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Draw arc in tikz when center of circle is specified

an very trivial question but I can't do it! Sorry but I find coordinates confusing.

I would like to draw an arc around a circle; both should have the same center of rotation. And I want to position the arc relative to the circle.

Here is the MWE to correct:

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}

\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\begin{document}
\begin{tikzpicture}
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\draw[thick] (sh1) arc (0:300:2cm);
\end{tikzpicture}
\end{document}

How do I get to have the same center for both shapes, with relative positioning of the arc? Any simple and brilliant idea?

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marked as duplicate by Jake, percusse, Loop Space, Tom Bombadil, lockstep Aug 30 '12 at 16:00

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
You can replace your \draw ... command with \draw[thick] (sh1) ++(0:2cm) arc (0:300:2cm); to move to a point on the circle before drawing the arc. See Leo Liu's answer to Draw arc in tikz when center of circle is specified –  Jake Aug 30 '12 at 11:14
    
Thanks Jake. I did have a look at the post you mention before posting mine but I could not make the code work. I am not familiar with this way of coding. –  jejuba Aug 30 '12 at 12:42
    
No problem. It's always a good idea to include the things you've tried and the questions you've already looked at in your question. That way, people will be able to offer more specific advice. –  Jake Aug 30 '12 at 12:45

2 Answers 2

up vote 7 down vote accepted

The problem is that for arc you specify the start point of the arc, not the cirle's center. You could define a custom command like this:

The Code

\documentclass{article}
\usepackage{tikz}


\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\def\centerarc[#1] (#2) (#3:#4:#5)% [draw options] (center) (initial angle:final angle:radius)
{ \draw[#1] (#2) ++(#3:#5) arc (#3:#4:#5);
}

\begin{document}
\begin{tikzpicture}
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\centerarc[thick] (sh1) (0:300:2cm)
\end{tikzpicture}
\end{document}

Special thanks to Paul Gaborit and Jake for simplifying this.

The Result

enter image description here

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1  
To specify the start point, the following notation is simpler: ($(#2) + (#3:#5)$)... –  Paul Gaborit Aug 30 '12 at 11:29
    
@Jake, thanks, just saw that, I will update. –  Tom Bombadil Aug 30 '12 at 11:34
    
Well, just great! –  jejuba Aug 30 '12 at 12:50

Well, I don't a have brilliant idea ... but note, that arc always starts at given coordinate - in your case at the center of the circle sh1, and its center is shifted by the amount specified in last argument for arc - in your case by 2cm. So, if you shift the start of the arc in the appropriate direction and amount, you will get what you desired (if I understand your problem correctly).

In your case try:

\documentclass{article}
\usepackage{tikz}
\usepackage{graphicx}

\tikzset{state/.style={circle,draw=black, very thick,minimum size=4em}}

\begin{document}
\begin{tikzpicture}
\draw[help lines] (-3,-3) grid + (9,9);
\node[state, fill=black!20] (sh1) at (1,3) {Circle};
\draw[thick] ([xshift=20mm] sh1.center) arc (0:300:2cm);
\end{tikzpicture}
\end{document}
share|improve this answer
    
Thanks Zarko. It works well. The only (minimal) issue I see is that if I change the angles in the arc, the arc and the circle do not share automatically the same center of rotation. But this can be remedied. Thanks again. –  jejuba Aug 30 '12 at 12:52

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