How to place a node in the middle of an arc?

Question

I think it is a simple question, but I didn't find the answer yet. The code I'm using is the following:

\draw[->](1,0) arc(0:-30:1) node[midway]{$30$};

But in this way the node is placed at the origin intead of at the middle of the arc.

Note: it must be done with arc.

Answer 1

As of 2012-03-01, thanks to Till Tantau, it is now possible to do it straightforwardly with the cvs version of pgf-tikz.

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw[->](1,0) arc(0:-30:1) node[midway]{$30$};
\end{tikzpicture}
\end{document}

Answer 2

The reason why it is shown at the origin is because there is no explicit second coordinate for TikZ to interpolate via pos. One solution is to, roughly speaking, parameterize the arc path via markings library. This is simply a modification of my previous answer for marking a path with a node.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.markings}
\begin{document}
\begin{tikzpicture}[
arcnode/.style 2 args={                
            decoration={
                        raise=#1,             
                        markings,   
                        mark=at position 0.5 with { 
                                    \node[inner sep=0] {#2};
                        }
            },
            postaction={decorate}
}
]
\draw[->, arcnode={20pt}{$30$} ] (0,0) arc (0:-30:2cm) ;
\end{tikzpicture}
\end{document}

For some reason, when the curved paths become too short(e.g. your example path starts working after 1.095cm for radius of the arc) it gives a Dimension too large error, so probably, there is a detail that I don't know yet here. This is not the case for straight paths.

Answer 3

Problem taken from: Draw centered label above arc in TikZ

---

This is another approach to this problem. In this way, there is another second coordinate.

\documentclass[tikz,border=5pt]{standalone}
\begin{document}
\begin{tikzpicture}
    \draw[green] (1,0) arc[radius = 1, start angle=0, end angle=90] node[above] {H} arc[radius = 1, start angle=90, delta angle=90];
\end{tikzpicture}
\end{document}