AutoCAD Basics - Section 1

CHAPTER 3: UNITS AND COORDINATES

We have already mentioned that with Autocad we can make drawings of very different types, from architectural plans of an entire building, to drawings of pieces of machinery as fine as those of a clock. This imposes the problem of the units of measure that one drawing or the other requires. While a map can have meters, or kilometers, as the case may be, a small piece can be millimeters, even tenths of a millimeter. In turn, we all know that there are different types of units of measurement, such as centimeters and inches. On the other hand, inches can be reflected in decimal format, for example, 3.5 ″ although it can also be seen in fractional format, such as 3 ½ ”. The angles on the other hand, can be reflected as decimal angles (25.5 °), or in degrees minutes and seconds (25 ° 30 ′).

All this forces us to consider some conventions that allow us to work with the units of measurement and the appropriate formats for each drawing. In the next chapter we will see how to choose the formats of the units of measure and their precision. Consider for the moment how the problem of the measures itself in Autocad is raised.

3.1 Units of measurement, drawing units

The units of measure that Autocad handles are simply "drawing units". That is, if we draw a line that measures 10, then it will measure 10 drawing units. We could even colloquially call them "Autocad units", although they are not officially called that. How much do 10 drawing units represent in reality? That's up to you: if you need to draw a line representing the side of a 10-meter wall, then 10 drawing units will be 10 meters. A second line of 2.5 drawing units will represent a distance of two and a half meters. If you are going to draw a road map and make a road segment of 200 drawing units, it is up to you whether those 200 represent 200 kilometers. If you want to consider a drawing unit equal to one meter and then want to draw a line of one kilometer, then the length of the line will be 1000 drawing units.

This then has 2 implications to consider: a) You can draw in Autocad using the actual measurements of your object. A real unit of measurement (millimeter, meter or kilometer) will be equal to a drawing unit. Strictly speaking, we could draw incredibly small or incredibly large things.

b) Autocad can handle a precision of up to 16 positions after the decimal point. Although it is convenient to use this capacity only when it is strictly necessary to take better advantage of the computer resources. So here is the second element to consider: if you are going to draw a building of 25 meters high, then it will be convenient to establish a meter equal to a drawing unit. If that building is going to have details in centimeters, then you must use a precision of 2 decimals, with which one meter and fifteen centimeters will be 1.15 drawing units. Of course, if that building, for some strange reason, required millimeter detail, then 3 decimal places would be required for precision. One meter, fifteen centimeters, eight millimeters would be 1.158 drawing units.

How would the drawing units change if we establish as a criterion that a centimeter is equal to a drawing unit? Well, then one meter fifteen inches eight millimeters would be 115.8 drawing units. This convention would then require only a decimal precision position. Conversely, if we say that a kilometer is equal to a drawing unit, then the previous distance would be 0.001158 drawing units, which requires 6 precision decimal positions (although handling centimeters and millimeters would not be so handy).

From the above it is clear that the decision of equivalence between the units of drawing and the units of measure depends on the needs of its drawing and the precision with which it must work.

On the other hand, the problem of the scale that the drawing must have to be printed on a certain size of paper is a different problem from what we have exposed here, since the drawing can later be “scaled” to fit the different sizes of paper. paper, as we will show later. So the determination of "drawing units" equal to "x units of measurement of the object" has nothing to do with the scale of printing, a problem that we will attack in due course.

 

3.2 Absolute Cartesian Coordinates

Do you remember, or have you heard of, the French philosopher who in the XNUMXth century said “I think, therefore I am”? Well, that man named Rene Descartes is credited with developing the discipline called Analytic Geometry. But don't be scared, we are not going to relate mathematics to Autocad drawings, we only mention it because he invented a system for the identification of points in a plane that is known as a Cartesian plane (although if this is derived from its name , should be called "Descartesian plane" right?). The Cartesian plane, made up of a horizontal axis called the X axis or abscissa axis and a vertical axis called the Y axis or ordinate axis, allows locating the unique position of a point with a pair of values.

The point of intersection between the X axis and the Y axis is the origin point, that is, its coordinates are 0,0. The values ​​on the X axis on the right are positive and the values ​​on the left are negative. The values ​​on the Y axis upwards from the point of origin are positive and down negative.

There is a third axis, perpendicular to the X and Y axes, called the Z axis, which we use fundamentally for the three-dimensional drawing, but we will ignore it for the time being. We will return to it in the section corresponding to the drawing in 3D.

In Autocad we can indicate any coordinate, even those with negative X and Y values, although the drawing area is mainly in the upper right quadrant, where both X and Y are positive.

Thus, to draw a line with all accuracy, just indicate the coordinates of the endpoints of the line. Let's look at an example using the coordinates X = -65, Y = -50 (in the third quadrant) for the first point and X = 70, Y = 85 (in the first quadrant) for the second point.

As you can see, the lines that represent the X and Y axes are not shown in the screen, we must imagine them for the time being, but in Autocad the coordinates were considered to draw exactly that line.

When we enter exact X, Y coordinates with respect to the origin (0,0), then we are using absolute Cartesian coordinates.

To draw lines, rectangles, arcs or any other object in Autocad we can indicate the absolute coordinates of the necessary points. In the case of the line, for example, of its starting point and its ending point. If we remember the example of the circle, we could create one with exactitude by giving the absolute coordinates of its center and then the value of its radius. It is worth saying that when typing the coordinates, the first value without exception will correspond to the X axis and the second to the Y axis, separated by a comma and this capture can occur both in the command line window and in the boxes of the dynamic capture of parameters, as we saw in chapter 2.

However, in practice, the determination of absolute coordinates is often complex. Therefore, there are other methods to indicate points in the Cartesian plane in Autocad, such as those that we will see next.

3.3 Absolute polar coordinates

Absolute polar coordinates also have the origin coordinates as the reference point, ie 0,0, but instead of indicating the X and Y values ​​of a point, only the distance to the origin and the angle is required. The angles are counted from the X axis and counterclockwise, the vertex of the angle coincides with the origin point.

In the Command Window or the capture boxes next to the cursor, depending on whether or not you are using dynamic parameter capture, the absolute polar coordinates are indicated as distance <angle; for example, 7 <135, is a distance of 7 units, at an angle of 135 °.

Let's look at this definition in video to understand the use of absolute polar coordinates.

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4 Comments

  1. It is very good free teaching, and share it with people who do not have enough economy to study the autocad program.

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