Multiview orthographic projection
Symbols used to define whether a
projection is either First Angle (left) or Third Angle (right).
In technical drawing
and computer graphics, a multiview orthographic projection is an
illustration technique in which up to six pictures of an object are produced,
with each projection plane parallel to one of the coordinate axes of the
object.
The views are positioned relative to
each other according to either of two schemes: first-angle or third-angle
projection. In each, the appearances of views may be thought of as being projected
onto planes that form a 6-sided box around the object.
Overview
Quadrants in descriptive geometry
Modern orthographic projection is derived from Gaspard Monge's descriptive geometry. Monge defined a reference system of two viewing planes, horizontal H ("ground") and vertical V ("backdrop"). These two planes intersect to partition 3D space into 4 quadrants, which he labeled:- I: above H, in front of V
- II: above H, behind V
- III: below H, behind V
- IV: below H, in front of V
The 3D object of interest is then placed into either quadrant I or III (equivalently, the position of the intersection line between the two planes is shifted), obtaining first- and third-angle projections, respectively. Quadrants II and IV are also mathematically valid, but their use would result in one view "true" and the other view "flipped" by 180° through its vertical centerline, which is too confusing for technical drawings. (In cases where such a view is useful, e.g. a ceiling viewed from above, a reflected view is used, which is a mirror image of the true orthographic view.)
Monge's original formulation uses two planes only, and obtains the top and front views only. The addition of a third plane to show a side view (either left or right) is a modern extension. The terminology of quadrant is a mild anachronism, as a modern orthographic projection with three views corresponds more precisely to an octant of 3D space.
First-angle projection
In first-angle projection, the object is conceptually located in quadrant I, i.e. it floats above and before the viewing planes, the planes are opaque, and each view is pushed through the object onto the plane furthest from it. (Mnemonic: an "actor on a stage".) Extending to the 6-sided box, each view of the object is projected in the direction (sense) of sight of the object, onto the (opaque) interior walls of the box; that is, each view of the object is drawn on the opposite side of the box. A two-dimensional representation of the object is then created by "unfolding" the box, to view all of the interior walls. This produces two plans and four elevations. A simpler way to visualize this is to place the object on top of an upside-down bowl. Sliding the object down the right edge of the bowl reveals the right side view.Third-angle projection
In third-angle projection, the object is conceptually located in quadrant III, i.e. it is positioned below and behind the viewing planes, the planes are transparent, and each view is pulled onto the plane closest to it. (Mnemonic: a "shark in a tank", esp. that is sunken into the floor.) Using the 6-sided viewing box, each view of the object is projected opposite to the direction (sense) of sight, onto the (transparent) exterior walls of the box; that is, each view of the object is drawn on the same side of the box. The box is then unfolded to view all of its exterior walls. A simpler way to visualize this is to place the object in the bottom of a bowl. Sliding the object up the right edge of the bowl reveals the right side view.Here is the construction of third angle projections of the same object as above. Note that the individual views are the same, just arranged differently.
Additional information
First-angle projection is as if the object were sitting on the paper and, from the "face" (front) view, it is rolled to the right to show the left side or rolled up to show its bottom. It is standard throughout Europe and Asia (excluding Japan). First-angle projection was widely used in the UK, but during World War II, British drawings sent to be manufactured in the USA, had to be drawn in third-angle projection. This historical position of the UK means that some British companies completely adopted third angle projection. BS 308 (Part 1) Engineering Drawing Practice, gave the option of using both projections, but generally every illustration (other than the ones explaining the the difference between first and third-angle) were done in first-angle. After the withdrawal of BS 308 in 1999, BS 8888 offered the same choice since it referred directly to ISO 5456-2, Technical drawings – Projection methods – Part 2: Orthographic representations.Third-angle is as if the object were a box to be unfolded. If we unfold the box so that the front view is in the center of the two arms, then the top view is above it, the bottom view is below it, the left view is to the left, and the right view is to the right. It is standard in the USA (ASME Y14.3-2003 specifies it as the default projection system), Japan (JIS B 0001:2010 specifies it as the default projection system), Canada, and Australia.
Both first-angle and third-angle projections result in the same 6 views; the difference between them is the arrangement of these views around the box.
A great deal of confusion has ensued in drafting rooms and engineering departments when drawings are transferred from one convention to another. On engineering drawings, the projection angle is denoted by an international symbol consisting of a truncated cone, respectively for first-angle and third-angle:
The 3D interpretation of the symbol can be deduced by envisioning a solid truncated cone, standing upright with its large end on the floor and the small end upward. The top view is therefore two concentric circles ("donut"). In particular, the fact that the inner circle is drawn with a solid line instead of dashed disambiguates this view as the top view, not the bottom view.
- In first-angle projection, the "top" view is pushed down to the floor, and the "front" view is pushed back to the rear wall; the intersection line between these two planes is therefore closest to the large end of the cone, hence the first-angle symbol shows the cone with its large end open toward the donut.
- In third-angle projection, the "top" view is pulled up to the ceiling, and the "front" view is pulled forward to the front wall; the intersection line between the two planes is thus closest to the small end of the cone, hence the third-angle symbol shows the cone with its large end away from the donut.
Multiviews without rotation
Orthographic multiview projection is derived from the principles of descriptive geometry and may produce an image of a specified, imaginary object as viewed from any direction of space. Orthographic projection is distinguished by parallel projectors emanating from all points of the imaged object and which intersect a plane of projection at right angles. Above, a technique is described that obtains varying views by projecting images after the object is rotated to a desired position.Descriptive geometry customarily relies on obtaining various views by imagining an object to be stationary, and changing the direction of projection (viewing) in order to obtain the desired view.
Views
Section
A section, or cross-section, is a view of a 3-dimensional object from the position of a plane through the object.A cross section is a common method of depicting the internal arrangement of a 3-dimensional object in two dimensions. It is often used in technical drawing and is traditionally crosshatched. The style of crosshatching indicates the type of material the section passes through.
With computed axial tomography, computers construct cross-sections from x-ray data.
Elevation
An elevation is a view of a 3-dimensional object from the position of a horizontal plane beside an object. In other words, an elevation is a side-view as viewed from the front, back, left or right (and referred to as a front elevation, [left/ right] side elevation, and a rear elevation). It is the corollary to the concept of a "view" (which is always overhead and is therefore referred to as an overhead view).An elevation is a common method of depicting the external configuration and detailing of a 3-dimensional object in two dimensions. Building façades are shown as elevations in architectural drawings and technical drawings.
Elevations are the most common orthographic projection for conveying the appearance of a building from the exterior. Perspectives are also commonly used for this purpose. A building elevation is typically labeled in relation to the compass direction it faces; the direction from which a person views it. E.g. the North Elevation of a building is the side that most closely faces true north on the compass.
Interior elevations are used to show detailing such as millwork and trim configurations.
In the building industry elevations are a non-perspective view of the structure. These are drawn to scale so that measurements can be taken for any aspect necessary. Drawing sets include front, rear and both side elevations. The elevations specify the composition of the different facades of the building, including ridge heights, the positioning of the final fall of the land, exterior finishes, roof pitches and other architectural details.
Developed Elevation
A developed elevation is a variant of a regular elevation view in which several adjacent non-parallel sides may be shown together, as if they have been unfolded. For example, the north and west views may be shown side-by-side, sharing an edge, even though this does not represent a proper orthographic projection.Plan
A plan is a view of a 3-dimensional object from the position of a horizontal plane through, above, or below the object. In such views, the portion of the object in front of the plane is omitted to reveal what lies beyond. In the case of a floor plan, the roof and upper portion of the walls may be omitted. Elevations, top (roof) plans, and bottom plans are orthographic projections, but they are not sections as their viewing plane is outside of the object.A plan is a common method of depicting the internal arrangement of a 3-dimensional object in two dimensions. It is often used in technical drawing and is traditionally cross-hatched. The style of crosshatching indicates the type of material the section passes through.
Auxiliary view
An auxiliary view is a view taken from an angle that is not one of the primary views. An auxiliary view is a view at an angle used to give deeper insight into the actual shape of the object. An auxiliary view is used to show a slanted surface in true size and shape. This is accomplished by providing a view that is perpendicular to the slanted surface.SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
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