Unit IIInterior Graphics I

Orthographic Projection

Plan, elevation and section — and the first-angle convention.

≈ 55 min + projection platesByAmogh N P· Architect & interior designer

One object, described completely by 2-D views. Parallel projectors striking a plane at 90° give true-shape views — the plan (top), the elevation (front/side) and the section (a cut). The convention that trips everyone is which way the views fall: India and ISO use first-angle, the US uses third — and a small symbol declares which. Reading one system as the other mirrors the object.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Interior Graphics I:

CO2 · Understand

Explain how parallel projectors give true-shape plan, elevation and side views.

CO2 · Apply

Lay out views in the correct first-angle arrangement, with hidden lines.

CO2 · Apply

Draw a section view — cutting plane, arrows and 45° hatching.

CO6 · Understand

Read the projection symbol and scale, and choose the minimum set of views.

Parallel projectors, true shape

The principle & the planes

Views are true shape because the projectors strike the plane at 90°, and are laid out by projecting points between them on the HP and VP.^[1]

DiagramThe reference planes — the Horizontal Plane and Vertical Plane meeting at the XY ground line, object in the first quadrant

DiagramThree orthographic views projected from one another with thin projection lines, in first-angle arrangement

Parallel projectors at 90°

An ORTHOGRAPHIC projection represents a 3-D object by 2-D views made with parallel projectors that strike the projection plane at exactly 90° — no convergence, no distortion, so each view is TRUE SHAPE in its plane. Mutually perpendicular planes give a complete description: the PLAN (top view, looking down), the ELEVATION (front/side, looking horizontally), and the SECTION (a cut view exposing the interior).^[1]

The conventions that decide a reading

First-angle, the symbol & sections

Which system a drawing uses, the truncated-cone symbol that declares it, and how a section cuts through with 45° hatching.^{[1, 2]}

DiagramFirst-angle versus third-angle projection — the same views arranged differently, and the truncated-cone symbol for each

The same views, arranged differently

In FIRST-angle the object sits between the observer and the plane, casting its view onto the plane behind — so the views 'fall away': the plan sits BELOW the front, the left-side view goes to the RIGHT. In THIRD-angle the plane is between observer and object (a glass box unfolded) — views 'fall toward' the observer: plan ABOVE, right-side to the right. INDIA and ISO use FIRST-angle; the USA and Canada use third. Read the drawing's system before you read the drawing.^[1]

DiagramA section view — a cutting plane A-A with direction arrows and 45-degree hatching meaning material cut here

Myth vs reality

At a glance

Aspect	One side	The other
Plan	Myth: the view from the front	Reality: the TOP view (looking down)
First vs third angle	Myth: interchangeable styles	Reality: reading one as the other mirrors it
Which system	India & ISO: first-angle	USA & Canada: third-angle
Hidden lines	Myth: drop them to look clean	Reality: they carry required information
Hatching	Myth: shading / decoration	Reality: 'material was cut here'

Vocabulary

Key terms

Plan

The TOP view — looking straight down. (Not the front view.)

Elevation

The front or side view — looking horizontally at the object.

First-angle projection

The India/ISO system: plan below the front, left-side view to the right.

Projection symbol

The truncated-cone mark in the title block declaring first- or third-angle.

Cutting plane

The chain line (with arrows, labelled A–A) marking where a section is taken.

Hatching

Thin 45° lines on a cut face meaning 'material cut here' — not shading.

Apply it

Projection plate

Given the isometric of a simple hollow solid, draw its front elevation, top plan and one side view in the correct first-angle arrangement, hidden edges dashed, and add the first-angle projection symbol in the title block. Then take a full section A–A through the hollow, with the cutting-plane line, direction arrows and 45° hatching — omitting hidden lines in the section.

Check your understanding

Self-assessment

1. In an orthographic drawing, the 'plan' is the —

2. India uses which projection system, and how is it declared?

3. 45° hatching on a section view means —

In a nutshell

Recap

Orthographic views use parallel projectors at 90°, so each view (plan, elevation, section) is true shape.

Views are laid out by projecting points between them on the HP/VP planes about the XY line.

India and ISO use first-angle (plan below front); the truncated-cone symbol declares the system, and the two are not interchangeable.

A section uses a cutting plane with arrows (A–A) and 45° hatching meaning 'material cut here'; hidden lines are omitted in the section.

Draw the minimum set of views to a stated scale — but always dimension the true size.

The evidence

References & further reading

[1]N.D. Bhatt & V.M. Panchal, Engineering Drawing, Charotar (orthographic and first-angle projection, sections, hatching).
[2]BIS SP 46: Engineering Drawing Practice — projection symbol, sectioning and material-hatching conventions (confirm current edition).
[3]Francis D.K. Ching, Architectural Graphics, Wiley (plan, section and elevation as working drawings).
[4]M.G. Shah, C.M. Kale & S.Y. Patki, Building Drawing, Tata McGraw-Hill (orthographic building drawings in the Indian context).