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Exploring Drafting
of “overlays” similar to the drawing sheets
used in manual drafting. Layers are groupings
of objects that can be turned on or off to
distinguish drawing content. See Figure 7-11.
Using layers, several different displays can be
shown within a single drawing fi le. It is very
common, for example, to create separate layers
for visible lines, construction lines, section lines,
text, and dimensions. The display of each layer
can be turned on or off as desired. In this
way, certain portions of the drawing can be
“hidden” while displaying other features.
In architectural projects, different plans are
commonly placed on different layers within a
single drawing fi le. The fl oor plan, foundation
plan, and plumbing plan, for example, may
each be assigned to a separate layer. This
provides a way to plot different displays from
a single drawing.
Layers are typically named to refl ect their
content. For example, all of the dimensions in
a drawing may be assigned to a layer named
Dims. In addition, each layer may be assigned
its own color. In many cases, company or school
standards determine layer naming conventions
and specify how to organize drawing content.
Layers may also be assigned different
linetypes to distinguish content. A linetype is
a setting used to describe a line defi nition in
the Alphabet of Lines. Examples of linetypes
include the Object, Centerline, and Hidden
linetypes. When a line is drawn with a specifi ed
linetype, it has the same characteristics as the
equivalent line in the Alphabet of Lines. Each
linetype may have its own lineweight setting
to refl ect the plotting thickness desired. When
using a plotter with pens, the line thickness is
determined by the size of the plotter pen.
Drawing specifi cations such as layer and
linetype settings should be determined before
starting a project. Saved settings for layers,
linetypes, object snaps, and other CAD functions
can be specifi ed in a drawing template and used
each time a new drawing is started. Templates
and setup commands are discussed later in
this chapter.
Object snap is a function that allows the
cursor to be “snapped” to specifi c locations on
an existing object. It can be set to one or more
modes that determine the type of location.
This is useful for drawing objects using points
on an existing object, such as a line or circle.
For example, the Endpoint object snap mode
allows you to snap the cursor to an endpoint
of an object, such as a line. The Midpoint object
snap mode is used to snap the cursor to the
midpoint of an object. The Center object snap
mode is used to snap the cursor to the center of
a circle or an arc. Object snap can also be used
to draw objects that are parallel, perpendicular,
or tangent to other objects.
Orthogonal mode simplifi es the task of
drawing horizontal and vertical lines. When
orthogonal mode is enabled, the cursor is
confi ned to horizontal and vertical movement
on the drawing plane. Inclined lines cannot
be drawn when using this mode. Orthogonal
mode is useful for drawing lines at 90° angles,
such as the outlines making up a drawing
border.
Direct distance entry is used to specify
accurate distances when drawing lines by
moving the mouse in the desired direction and
entering a measurement value. This function is
commonly used in conjunction with orthogonal
mode to draw horizontal or vertical lines at a
specifi c distance. However, direct distance entry
can also be used to draw a line at a specifi c
distance and angle by using the appropriate
angular entry. This is often faster than using
coordinate values to locate points when drawing
lines.
Layers and Linetypes
In manual drafting, it is common to have
different plan drawings on separate sheets
for complex drawing projects, such as an
architectural building project. When this is the
case, the sheets are overlaid on top of each other
so that the different drawings can be viewed
separately. In CAD drafting, drawings can be
managed in a similar way through the use of
layers. Layers can be thought of as a system
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