4

Understanding

Dimensions

4.1 TYPES OF DIMENSIONS.

A dimension is a numerical value expressed in an appropriate unit of measure. It is indicated on draw-

ings along with lines, arrows, symbols, and notes to define the size and specification of an object.

Dimensioning is thus a process of incorporating numerical values onto a drawing to enable the siz-

ing of different elements and the location of parts of a building or object. Drawings should be fully dimen-

sioned so that a minimum of computation is required and all the parts can be built without having to scale

the drawings to determine an object’s size. Duplication of dimensions should whenever possible be

avoided unless it adds clarity. Figure 4.1 shows examples of different types of dimensions.

Distances may be indicated with either of two standardized forms of dimension: linear and ordinate.

Linear Dimensions are used for displaying and measuring length along the X or Y axis. As the name sug-

gests it can only be aligned along the X or Y axis. These dimensions are typically used to show absolute

lengths along the X or Y axis (Figure 4.1A). With linear dimensions, there are two parallel lines, called

“extension lines,” that are spaced at the distance between two features and shown at each of the fea-

tures (Figure 4.2A). The dimension line consists of a line perpendicular to the extension lines; it is shown

between the extension lines and terminates at these lines typically with arrows, slashes, or dots. (For

examples of different endpoints see Figures 4.2A, 4.2B, and 3.8C.) The distance is indicated numeri-

cally at the midpoint of the dimension line, either adjacent to it or in a gap provided for it.

Ordinate dimensions are used for measuring the length along any X or Y axis and displaying length

as a text with the use of a leader. Generally, these dimensions are used to show lengths of entities us-

ing leader lines (Figure 4.1B).

Radial Dimensions are used for measuring the radius of arcs, circles, and ellipses and displaying it

with a leader line. Radial dimensions often use an “R” followed by the value for the radius (Figure 4.3); Di-

ametral dimensions generally use a circle with a forward-leaning diagonal line through it, called the diam-

eter symbol, followed by the value for the diameter. A radially aligned line with arrowhead pointing to the

circular feature, called a leader, is used in conjunction with both diametral and radial dimensions. All types

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