📏 Types of Scales

In technical drawing and planning, a scale is used to represent large or small objects accurately on paper. Since it is not possible to draw everything in actual size, scales help convert real dimensions into manageable drawing sizes while preserving accuracy.

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1️⃣ Plain Scale

• Definition: A plain scale can represent only two units of measurement (for example: meters and decimeters, or kilometers and hectometers).
• Construction: It consists of a straight line divided into main units and further subdivided into smaller parts.
• Use: Suitable for readings up to one decimal place.

📌 Example: A plain scale might show meters on the main divisions and decimeters on the subdivisions.

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2️⃣ Diagonal Scale

• Definition: A diagonal scale can represent three units of measurement (for example: meters, decimeters, and centimeters).
• Construction: A rectangle is drawn, divided horizontally into main units, and vertically into subdivisions. Diagonals are drawn across the small divisions, allowing very fine readings.
• Use: Suitable for readings up to two decimal places, hence more precise than a plain scale.

📌 Example: A diagonal scale might show meters, decimeters, and centimeters all together, allowing accurate measurements.

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3️⃣ Comparative Scale

• Definition: Used to compare measurements in different systems of units (e.g., kilometers vs. miles, meters vs. yards).
• Use: Helpful in international or interdisciplinary projects where unit systems differ.

📌 Example: A comparative scale could show kilometers and nautical miles side by side for transport planning.

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4️⃣ Vernier Scale

• Definition: A precise scale that uses a vernier device for measuring up to very fine accuracy.
• Use: Allows readings much smaller than what a plain or diagonal scale can provide (used in instruments like vernier calipers, theodolites, etc.).

📌 Example: In surveying or detailed engineering drawings, a vernier scale helps achieve millimeter-level precision.

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5️⃣ Scale of Chords

• Definition: Used to measure and construct angles in drawings.
• Use: Mostly in geometry and navigation-related drafting.

📌 Example: In absence of a protractor, a scale of chords can construct angles like 30°, 45°, 60°, etc.

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🔑 Key Difference Between Plain & Diagonal Scales

Feature	Plain Scale	Diagonal Scale
Units represented	2 (main unit + subdivision)	3 (main unit + two subdivisions)
Accuracy	Up to 1 decimal place	Up to 2 decimal places
Construction	Simple divisions on a line	Rectangle with diagonals
Use	Quick, less detailed measurements	Precise measurements

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✅ In summary:

• Plain scales → simple, show two units.
• Diagonal scales → more precise, show three units.
• Comparative, vernier, and chord scales → used for specialized needs.