Understanding the coordinate system

Understanding the coordinate system is essential to understanding the way AutoCAD works. In AutoCAD, you can assign length and angles, as well as coordinate values, to make drawings, but to do all this, knowledge of the coordinate system is essential.

Primarily, these are two types of coordinate systems that we will use to make geometries in AutoCAD, and they are Cartesian and polar coordinates. First let's have a look at what Cartesian coordinates are.

Cartesian coordinates

AutoCAD follows the Cartesian coordinate system, which is a graphical method of assigning coordinates to a point in space. The simple three-dimensional space has three coordinates, namely X, Y, and Z, which are mutually perpendicular to each other, as in the following diagram. The point of intersection of the three mutually perpendicular axes is the origin, which is represented as (0,0,0):

Figure 2.1: Mutually perpendicular coordinates

The position of any point in a three-dimensional space can be specified using these three axes, which are represented by the X, Y, and Z axes in the preceding diagram. But for a two-dimensional space, we only need to use the X and Y axes to define the position of any point.

In a two-dimensional space, the simple (X,Y) coordinate system is used and any point in a two-dimensional space can be defined using these two coordinates only. Take the example of the following graph. Here, the origin is mentioned as (0,0), which is also the point of intersection of the X and Y axes, represented by horizontal and vertical lines, respectively:

Figure 2.2: Cartesian coordinates

The A (7,8) point is at 7 units from the origin along the X axis and at 8 units along the Y axis. Similarly, the B (-6,3) point is at 6 units along the negative side of the X axis and at 3 units along the positive side of the Y axis. In the case of the C (4,-5) point, the distance from the positive side of the X axis is 4 units, and its distance along the negative side of the Y axis is 5 units.

The X axis points to the right of the origin are positive and the points to the left of the origin are negative. Similarly, on the Y axis, the points on top of the origin are positive and the points below the origin are negative.

Polar coordinates

Using polar coordinates, we can also represent points in a two-dimensional space. In this case, one polar distance and an angle with respect to the X axis are required instead of the X and Y coordinate values. To understand this clearly, have a look at the following graph:

Figure 2.3: Polar coordinates

In this case, the B point is represented by (8<30), where 8 is the distance between the A and B points. Here, A is the origin and 30 is the angle between line AB and the positive X axis in an anticlockwise direction.

This type of coordinate representation, where a point in space is represented by an angle with respect to the positive X axis and the distance from the origin, is known as a polar coordinate system.

Throughout this book, we will use both methods of coordinates to make our drawing. Drawings in AutoCAD are not essentially made only with coordinate values. For most of the cases, we use a general approach of direct distance entry and we use coordinates only in specific situations.

In the next section, we will start making our first drawing with the Line command using direct distance entry as well as different coordinate values.