# Symmetric dashed circle in tikz

I'm trying to draw a dashed circle. However, the one I created is not symmetric so that, at different symmetric positions with respect to the center, the dashed elements do not correspond. I would like to have a perfectly centered element (or space) at the positions 0 and 180 degrees (and also 90 and 270). Here the code I'm using

\documentclass[tikz]{standalone}
\usetikzlibrary{decorations.markings}
\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}
\draw [ultra thick, blue, dash pattern=on 0.39793506944cm off 0.09948376736cm]circle(2.85cm);

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

\centerarc[black,ultra thick](0,0)(20:160:3)
\centerarc[black,ultra thick](0,0)(200:340:3)

\draw[black, ultra thick] (-2.80,1.02606042998)  -- (-4.178,1.02606042998);
\draw[black, ultra thick] (-2.81907786236,-1.02606042998) -- (-4.178,-1.02606042998);

\draw[black,ultra thick] (2.81907786236,1.02606042998)  -- (4.178,1.02606042998);
\draw[black,ultra thick] (2.81907786236,-1.02606042998) -- (4.178,-1.02606042998);

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

\end{tikzpicture}
\end{document}


Instead of a dashed pattern you can draw multiple arcs to simulate it. This way you can control start and ending angles of every fragment.

\documentclass[tikz,border=3mm]{standalone}

\begin{document}

\begin{tikzpicture}

\foreach \i [evaluate=\i as \start using (\i-1)*5] in {1,2,...,72}
\draw[blue, ultra thick] (\start+.5:2.85cm) arc[start angle=\start+.5, delta angle=4, radius=2.85cm];

\draw[ultra thick] (20:3) arc[start angle=20, end angle=160, radius=3cm];
\draw[ultra thick] (200:3) arc[start angle=200, end angle=340, radius=3cm];
\draw[ultra thick] (20:3)--++(0:3cm);
\draw[ultra thick] (160:3)--++(180:3cm);
\draw[ultra thick] (200:3)--++(180:3cm);
\draw[ultra thick] (340:3)--++(0:3cm);

\draw[red] (-90:4)--(90:4) (0:4)--(180:4);

\end{tikzpicture}

\end{document}


• Simple and effective idea Ignasi, Thank you a lot.
– Fly
Mar 8, 2021 at 10:31

The example from Ignasi's answer with the following changes:

• The black lines are properly connected
• The black lines start and ends at the same horizontal position as the red lines.

Example code:

\documentclass[tikz,border=3mm]{standalone}

\begin{document}

\begin{tikzpicture}

\draw[blue, ultra thick]
\foreach \i [evaluate=\i as \start using (\i-1)*5] in {1,2,...,72} {
(\start+.5:2.85cm) arc[start angle=\start+.5, delta angle=4, radius=2.85cm]
};

\draw[ultra thick]
(20:3) coordinate (tmp) (4, 0 |- tmp) -- (tmp)
arc[start angle=20, end angle=160, radius=3cm] to (-4, 0 |- \tikztostart)
(200:3) coordinate (tmp) (-4, 0 |- tmp) -- (tmp)
arc[start angle=200, end angle=340, radius=3cm] to (4, 0 |- \tikztostart)
;

\draw[red] (-90:4)--(90:4) (0:4)--(180:4);

\end{tikzpicture}

\end{document}


• Thank you a lot Heiko, it's much better now!
– Fly
Mar 14, 2021 at 12:37
• Thank you for your correction. It's much better now. Just a question. Is there any reason for using to (-4, 0 |- \tikztostart) instead of -- (-4, 0 |- tmp)? Mar 14, 2021 at 17:34
• @Ignasi This would also work. Mar 14, 2021 at 17:49