filling an arc in a circle

I don't know how to fill an arc in the chart below

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}[>=latex]

% Draw the lines at multiples of pi/12
\foreach \ang in {0,...,31} {
\draw [lightgray] (0,0) -- (\ang * 180 / 16:3);
}

% Add the labels at multiples of pi/4
\foreach \ang/\lab/\dir in {
0/0/right,
1/{\pi/4}/{above right},
2/{\pi/2}/above,
3/{3\pi/4}/{above left},
4/{\pi}/left,
5/{5\pi/4}/{below left},
7/{7\pi/4}/{below right},
6/{3\pi/2}/below} {
\draw (0,0) -- (\ang * 180 / 4:3.1);
\node [fill=white] at (\ang * 180 / 4:3.2) [\dir] {\scriptsize $\lab$};
}

% The double-lined circle around the whole diagram
\draw [style=thick] (0,0) circle (3);

% Concentric circles and radius labels
\foreach \s in {0, 1, 2} {
\draw [SteelBlue3, thick] (0,0) circle (\s + 0.5);
\draw (0,0) circle (\s);
\node [fill=white] at (\s, 0) [below] {\scriptsize $\s$};
}

\draw[fill=red!50 ] (0,0) -- (0.5 * 180 :1);
\end{tikzpicture}
\end{document}


My attempt is in the end of the diagram

\draw[fill=red!50 ] (0,0) -- (0.5 * 180 :1);


but it does not do anything. I would like to draw something like the image below

• What arc? I think there was a similar question though... – Alenanno Sep 7 '16 at 21:20
• The command you use is a line, it won't fill because it's not an area. If you did a curve, it would fill that area. Can you draw the area you want to fill with some external program? Just to show what you want to achieve, because I'm not sure. – Alenanno Sep 7 '16 at 21:25

If you want the outer border to be an arc, then you can modify your filling command to something like

\draw[fill=red!50 ] (0,0) -- (0.5 * 180 : 2cm) arc[start angle=90,delta angle=-30,radius=2cm] -- cycle;


If you want a triangle, as in your image, you can do something like

\draw[fill=red!50 ] (0,0) -- (180 : 2cm) -- (150:2cm)  -- cycle;


In both cases, modify the angles and radius to suit your need.

Complete code:

\documentclass{standalone}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}[>=latex]

% Draw the lines at multiples of pi/12
\foreach \ang in {0,...,31} {
\draw [lightgray] (0,0) -- (\ang * 180 / 16:3);
}

% Add the labels at multiples of pi/4
\foreach \ang/\lab/\dir in {
0/0/right,
1/{\pi/4}/{above right},
2/{\pi/2}/above,
3/{3\pi/4}/{above left},
4/{\pi}/left,
5/{5\pi/4}/{below left},
7/{7\pi/4}/{below right},
6/{3\pi/2}/below} {
\draw (0,0) -- (\ang * 180 / 4:3.1);
\node [fill=white] at (\ang * 180 / 4:3.2) [\dir] {\scriptsize $\lab$};
}

% The double-lined circle around the whole diagram
\draw [style=thick] (0,0) circle (3);

% Concentric circles and radius labels
\foreach \s in {0, 1, 2} {
\draw [SteelBlue3, thick] (0,0) circle (\s + 0.5);
\draw (0,0) circle (\s);
\node [fill=white] at (\s, 0) [below] {\scriptsize $\s$};
}

\draw[fill=red!50 ] (0,0) -- (0.5 * 180 : 2cm) arc[start angle=90,delta angle=-30,radius=2cm] -- cycle;

\draw[fill=blue!50 ] (0,0) -- (180 : 2cm) -- (150:2cm)  -- cycle;
\end{tikzpicture}
\end{document}