AutoCAD was initially developed in the early 1980's by Autodesk Inc., Sausalito, California and up until Release 10, it was essentially a two-dimensional drawing system. Following that release, it now supports a full three-dimensional database. A short section in these notes explain how the 3D features have been incorporated into AutoCAD, but provide little detail of the 3D operations. This tutorial is focused on the 2D drafting aspects of AutoCAD.

In order to understand AutoCAD,
we should focus on a few specific concepts.

Vector files store an image by mathematically defining the objects that make up the image. A line, for example, is defined and displayed using its two end points. A circle is defined only by a center point and a radius. Vector file formats can store large quantities of drawing file information at practically infinite resolution. AutoCAD stores drawings in a vector format by default, because vector files are smaller, more accurate and more flexible when compared to raster files.

Raster files store images as a series of pixel color and locations. The color of each point on the picture is specified using an 8, 16 or 32 character value. As the picture grows larger, so does the file size required to store all the characters representing the color of each pixel. Raster files are useful for storing images that would be difficult or time consuming to convert into a vector format, like photographs, scanned in maps or corporate logos.

Recent AutoCAD releases allow you to place, edit and plot raster images in vector drawings. For example, civil engineers, site planners and architects can use AutoCAD's raster tools to combine raster images of aerial photographs of locations, tax and planning maps with their vector drawing files to analyze and communicate how their designs will work in the real world.

positive angle direction

The origin of the coordinate system, or it�s 0,0 point, is located in the lower left corner of your screen. All coordinate points that you place in a drawing are related to the origin. In architectural drafting, you can enter points using four different coordinate methods.

Absolute rectangular coordinates define the location of a point by stating its position along two axes in relation to the origin. You enter absolute rectangular coordinates by typing the x and y location separated by a comma.

Relative rectangular coordinates define a point in relation to the previous point. You enter relative rectangular coordinates by typing an @ sign followed by the x and y distance from the previous point.

Absolute polar coordinates define a point by specifying its distance and angle from the origin. The angle sweeps counterclockwise from zero degrees at 3 o�clock. You enter polar coordinates by typing a distance, a "less than" sign, and an angle.

Relative polar coordinates define a point by specifying its distance from the previous point. You enter relative polar coordinates by typing an @ sign, a distance, a "less than" ("<") sign, and an angle.

Setting an initial point with absolute rectangular coordinates and continuing with relative polar coordinates is usually the most efficient method of drawing wall lines and other architectural components.

Model space can be pictured as an as-large-as-you-wish-size piece of paper (or space, depending on whether a two-dimensional or three-dimensional drawing is considered) on which a real-world entities are drawn at full scale. This means that if a line in the real world measures one inch or one thousand feet, you tell AutoCAD to draw a line that is one inch or one thousand feet long. (In fact all the drawing are done in AutoCAD units to which real units of measurement are assigned only when the drawing is to be plotted.) Hence, 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 paper (or space). 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.

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.

Although it is possible to plot from model space, it is normal to set up a sheet in paper space. Paper space can be thought of as a model of a standard-size piece of paper, at a scale of 1:1, on which you cut holes (so called viewports) to let different views of the drawing in model space show through (as shown in Figure on the left). To fit the drawings into the holes, and all together on the page, each view need to be scaled to the desired size.

Generaly speaking layers can be imagined as transparent sheets that contain the components of your drawing. All entities on a certain layer are usually plotted with the same pen and the same line type.

The prototype drawings are used as templates in the production of other drawings. This is a powerful concept as it permits firms to set up office standards which are accessible to all users of the system.

In order to maintain a clean and accurate representation of the drawing, the drawing display needs to be refreshed from time to time. To refresh the drawing display, either screen REDRAWs or drawing REGENerations can be done.

REDRAWing cleans up only the display by removing temporary markers that indicate points you have specified. Screen redraws are faster but less thorough than drawing regeneration.

REGENerating, however, not only cleans up the display, but also regenerates the entire drawing and updates the drawing database with the recomputed screen coordinates for all objects in the drawing. Because regeneration can take a long time in complex drawings, it is more usual to redraw. Some commands, though, automatically initiate the drawing regeneration. When this happens, AutoCAD displays a message.

All graphic entities in AutoCAD are defined fully within 3D space. However, in the normal case, where the user makes no attempt to define them as 3D objects, they are assumed to lie on the base (Z=0) plane of the coordinate system. In that case, the z- value of every point is assumed to be zero.

• firstly, most of the basic graphic entities (lines, arcs and curves) can have a thickness associated with them which is effectively a height in the z-direction (this has always been a feature of AutoCAD and is sometimes referred to as 2.5D);
• secondly, when specifying the position of any point during the drawing process, all three coordinate values can be entered by the user (x,y and z);
• thirdly, AutoCAD supports a number of specifically 3D graphic entities, including 3D polylines, faces and meshes, along with a set of commands that can be used to create 3D objects with them;
• fourthly, and most significantly, AutoCAD allows the draftsman to set up a temporary user coordinate system (UCS) which can be positioned and orientated anywhere in space (relative to the WCS). Once a UCS is established, all 2D drawing or 3D model is done relative to those coordinates until a new UCS is defined or the user switches back to the WCS;
• lastly, from release 11 onwards, AutoCAD has provided an extension package called AME (Advanced Modelling Extension) which provides 3D solid modelling capabilities. These allow you to form complex solid objects by adding, subtracting and intersecting a set of simple solid primitives such as, boxes, cone, cylinders, spheres, etc. For example, you can form a box with a circular hole by subtracting a cylinder from a box.We will not be concerned with these techniques in these tutorials.

That now concludes the discussion of general concepts in AutoCAD. The remainder of this tutorial is concerned with how to interact with AutoCAD in order to produce drawings.