Avec l'extension `printlen` :
\documentclass[french,a4paper]{article}
\usepackage[]{tikz}
\usepackage{printlen}
\uselengthunit{cm}
\begin{document}
\begin{tikzpicture}
\draw (1,0) arc (0:90:1);
\pgfgetlastxy{\ax}{\ay} % (pas de point virgule pour commande pgf)
\draw (3,0) node[draw]{\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\fill[red] (\ax,\ay) circle (5pt);
\end{tikzpicture}
\end{document}
![alt text][1]
Ou plus généralement :
\documentclass[french,a4paper]{article}
\usepackage[]{tikz}
\usepackage{printlen}
\uselengthunit{cm}
\begin{document}
\begin{tikzpicture}
\draw (1,0) arc (0:90:1);
\pgfgetlastxy{\ax}{\ay} % (pas de point virgule pour commande pgf)
\draw (3,0) node[draw]{\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\fill[red] (\ax,\ay) circle (5pt);
\draw (3,-1) node[draw]{\uselengthunit{mm}\rndprintlength{\dimexpr\ax}, \foreach \n [count=\xi] in {cm,mm,pt,pc,in,bp,dd,cc}{
\draw (3,-\xi) node[draw]{\uselengthunit{\n}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-2) node[draw]{\uselengthunit{pt}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-3) node[draw]{\uselengthunit{pc}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-4) node[draw]{\uselengthunit{in}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-5) node[draw]{\uselengthunit{bp}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-6) node[draw]{\uselengthunit{dd}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
\draw (3,-7) node[draw]{\uselengthunit{cc}\rndprintlength{\dimexpr\ax}, \rndprintlength{\dimexpr\ay}};
}
\end{tikzpicture}
\end{document}
![alt text][2]
[1]: https://texnique.fr/osqa/upfiles/Screen_Shot_2020-07-07_at_19.51.37.png
[2]: https://texnique.fr/osqa/upfiles/Screen_Shot_2020-07-07_at_19.58.31_Hf8rdwh.png
