AutoCAD Tutorial : Session Four

grid, snap, ortho, plot, tablet, move and rotate with reference angle

Open your model again. Make sure you have a plan view.


* Type grid and on. You should see small dots indicating the corners of one-unit squares; it is possible to specify the grid spacing, but one-unit squares is the default.

Note that this grid is an on-screen grid only; it will not print out. (If you have not set the limits, the grid will only cover a small portion of the model; if you have set them, the grid will cover the area defined by the limits.)


* With the grid turned on, you can force the system to choose only points on the grid. Type snap and choose on. The cursor will now snap to the grid corners, but you still can specify a point not on the grid with the keyboard.

You can determine the size of the grid and the configuration of the snap points independently. They will be the same if you set the grid spacing to 0; that tells the system to set the grid spacing to equal the snap spacing. You can also choose snap as a choice in the sub-menu after typing grid. But if you want the grid and snap spacing to be different, then set the grid spacing to any number except 0, and your snap spacing to a different number.

To use the grid and snap features, now use the mouse to draw a polyline (staying on layer EXP from 1,1 to 3,4 to -5,4 to 5,6. (If you haven't noticed before, the coordinates of the current cursor location show in the box at the top of the model, more or less in the center.) Put the new polyline on the layer trash.

* Now turn off the grid and the snap requirement. Type grid off and then snap off.


* This might seem a good way to draw a horizontal or vertical line, but it's easier to use the ortho command. Type ortho and select on, all lines drawn with the mouse or a digitizer will be horizontal or vertical. Draw a few lines to see how this works. (Control-O serves as a "toggle" switch. If you type it, it will turn on or off the ortho mode.)

Save your work.

LAYERS (again)

The importance of layer can't be overstated. Let's try a couple of layer manipulations to see how we can use the layers creatively.

Make sure you have a plan view. Make your current layer VL. Freeze all others (type laf* and then again to complete the command. You will have instructed the system to freeze all layers, and that may seem pointless, but the system will not freeze the current layer; so VL will still be on.

You have a letter I. Thaw layers H? (layers with two letters, the first of which is an H. You have an E.

Freeze layer HB; you have an F.

Freeze layer HT and thaw VR; you have an H.

For more information about layers and naming them, see the CSA Web document, CSA Layer Naming Convention.


Skip this if you don't have a plotter attached to your system. You may need help from someone to set up your plotter and to make sure the paper and pens are properly installed.

* To make your first paper drawing, first set the current layer to VL; then freeze all other layers.
* Now type plot; you will get a dialog box with many choices. For now, you can ignore many of them. First, make sure that there is no check mark in the plot-to-file box.
* Then select the window push button so that you can specify the window to be plotted by outlining a window on your model with the rodent. (The window check-box will automatically be checked.) Select the pick choice and make a window around Rect. A, making the window only as large as necessary to include that rectangle. You have specified the part of the model to be drawn.
* Now you must specify the scale of the drawing. The system for doing so is odd. In the upper right is a place where you can indicate whether you are dealing in mm. or inches. Select mm. In the lower right, you will see two boxes, one for plotted mm. and one for drawing units. You need to fill in the boxes with the appropriate numbers. You should be able to say that 40 mm. on paper (plotted) equals 1 drawing unit in the model. (Assuming that our drawing units are meters; that means a scale of 1:25 - check the math to be sure you understand that.) Now, in the lower right, there are checkboxes for full or partial previews.
* Select full preview and then ask for a preview. Make certain that the rectangle will fit on the paper. If it doesn't, start over with the window specification. (Don't use the transparent pan command on your model while trying to specify the window. At least in my version of AutoCAD, it causes an error. If you need to pan, stop the plot process, do a regular pan, and then type plot again.) If all is set correctly, make the drawing. (I often get an error message telling me that the drawing won't fit on the paper, but it is usually a spurious message; so, if you get the same message, try going ahead with the drawing.)

* Measure the rectangle on your drawing. Apply the scale factor, and calculate the size of the rectangle. Compare that to the model. Forget how to get the size of the rectangle in the model? Use dist and endpoint to get the measurements. I think you will find that the long side came out to be a round number (6 meters) and, as a result, was measured more or less exactly, but the shorter side of the rectangle, not being a round number, may not be measured so accurately.


Skip this if you don't have a digitizer attached to your system.

Now let's use the drawing we just produced to make a copy of the rectangle. We'll see how accurately we can use a drawing to enter information. We will trace it on the digitizer. (If you don't have a drawing to use, make a scaled drawing of a rectangle that is 6m. high and .75m. wide and use that drawing instead.)

Note at the beginning that the digitizer has a sighting device, called a puck, that has crosshairs and perhaps some magnification. You will have to place the crosshairs precisely on the points you're trying to pick; so experiment now. Is it better to align the crosshairs with lines when picking a corner, or to try to have all the lines stand clear of one another? It will vary from person to person, but I have found that I prefer not to align the crosshairs with the drawing lines when I have a sharp, clear corner. But I do usually align them when I have a less clear corner. You must be the judge for your own style.

* First, you must get the system to recognize that the digitizer is working at a scale other than one-to-one. That is, the system must repond to a specified movement on the digitizer as equivalent to a larger movement in the real world. You do that by picking two points on the model and identifying the coordinates of the points in real-world terms (x and y only; you can't expect the digitizer to figure out an elevation when it's flat). To start the process, tape the drawing on the digitizer surface. Check your model and use the id command (with endpoint) to get the coordinates of two diagonally opposite corners of the rectangle. (If you are using a paper drawing, consider the lower left point to be 3,2 and the upper right to be 3.75,8.)
* Start the process of calibrating the tablet with the command tablet; then type cal (for calibrate). You will be asked for a point on the digitizer. Pick one of the corners for which you know the coordinates. Then you will be asked for the coordinates of the point. Type them in. Do the same for the other known point. The digitizer is now scaled to match the scale of the drawing.
* Before tracing the rectangle, make a new layer called trace. Make trace the current layer, and freeze VL. To trace the rectangle, you need only issue the command - line or pline or 3dpoly - and pick the four corners with the digitizer puck. (Make sure snap and osnap are off.) Close the line to make your rectangle. Now check your new rectangle (with endpoint and either id or dist) to see how close you are to the original size 6 m. x .75 m. Also check the coordinates of each point and compare them to the coordinates for Rect. A.

When you traced the rectangle, what did you do about the elevation of each point? If you only entered the point picked with the digitizer, the elevation was assumed to be the default elevation. (That can be changed with the command elevation.) If, however, you wanted to make a specific elevation for any of the points you selected, you could have typed .xy before picking the point. The system would have accepted the x- and y-coordinates chosen and prompted you for the z-coordinate, which you would have entered with the keyboard. So you can't enter all three coordinates from the digitizer, but you can use it for two and type the third coordinate.

We chose to calibrate the tablet in a specific way for a reason. Using the diagonally opposite corners of the rectangle, rather than one of the sides, for instance, meant that we used the longest distance between points available on the drawing. That, in turn, meant that the same absolute error in our digitizing process was spread over the largest distance possible. Thus, the error inevitably introduced when we tried to pick specific points was the smallest achievable percentage of the distance. When digitizing any drawing, it's best to calibrate with the best points you have (the ones you know most certainly) and the two such points farthest apart. (Note that the tablet calibration will not survive if you exit AutoCAD. You will have to recalibrate in that case; so you would want to start digitizing with points at the extremities of your drawing and recalibrate with them if you must quit and restart the program.)

* Although we calibrated with the actual coordinates from the model, we could have calibrated with the correct scale but not the actual model coordinates. We'll do that now. First find the distance between the two diagonally opposite points of Rect. A that you used before. (Get the computer to find the distance; it will be more accurate.) Then make another new layer called trace2; make it the current layer and freeze all others. Now recalibrate the tablet by picking the same two points on your drawing, but label the first point 0,0 and the second one x,0 (x being the distance from corner to diagonally opposite corner - I think it should be 6.0467 - 6.047 when rounded off to three decimal places, as we instructed the computer to do). The scale is correct but the position is arbitrary, and the axes are capriciously oriented. Draw the rectangle as before; now we will move it to the correct location, and rotate it to fit.

You may have trouble with the next command, because the tablet is calibrated. You can turn the calibration off and back on (so long as you don't quit AutoCAD in the interim - if you do, it will lose the calibration) with Control-T. It acts as a toggle, changing the tablet mode back and forth from calibrated to normal.


If you do not have a digitizing tablet and therefore do not have the new rectangle to use, please draw one, using the pline command, from 0.021,1.551,1, to -3.884,6.106, to -3.315,6.594, to .591,2.039. Don't forget to close the figure; use it in the following. Put it one layer trace.

* Now turn back on layer VL so you have Rect. A to guide you. Return to the isometric view. Move the new rectangle with the aid of the endpoint feature so that one corner lies precisely on the appropriate corner of Rect. A. Use the id or list command to check your move, making sure that both rectangles have one corner in exactly the same place.

* Now type rotate to start the process of rotation.

This time, let's match the angle from corner to corner of the new rectangle to the comparable angle in the original one, which we can't do with numbers, since we don't know the rotation required. We want to tell the system to use the angle from corner to corner of our new rectangle as a starting reference and to rotate the rectangle so it matches the angle from corner to corner of Rect. A.

* After you've picked the rectangle and located the base corner around which to rotate it, the prompt will be "/Reference:." Type ref to indicate that you will provide the reference angle. The new prompt is "Reference angle <90.000>:." Pick the ends of the diagonal on the new rectangle to define the reference angle. Start with the end at the base point, and be sure to use endpoint. Then the system will prompt you for the new angle. Again using endpoint, select the corner of Rect. A where the corner of your new rectangle belongs. That should complete the rotation.

Time to save your work again.

End of Session Four

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