Defining Commands

1. \makeatletter
\newcommand{\gettikzxy}[3]{%
     \tikz@scan@one@point\pgfutil@firstofone#1\relax
     \edef#2{\the\pgf@x}%
     \edef#3{\the\pgf@y}%
     \pgfmathparse{#2/28.3465} \let#2\pgfmathresult;
     \pgfmathparse{#3/28.3465} \let#3\pgfmathresult;
}
\makeatother
   
   Usage:

   TiKZ Listing: Points.tikz.tex. PDF   PNG   SVG   ZIP*   \begin{tikzpicture} \coordinate (O) at (0,0); \coordinate (i) at (1,0); \coordinate (j) at (0,1); \draw[-latex] (O) -- (i); \draw[-latex] (O) -- (j); %Polar coordinates: in degrees \coordinate (P) at (45:3); \coordinate (Q) at (-2,2); \coordinate (R) at (4,1); %Projection of R on PQ \coordinate (S) at ($(P)!(R)!(Q)$); \gettikzxy{(R)}{\sx}{\sy}; \filldraw (O) circle(2pt) node[anchor=north east] {$O$}; \filldraw (P) circle(2pt) node[anchor=north east] {$P$}; \filldraw (Q) circle(2pt) node[anchor=north east] {$Q$}; \filldraw (R) circle(2pt) node[anchor=north east] {$R$}; \filldraw (S) circle(2pt) node[anchor=north east] {$S$}; %Convex Combinations \draw ($(P)!-1!(Q)$) -- ($(P)!2!(Q)$); \end{tikzpicture}