Definition & types of technical drawing

1.0 Definition of technical drawing

Technical drawing can be defined as the graphic representation of an object, concept or idea using a universal language consisting of graphic symbols produced with the aid of drawing equipment/tools that can be used to measure straight and curved lines according to specified dimensions, scales and codes of practice.

Technical drawing is used in many professions to draft ideas and different views of physical objects like drainages, culverts, septic tanks, incinerators, houses, etc. Drawing—either artistic or technical—is one of the oldest forms of communication and is believed to be older than verbal communication. Generally, there are two types of drawings: artistic drawing, and technical drawing:

Artistic Drawing

Artistic drawing is a type of drawing that is abstract because its meaning is unique to the person/artist who created it. In order to understand the meaning of an artistic drawing, one has to understand the artist’s point of view or motivation for producing a specific artistic drawing.

Sometimes, it is necessary to understand an artist in order to understand their artistic drawing because artists often take a unique/abstract approach when communicating through their drawings. This type of approach gives rise to various interpretations when their drawings are exposed to the public.

Irrespective of how complex artistic drawings may appear, they express clear feelings, beliefs, philosophies and ideas of the artists who created them. Artistic drawings are generally freehand drawings or drawings made without the use of drawing instruments/tools.

Technical Drawing

Technical drawing is the type of drawing that is not abstract because it doesn’t require an understanding of what its creator has in mind; rather, it requires an understanding that can only be gained by studying and using universally accepted tools, codes and conventions applicable to technical drawing.

In addition to the previously stated definition on technical drawing, we can say that technical drawing clearly, precisely and concisely communicates all important information conveyed by an idea produced in graphic form by the use of universally accepted codes of practice, tools, dimensions, notes, symbols, and specifications.

Technical drawing can be done manually, or with the use of computers. When any idea or object is drawn/drafted on computer, it is called “computer aided drafting”: CAD. One major advantage of using CAD is that revisions can be easily and speedily carried out on any draft.

Any student, architect, engineer, etc., must understand the theory behind projections, dimensioning, and conventions if they hope to become proficient in drafting and interpreting drafts. It’s very important for people to be trained in manual or traditional drafting before they are exposed to CAD softwares, because the former would make it easier for them to use computers in drafting.

2.0 Types of technical drawing

Technical drawings are constructed on the basis of fundamental principles of projection. There are two main types of technical drawing or projection: parallel projection and perspective projection. (Note that each projection has various categories that will be illustrated as you read further below.)

A projection is any drawing, draft or representation of an idea or object after considering views from various imaginary planes. Projections, which are quite similar to the direct views that one can see on televisions, can be used to represent actual objects if the following are employed:

  • the eye of the viewer looking at the object
  • an imaginary plane of projection as dictated by the direction of the eye(s) of the viewer
  • projectors or imaginary lines of sight.

The theories behind projection have been widely used to draft 3-dimensional objects on 2-dimensional media such as papers and computer screens. The theory of projection is based on two variables:

  • line of sight
  • plane of projection: plane from which images can be projected—depending on the axis.
Figure 1: Lines of sight: parallel and perspective projections

Figure 1: Lines of sight: parallel and perspective projections

Chapter 1_Figure_2_ planes of projection for parallel and perspective projectionssss.jpg

Figure 2: Planes of projection: parallel & perspective projections

2.1   Parallel Projection

Parallel projection is the type of projection in which the lines of sight or projectors are parallel to each other, but perpendicular to the planes of objects or images. Parallel projection can be categorized or divided into orthographic, oblique, and axonometric projections.

(1) Orthographic projection

Orthographic projection (or drawing) is the type of projection in which 3-dimensional objects are represented in 2 dimensions by projecting planes (consisting of 2 major axes) of objects so that they are parallel with the plane of the media they are projected on.

Orthographic projection can also be defined as the type of projection in which views are taken on different planes of objects and drawn (or represented) in 2 dimensions as illustrated by the principal views shown in the figures below:

Chapter 1_Figure_3_three major views of orthographic projection.jpg

Figure 3: Three major views of orthographic projection

Chapter 1_Figure_4_six general views of orthographic projection.jpg

Figure 4: Six general views of orthographic projection

There are two types of orthographic projection: first angle projection, and third angle projection:

In first angle projection (i.e., European/international system) the front view is placed at the top of a medium (paper, computer screen, etc.) along with the right side view which is placed at the left side of the front view, and the left side view which is placed at the right side of the front view, while the plan is placed by itself beneath the front view.

In third angle projection (i.e., American system) the plan is placed by itself at the top, while the front view is placed beneath the plan, and the right side view is placed at the right side of the front view, while the left side view is placed at the left side of the front view. (Note that we will be employing the use of third angle projection.)

Chapter 1_Figure_5_first and third angle projections

Figure 5: First angle and third angle projections

(2) Oblique projection

Oblique projection is the type of projection in which any object is drawn in 3 dimensions, with each of the 3 dimensions (or major planes) consisting of two lines (or major axes: either xy, or yz, or xz) perpendicular to each other (i.e. 90°), and one of the 3 planes parallel to the plane of paper or computer screen, etc.

In addition, one of the 3 planes is projected at either 30°, 45° or 60° to the x axis. Oblique projection is of 2 types: cavalier and cabinet projections.

Chapter 1_Figure_6_oblique_cavalier_cabinet projections.jpg

Figure 6: Oblique projection: cavalier and cabinet projection

In cavalier projection, one of the 3 planes is drafted to represent a plane of an object “according to a given scale”, while in cabinet projection, one of the 3 planes is drafted to represent half of a plane of an object “according to half of a given scale”. A scale is any ratio (examples: 1:10, 1:100, 1:1000, etc.) of the size of an object on paper to the actual size of the same object in real life.

Chapter 1_Figure_7_oblique projection with orthographic views.jpg

Figure 7: Oblique projection with orthographic views

(3)   Axonometric Projection

Axonometric projection is the type of projection that consists of three-dimensional drawings in which each of the 3 major axes (x, y, and z) of an object is drawn perpendicular to each other by either 30°, 45°, or 60°, and no plane of the object is drawn parallel to the plane of media—paper, computer screen, etc. Axonometric projection/drawing can be categorized into three types: isometric, dimetric and trimetric projections.

Isometric projection is a method of projection/drawing in which the edges of 3-dimensional objects are represented by 3 axes perpendicular to each other and inclined to each other by 120° on the plane of media—paper or computer; also, 2 of the 3 axes are at 30° to any imaginary x axis on any media.

In dimetric projection, 2 angles between any 2 major axes are unequal, while in trimetric projection, the 3 angles between the 3 major axes are unequal. Two different angles are required to construct 2 planes of objects in dimetric projections, while 3 different angles are required to construct 3 planes of objects in trimetric projections.

Chapter 1_Figure_8_Axonometric projection_Isometric_dimetric_trimetric

Figure 8: Isometric, dimetric and trimetric projections

2.2   Perspective Projection

Perspective projection is the type of projection in which objects appear smaller as their distances from an observer increases: objects dimensions along a line of sight appear shorter than they actually are.

There are 3 types of perspective projections: 1 point, 2 point, and 3 point projections. One point perspective projections consists of 1 vanishing point, while 2 point and 3 point perspective projections consist of 2 and 3 vanishing points, respectively.

A vanishing point is a point of convergence where all lines of sight meet.

Chapter 1_Figure_9_one point perspective projection

Figure 9: One point perspective projection

Chapter 1_Figure_10_two point perspective projection

Figure 10: Two point perspective projection

Chapter 1_Figure_11_three point perspective projection

Figure 11: Three point perspective projection

3.0 Objectives of technical drawing

The general objectives of studying technical drawing include the following:

  • to develop skills in using universally accepted tools, symbols, scales and conventions to draw any visible object or invisible idea on paper, and computer.
  • to understand orthographic and isometric projections and employ it in drafting/drawing ideas and objects using both projections, respectively.
  • to understand and interpret technical drawings, sketches, and working drawings.
  • to develop the ability to use imagination to observe, visualize and draft objects, ideas or concepts.
  • to develop the ability to produce clean, accurate, neat, and informative drawings in a moderate amount of time.
  • to develop the ability to take on any projects and draw environmental health science, civil and environmental engineering objects/structures.

4.0 Purpose of technical drawing

To draft and design objects or structures, and assess how they would appear in real life after they are manufactured, fabricated, assembled, constructed or built. For example, houses, septic tanks, drainages, etc., must be designed and assessed before they are built.

5.0 Application of technical drawing

Technical drawings have wide applications in any field in which planning and designing are required, such as architecture, manufacturing, engineering, construction, estate management, etc.

Sanitarians, surveyors, and civil/environmental engineers use technical drawings to supervise construction of layouts, structures, objects, and boundaries for various types of properties (houses, etc.).

Technical drawings are also used in situations where ideas/designs for objects and structures need to be refined, and different 2-dimensional views need to be assembled into 3-dimensional views.

Generally, technical drawings are used by a variety of professions, including but not limited to:

  • engineers
  • architects
  • contractors
  • inventors
  • technicians
  • teachers
  • etc.

If you desire to develop your imagination (visualization) and be good at technical drawing, then you must view and study different shapes and sizes of objects in 2 and 3 dimensions respectively.

For further study of various kinds of objects, click the link below and open a page of images on a new tab:

Images of objects projected in 2 and 3 dimensions



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