How does third-angle orthographic projection communicate the exact shape and size of a component to a manufacturer?
Orthographic projection in third-angle: the six principal views, the front elevation, plan and end elevation, how they line up and project, and the use of the projection symbol and auxiliary views for complex features.
An SQA Higher Graphic Communication answer on orthographic projection, covering third-angle projection, the front elevation, plan and end elevation, how the views project and line up, the third-angle symbol, and auxiliary views for sloping faces.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this key area is asking
The SQA wants you to read and produce an orthographic drawing in third-angle projection: the front elevation, the plan and the end elevation, how they project and line up, the third-angle symbol, and the use of an auxiliary view for a sloping face. At Higher the components are more complex than at National 5, so accurate projection and view choice matter more.
The principal views and third-angle layout
The three views you use most are:
- The front elevation (the master view), the most informative view, chosen so it shows the object clearly.
- The plan, the view looking down from above. In third angle it is placed below the front elevation.
- The end elevation (side view), placed to the side the object is viewed from.
A full set has six principal views (front, rear, both ends, plan and bottom), but a drawing uses only the minimum number needed to describe the object without ambiguity.
How the views project and line up
This projection discipline is what makes the drawing reliable: a manufacturer can pick any feature in one view and find it in the others. Centre lines, hidden detail and outlines (covered in the line-types page) carry across the views too.
The projection symbol
A drawing states its convention with a projection symbol placed in or near the title block. The third-angle symbol is a small drawing of a truncated cone with its two end circles, arranged as they would appear in third angle (the larger circle on the left). It removes any doubt about whether a view is to the left or right, which matters because the same object drawn in first angle would put the plan above and the end elevation on the opposite side.
Auxiliary views for sloping faces
At Higher many components have inclined (sloping) faces. In the front, plan and end views a sloping face is foreshortened, so its true shape and sizes cannot be read.
Worked example
Examples in context
Orthographic third-angle drawings are the standard production drawing in UK engineering and manufacturing. A machinist or CNC programmer reads them to make a part, so the projection must be unambiguous. The same discipline appears in the related building drawings, where plans and elevations of a building line up in exactly the same way.
Try this
Q1. In third-angle projection, where is the plan placed relative to the front elevation? [1 mark]
- Cue. Directly below the front elevation.
Q2. State which dimension is shared between the plan and the end elevation, and how it is transferred. [2 marks]
- Cue. Depth; transferred with a 45 degree mitre line (or compasses).
Q3. State why an auxiliary view is used. [1 mark]
- Cue. To show the true shape and size of a sloping (inclined) face that the principal views foreshorten.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher (style)4 marksA component has a front elevation, a plan and an end elevation. Describe how these three views line up in a third-angle orthographic drawing and how their dimensions relate.Show worked answer →
In third-angle projection the views are arranged as if each view is folded out onto a sheet placed in front of the face it shows, so each view sits on the same side as the face you look at.
The front elevation is the master view. The plan is placed directly below the front elevation (in third angle the plan goes below, not above), so widths line up vertically: any width on the plan is directly below the same width on the front elevation. The end elevation is placed to the side of the front elevation, on the side from which it is viewed, so heights line up horizontally: any height on the end elevation is at the same level as the same height on the front elevation.
The shared dimensions therefore project across: width is common to the front elevation and the plan, height is common to the front elevation and the end elevation, and depth is common to the plan and the end elevation (transferred with a 45 degree mitre line or compass).
Markers reward: plan below the front elevation, end elevation to the side, widths projecting vertically, heights projecting horizontally, and depth shared between plan and end elevation.
SQA Higher (style)3 marksExplain why an auxiliary view is sometimes added to an orthographic drawing, and how it is produced.Show worked answer →
An auxiliary view is added when a component has a sloping (inclined) face. In the standard front, plan and end views that face appears foreshortened, so its true shape and true sizes cannot be seen or dimensioned accurately.
The auxiliary view is projected at right angles to the sloping face (not from one of the three principal directions). Projection lines are drawn perpendicular to the inclined surface, and true distances are transferred from a related principal view, so the sloping face is drawn at its true shape and size.
This lets the manufacturer read and dimension features on the slope correctly, which would otherwise be distorted.
Markers reward: auxiliary view shows the true shape of a sloping/inclined face, projected perpendicular to that face, with true sizes transferred so the slope is not foreshortened.
Related dot points
- British Standards (BS 8888) line types and conventions: continuous thick outlines, thin lines for dimensions and projection, dashed hidden detail, chain centre lines, cutting planes and the conventional representation of repeated features.
An SQA Higher Graphic Communication answer on British Standards line types and conventions, covering continuous thick outlines, thin dimension and projection lines, dashed hidden detail, chain centre and cutting-plane lines, and conventional representations under BS 8888.
- Dimensioning and tolerances: the rules for dimension and projection lines, leaders and arrowheads, dimensioning circles, radii, diameters and angles, datum and chain dimensioning, and stating tolerances (limits, bilateral and unilateral).
An SQA Higher Graphic Communication answer on dimensioning and tolerances, covering dimension and projection lines, arrowheads and leaders, dimensioning diameters, radii and angles, datum versus chain dimensioning, and stating tolerances as limits.
- Sectional views: the cutting plane and section labelling, hatching at 45 degrees, the half section and revolved/removed sections, and the parts conventionally left unsectioned (shafts, fasteners, ribs and webs).
An SQA Higher Graphic Communication answer on sectional drawings, covering the cutting plane and labelling, hatching at 45 degrees, half sections and removed sections, and the parts conventionally left unsectioned such as shafts, bolts and ribs.
- Assembly and production drawings: the assembly (and exploded) view, item numbers and the parts list, the title block and scale, and the difference between an assembly drawing and a single-part (detail) drawing.
An SQA Higher Graphic Communication answer on assembly and production drawings, covering assembly and exploded views, item numbers and the parts list, the title block, scale, and the difference between assembly and detail drawings.
- Building (architectural) drawings: the site plan, floor plan, elevations and sections, common scales, and the British Standard building symbols for doors, windows, sanitary fittings and services.
An SQA Higher Graphic Communication answer on building drawings and symbols, covering site plans, floor plans, elevations and sections, the common architectural scales, and the British Standard symbols for doors, windows, sanitary ware and services.