WHAT IS IN AN AUTOCAD DRAWING

An AutoCAD drawing is made up of entities. These can be either simple graphic primitives (such as lines, arcs, circles, text, and so on) or blocks (which are groups of entities).

The graphic primitives are defined geometrically in terms of the normal cartesian coordinate system (right-handed system with positive X-axis to the right, positive Y-axis up the screen and positive Z-axis coming out of the screen towards the user). Hence, for example, lines are defined by their end- point coordinates (x, y and z), while circles are defined by their centre coordinates and radius. Each entity also has certain attributes associated with it, such as line style, text font or colour.

A block is a group of entities that can be manipulated as a single unit. Once created, a block may be moved, scaled, rotated, copied or deleted. A block can be created by collecting together a group of entities from the current drawing (the drawing being currently edited) and assigning a single name to that group. Alternatively, an existing drawing (from disk) can be inserted into the current drawing as a block. Equally, a block from the current drawing can be written out to a file as a new drawing.

It is important to understand that a block, in AutoCAD, is unique to a specific drawing. That is, when an existing drawing is inserted into the current drawing as a block, AutoCAD simply copies the graphics from that drawing and includes it as a single object. The existing drawing is unaffected by the action. Indeed, if that existing drawing (from which the block was created) were subsequently edited then those changes would not affect the inserted block. It is possible, however, to update a block definition in a drawing by having AutoCAD redefine the block using the current version of the original drawing.

As an alternative to inserting a drawing into another drawing as a block, AutoCAD now provides the facility to attach a drawing to the current drawing as an external reference. The external drawing is then treated like a block except that it is automatically updated each time the drawing to which it is attached is loaded. Naturally, if the external drawing is altered in the meantime, then the external reference will be updated to reflect those changes.

Another important feature of a block (including external reference blocks) is that it can be duplicated many times within a drawing, with each copy pointing to the one graphic description, but having its own unique position, scale and rotation factor. This is known as instancing. There are two advantages to this approach:

space saving - the graphics description is only stored once; and ease of re-definition - the graphics description of a block can be altered and then re-defined in order to update each instance in the drawing.

It is also possible to associate named attributes with a block. For example, a block representing a door might have attributes such as type, manufacturer, finish, lock-type, etc. Each instance of the block would have a value associated with it for each defined attribute. AutoCAD provides facilities for scheduling the attribute data associated with inserted blocks in a drawing.

The final concept to be explained about blocks is the notion of exploding. Any instance of a block may be exploded in order to reduce it back to its separate graphic entities. Similarly, an external reference block can be unbound so that it becomes a local block and then, in turn, can be exploded. Units, scales and paper sizes.

A clear understanding of the way that AutoCAD handles units of measurement, scale and paper size is necessary before a drawing can be created or edited. AutoCAD provides complete control over these things by distinguishing between model space and paper space. All drawings, whether two-dimensional or three-dimensional should be thought of as models of a real-world entity (either a drawing or a 3D description of a real-world object). In either case, the description is maintained in model space. Paper space can be thought of as a model of a standard-size piece of paper on which you can establish viewports (rectangular regions) in which AutoCAD will display scaled views of the drawing you have created in model space. You can, of course, add additional linework in paper space to form borders and annotation.

In order to understand this, it is best to picture model space as a very large piece of paper located somewhere in two-dimensional space. (AutoCAD can actually draw anywhere in 3D space, but let's ignore that for the moment!) Since, the size and location of that piece of paper is entirely up to you, it would make sense to choose something that makes drawing as simple as possible. Now, Australian architects typically use millimetres as their unit of measurement and would draw objects in their true size. This means that when drawing a plan of a building that is, say, 15 metres square, the "paper" size (in model space) would be set at something greater than 15,000 x 15,000 units (where "units" equals millimetres). Strictly speaking, it is not necessary to set the model space drawing size, but it is helpful to do so in order to establish a context for the drawing.

When working on the drawing in model space, you should think of the graphics screen as a window through which you can view all or part of that drawing sheet. Thus, when viewing a drawing, a larger display scale will allow you to view only a part of the drawing on the screen. This is referred to as zooming in on the drawing: as you increase the scale, you can see less of your drawing, but what you can see will be in greater detail. If you zoom out, then you will be able to see more of the drawing, but in lesser detail. AutoCAD provides some fairly sophisticated tools for zooming in and out of your drawing and for panning back and forth across it. These are described later.

Although it is possible to plot from model space, it is normal to set up a sheet in paper space, and create one or more viewports showing different views of the drawing in model space. Each view is displayed at a fixed scale, and the paper space drawing is then plotted at a scale of 1:1. In this case, text notes and other annotation on the drawing is best added in paper space, and plotted at its actual size. Any text that is entered in model space has to be drawn at an artificially large size so that when it is scaled down for plotting, it ends up at the size required. For example, text on a 1:100 drawing might be set at 250 millimetres high in model space to create plotted text that is 2.5mm high on the plot.