Projection Methods and Dimension Types¶

In one sentence: Understanding first-angle vs third-angle projection determines where views appear on a multi-view drawing; understanding projected vs 3-D referenced dimensions determines whether a measurement is geometrically correct or only visually approximate.

Orthographic projection¶

An orthographic view is a 2-D projection of a 3-D object — every line of sight is parallel to the projection direction, so there is no perspective foreshortening. Each face of the object is seen at true shape when the projection direction is perpendicular to that face.

A standard engineering drawing uses multiple orthographic views — typically front, top, and right — to fully describe a 3-D object. The views are arranged on the page according to a projection convention.

First-angle vs third-angle projection¶

The two conventions differ in which quadrant the object is projected into. The practical effect is where each view lands on the page.

First-angle projection (ISO / European)¶

The object is between the viewer and the projection plane. The projected image lands on the opposite side from the viewer.

Viewer  →  Object  →  Projection plane

View arrangement:

                  [Left view]
[Top view]  [Front view]  [Right view]  [Back view]
                  [Bottom view]

In first angle: the top view is below the front view; the right view is to the left of the front view. This is the opposite of what intuition might suggest.

Third-angle projection (ANSI / American)¶

The projection plane is between the viewer and the object. The image lands on the same side as the viewer.

Viewer  →  Projection plane  →  Object

View arrangement:

                  [Top view]
[Left view]  [Front view]  [Right view]  [Back view]
                  [Bottom view]

In third angle: the top view is above the front view; the right view is to the right of the front view — matching natural intuition.

Comparison table¶

	First angle (ISO)	Third angle (ANSI)
Standard	ISO 128-30	ANSI Y14.3
Common in	Europe, Asia, international	USA, Canada, Mexico
Top view position	Below front	Above front
Right view position	Left of front	Right of front
Projection symbol	●→○ (filled cone toward circle)	○→● (circle toward filled cone)

The projection symbol (a truncated cone drawn in the title block) tells the reader which convention is used. Always include it on drawings that will be read internationally.

Setting the projection in TechDraw¶

Set the projection type in one of two places:

Per Projection Group: In the Projection Group's Data panel, change Projection Type to First Angle or Third Angle.

Document default: Edit → Preferences → TechDraw → General → Projection Angle

When you change the Projection Type on an existing group, all dependent views automatically reposition to the new arrangement.

Recommended view sets¶

Most engineering drawings use a subset of the six possible views. Choose the minimum number that unambiguously describes the part:

Part type	Typical view set
Prismatic / block	Front + Top + Right (3 views)
Symmetric / turned part	Front + Right only (2 views)
Flat plate	Top + section (2 views)
Complex casting	Front + Top + Right + 1–2 sections
Sheet metal	Front + unfolded flat pattern

Add an isometric view for assembly instructions or 3-D context — but note that isometric views are not suitable for dimensioning.

Projected vs 3-D referenced dimensions¶

This is the most important dimension concept in TechDraw, and the most common source of incorrect drawing measurements.

Projected dimensions (2-D)¶

When you dimension by selecting edges in the drawing view (2-D geometry), TechDraw measures the projected length on the drawing plane — the foreshortened distance as seen in the view, not the true 3-D distance.

For edges that are parallel to the drawing plane, projected length equals true length — the measurement is correct. For edges that are angled toward or away from the viewer, projected length is shorter than the true 3-D length — the measurement is wrong.

3-D referenced dimensions (true distance)¶

When you dimension by selecting 3-D vertices (via Shift-click to access the 3-D geometry), TechDraw measures the true 3-D distance between those vertices — regardless of the view angle or foreshortening.

This is the correct approach for all dimensions that must be manufacturable.

How to tell which type you have¶

Projected: the dimension shows the 2-D distance on the drawing plane. It can change if you rotate the view.
3-D referenced: the dimension shows the true 3-D distance. It does not change if you rotate the view. A small (∅) or ☞ annotation may appear on the dimension line.

Practical rules¶

Situation	Use
Edge is parallel to the view plane	Either type — they give the same result
Edge is perpendicular to the view plane (foreshortened)	3-D referenced only
Diagonal edge at angle to the view plane	3-D referenced only
Circle / arc in correct view	Projected diameter/radius — fine
Axonometric / isometric view	Use Axonometric Length Dimension

Projected dimensions on angled edges are misleading

A wall at 45° that appears 70.7 mm long in a top view is actually 100 mm long. Never use projected dimensions for edges that are not parallel to the drawing plane — the dimension will be wrong.

Use Insert Dimension (smart) for correct type selection

The Insert Dimension tool analyses the selected geometry and picks the right type automatically. For explicit control, hold Shift to access 3-D vertices when placing a dimension.

Broken views and partial views¶

For long or repetitive parts, use:

Broken View — removes a section of the middle of a long part; the break lines indicate that the part continues.
Detail View — magnified inset of a small region at a larger scale.
Section View — cross-section cut at a specified plane to reveal internal geometry.

Python API¶

import FreeCAD as App
import TechDraw

doc = App.ActiveDocument
page = doc.getObject("Page")
body = doc.getObject("Body")

# Create a projection group (third-angle)
group = doc.addObject("TechDraw::DrawProjGroup", "Views")
page.addView(group)
group.Source = [body]
group.ProjectionType = "Third Angle"   # or "First Angle"
group.Scale = 1.0

# Add front, top, and right views
group.addProjection("Front")
group.addProjection("Top")
group.addProjection("Right")
group.Anchor.Direction = App.Vector(0, -1, 0)   # Front looks along -Y
doc.recompute()

# Add a 3-D referenced length dimension (true distance)
dim = doc.addObject("TechDraw::DrawViewDimension", "TrueLength")
dim.Type = "Distance"
# References2D references 3-D vertices via "Vertex0" etc. (true 3-D)
view = doc.getObject("Views_Front")
dim.References2D = [(view, "Vertex0"), (view, "Vertex3")]
page.addView(dim)
doc.recompute()