Pyramide régulière à base carrée

\documentclass[border=5pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=1.8]
  \coordinate (A) at (0,0);
  \coordinate (B) at (3,0);
  \coordinate (C) at (3.8,1.2);
  \coordinate (D) at (0.8,1.2);
  \coordinate (S) at (1.9,3.5);
  \coordinate (O) at (1.9,0.6);
  \coordinate (M) at (1.5,0); % milieu AB
  \draw[thick] (A) -- (B) -- (C);
  \draw[dashed] (C) -- (D) -- (A);
  \draw[thick] (A) -- (S);
  \draw[thick] (B) -- (S);
  \draw[thick] (C) -- (S);
  \draw[dashed] (D) -- (S);
  \draw[dashed,blue] (S) -- (O) node[midway,right,font=\small] {$h$};
  \fill[blue] (O) circle (1pt) node[below right,font=\small] {$O$};
  \draw[dashed,red] (O) -- (M) node[midway,below,font=\small] {$a$};
  \node[below left,font=\small] at (A) {$A$};
  \node[below right,font=\small] at (B) {$B$};
  \node[right,font=\small] at (C) {$C$};
  \node[left,font=\small] at (D) {$D$};
  \node[above,font=\small] at (S) {$S$};
\end{tikzpicture}
\end{document}