How to convert TikZ coordinates into PGF coordinates?

Question

I have several coordinate specified in TikZ, say

\coordinate (a) at (0.4, 1.7);
\coordinate (b) at (3.2, 1.7);
\coordinate (c) at (1.7, 4.7);
\coordinate (cog) at ($1/3*(a) + 1/3*(b) + 1/3*(c)$);

Now I'm trying to use the (cog) coordinate in PGF commands, for example:

\pgftransformshift{<the (cog) coordinate>}

I already tried putting in (cog) or cog but it doesn't work as desired. So my question is: How do I convert my coordinate specified in TikZ so that I can use it inside PGF commands?

Answer 1

Coordinates also have anchors (well, only center anchor) and that can be used for referencing.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
\draw[style=help lines] (0,0) grid[step=1cm] (5,5);
\coordinate (a) at (0.4, 1.7);
\coordinate (b) at (3.2, 1.7);
\coordinate (c) at (1.7, 4.7);
\coordinate (cog) at ($1/3*(a) + 1/3*(b) + 1/3*(c)$);
\pgftransformshift{\pgfpointanchor{cog}{center}}
\draw (0,0) -- (2,2);
\end{tikzpicture}
\end{document}