AutoCAD: Relative coordinates

Remember some of the basics to make your drafting experience easier. The @ sign is your friend
Català - Castellano
Some of the basics of how AutoCAD works tend to be forgotten. One of this is the capacity to enter relative coordinates by using the @ sign before the coordinates. This will allow us to set a point, line end point, etc a certain distance from the point we clicked before.
This is specially useful when we draw rectangles using the RECTANGLE command. If we know the dimensions of the rectangle we want to draft, we can just click on the point we want one of its corners to be and enter @X,Y (being X=width and Y=height). By entering the @ sign the coordinates of the second corner will be relative to the first one, while if we didn't enter the @ sign, entering X,Y only, we would be entering coordinate points related to the UCS 0,0,0.
As a quick example, if we want to draw a rectangle 1 unit high by 2 units wide, we will do the following:

• Enter RECTANGLE command (or click rectangle icon if you use icons)
  
• click to select the point of the first corner
• enter @2,1
  

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AutoCAD: PROJMODE system variable

Control the projection mode for TRIM and EXTEND commands. Recently a colleague asked me: "When I am unable to trim lines, because “EDGE IS PERPENDICULAR TO UCS XY PLANE”.. what do I do?". I sincerely had no idea, but since "I don't know" is not the kind of answer I like to give I started looking for an answer. Googleling the problem showed that the quick solution was to set PROJMODE to 0. Tried and it worked. The next step was to know what was PROJMODE.
PROJMODE is a system variable that allows us to control the projection settings for the TRIM and EXTEND Commands. The default option for PROJMODE is 1. These are the three states of the variable and their result:

• PROJMODE (0): True 3D mode. This means that if two objects are not coplanar, you will not be able to TRIM or EXTEND using them as a reference.
• PROJMODE (1): Default setting. It projects to the XY plane of the current UCS. See that this will allow you to trim or extend lines that have different Z values, but the lines will remain on different planes. Remember that the command FLATTEN is an Express Tool that allows you to bring all geometry to Z=0
• PROJMODE (2): It projects to the XY plane of the current view. If your view is alligned with the current UCS (using the PLAN command or alligning the view by default to the UCS) there is no difference between setting the variable to 1 or to 2. If the view has been rotated the results on trimming will vary.

I still have not very clearly found out the reasons for initial error message, but at least we found a work around and we learned something new.
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AutoCAD Command: Snapang.

Is your Crosshair on a different angle than the UCS? SnapAng command will help you change this.Sometimes you might find a file where the crosshair is tilted although the UCS is aligned to the view. You might get crazy trying to align the crosshair with the UCS if you don´t know the SNAPANG command.
SNAPANG controls the angle of the grid so the crosshair is aligned to it. If you find a file where the crosshair is tilted although the UCS is squared with the view type SNAPANG and type "o" as the angle. THe crosshair will automatically go bacK to its "normal" position.
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AutoCAD: Automatic view rotation with the UCS command

Why does the view rotate automatically when changing the UCS?
Català - Castellano
When turning the UCS or setting a new one, we have the option to just create the UCS or to rotate the view together with the UCS. Some people like it automatic, some like me hate it and prefer to see the UCS rotate but stay with the same view.
The way to toggle between this too options relies on the UC command. If we type UC on the command line, it will pop up a window with the UCS options.

We go to the settings tab and we check or uncheck the box beside "Update view to Plan when UCS is changed". If we keep it unchecked, the view will not update automatically. If we want to align the view to the new UCS we will have to use the "plan" command.
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