• Centimeters and millimeters

A plane scale provides greater accuracy than a plain scale because it can show both a unit and its subdivisions.

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What is a Plane Scale?

A plane scale is a scale that represents:

A primary unit and its immediate subdivision.

For example:

• 1 meter divided into decimeters
• 1 kilometer divided into meters
• 1 foot divided into inches

It allows measurements to be read more precisely than a simple plain scale.

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Definition

A plane scale is defined as:

“A scale on which it is possible to measure two dimensions, a unit and its subunit.”

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Purpose of a Plane Scale

The main purposes are:

• Represent large distances on small drawings
• Measure lengths accurately
• Read units and subdivisions
• Convert actual dimensions into scaled dimensions
• Improve drawing precision

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Principle of Plane Scale

A plane scale is constructed based on the Representative Fraction (R.F.).

Formula

The Representative Fraction indicates the ratio between the drawing size and the actual size.

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Components of a Plane Scale

1. Scale Length

The total length of the scale drawn on paper.

Formula

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2. Main Divisions

Represent the primary unit.

Examples:

• Kilometers
• Meters
• Feet

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3. Subdivisions

Represent smaller units.

Examples:

• Meters
• Decimeters
• Inches

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4. Zero Point

The reference point from which measurements are taken.

Usually placed between the main divisions and subdivisions.

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Construction of a Plane Scale

Step 1: Determine the Representative Fraction (R.F.)

Example:

R.F. = 1/500

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Step 2: Determine Maximum Length to Be Measured

Example:

Maximum length = 50 m

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Step 3: Calculate Scale Length

Using:

Length of Scale = R.F × Actual Length

Convert actual length into the same unit first.

Example:

50 m = 50000 mm

Then:

Length of Scale = 1/500 × 50000

= 100 mm

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Step 4: Draw the Scale

Draw a rectangle of calculated length.

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Step 5: Divide into Main Units

For example:

50 m ÷ 5

Each division represents:

10 m

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Step 6: Subdivide the First Main Division

Divide it into:

10 equal parts

Each part then represents:

1 m

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Types of Measurements Using Plane Scale

Unit and Subunit

Examples:

Kilometers and Meters

Used in maps.

Meters and Decimeters

Used in civil engineering.

Feet and Inches

Used in architectural drawings.

Centimeters and Millimeters

Used in machine drawings.

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Example of Plane Scale Construction

Given

R.F. = 1/100

Maximum distance = 5 m

Required:

Measure meters and decimeters.

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Step 1

Convert:

5 m = 5000 mm

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Step 2

Calculate scale length:

Length = 1/100 × 5000

= 50 mm

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Step 3

Draw a 50 mm line.

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Step 4

Divide into 5 equal parts.

Each division:

1 m

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Step 5

Subdivide first division into 10 parts.

Each subdivision:

0.1 m

or

1 decimeter

The scale is now complete.

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Applications of Plane Scale

Civil Engineering

Used for:

• Road layouts
• Building plans
• Site maps

Mechanical Engineering

Used for:

• Machine layouts
• Equipment drawings

Surveying

Used for:

• Land maps
• Topographical plans

Architecture

Used for:

• Floor plans
• Elevations

Transportation Engineering

Used for:

• Highway plans
• Railway layouts

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Advantages of Plane Scale

Higher Accuracy

Can read units and subdivisions.

Easy Construction

Simple geometric procedure.

Better Precision

More precise than a plain scale.

Widely Applicable

Useful in many engineering fields.

Convenient Reading

Measurements can be read directly.

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Limitations of Plane Scale

Limited Precision

Cannot measure very small fractions accurately.

Manual Reading Errors

Human error may occur.

Less Accurate Than Diagonal Scale

Diagonal scales provide greater precision.

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Plane Scale vs Plain Scale

Feature	Plain Scale	Plane Scale
Units Measured	One unit and fraction	Unit and subdivision
Accuracy	Moderate	Higher
Construction	Simpler	Slightly more complex
Usage	Basic measurements	Precise measurements
Reading Capability	Limited	Better

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Plane Scale vs Diagonal Scale

Feature	Plane Scale	Diagonal Scale
Precision	Good	Very High
Subdivision	Unit and subunit	Smaller fractions possible
Complexity	Simple	More complex
Application	General engineering	High-precision work

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Common Examination Questions

What is a Plane Scale?

A plane scale is a scale that represents a unit and its subdivision, allowing more accurate measurement than a plain scale.

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What is the main use of a Plane Scale?

To measure and represent two units or a unit and its subunit accurately on engineering drawings.

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What is the difference between Plain Scale and Plane Scale?

A plain scale measures one unit and its fraction, whereas a plane scale measures a unit and its subdivision more accurately.

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Importance in Engineering Drawing

Plane scales are important because they:

• Enable accurate representation of dimensions
• Reduce measurement errors
• Improve drawing quality
• Assist in reading scaled distances
• Form the basis for advanced scales such as diagonal scales

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Conclusion

A Plane Scale is an engineering drawing scale used to represent a unit and its subdivision, such as meters and decimeters or feet and inches. It is constructed using the Representative Fraction (R.F.) and provides greater accuracy than a plain scale. Plane scales are widely used in civil engineering, mechanical engineering, architecture, surveying, and mapping for accurately measuring and representing dimensions on drawings and plans. Understanding plane scales is essential for mastering engineering graphics and technical drawing.

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