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someone (I sincerely hope it's not you!) who doesn't use precision techniques when cre-
ating drawings in AutoCAD.
Controlling Your Precision
In AutoCAD, lack of precision makes later editing, hatching, and dimensioning tasks
much more difficult and time consuming. Keep these facts in mind:
Small errors in precision in the early stages of creating or editing a drawing often
have a big effect on productivity and precision later.
Drawings may guide manufacturing and construction projects; drawing data may
drive automatic manufacturing machinery. Huge amounts of money and even lives
can ride on a drawing's precision.
In recognition of these facts, a passion for precision permeates the profession. Precision
is one of the characteristics that separates CAD from ordinary illustration-type drawing
work. The sooner you get fussy about precision in AutoCAD, the happier everyone is.
CAD precision versus accuracy
You often hear the words precision and accuracy used interchangeably, but it's useful to understand
the difference. In this topic, I use precision to mean controlling the placement of objects so they lie ex-
actly where you want them to lie in the drawing. For example, lines whose endpoints meet must meet
exactly, and a circle that's supposed to be centered on the coordinates 0,0 must be drawn with its cen-
ter exactly at 0,0. I use accuracy to refer to the degree to which your drawing matches its real-world
counterpart. An accurate floor plan is one in which the dimensions of the CAD objects equal the di-
mensions of the as-built house. In a sense, then, it's not the drawing that should be accurate — it's the
house!
CAD precision usually helps produce accurate drawings, but that's not always the case. You can pro-
duce a precise CAD drawing that's inaccurate because you started from inaccurate information (for
example, the contractor gave you a wrong field measurement). Or you might deliberately exaggerate
certain distances to convey the relationship between objects more clearly on the plotted drawing. Even
where you must sacrifice accuracy, aim for precision.
When I talk about drawing things precisely, I mean using precision techniques and tools
to specify points and distances with as much exactness as the program allows. Luckily,
AutoCAD provides a comprehensive package of tools for doing just that. Table 7-1 lists
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