How can I use fork style edges with explicitely positioned TikZ tree nodes?

Question

As part of a graphical export facility, I try to render a tree in TikZ were all the node positions are exactly given and were the edges are fork-style.

Doing that without the trees package demands that I calculate the edge 'corners' by myself, based on the given node coordinates, which I would like to avoid.

Adding explicitly positioned nodes to a TikZ tree is messing up the layout, since (according to the manual), TikZ is changing the coordinate system for each child node.

Without trees:

\documentclass{article}
\usepackage{tikz}
\begin{document}

\begin{figure}
\begin{tikzpicture}
\node at (10, 10) (root) {root} ;
\node at (10, 7) (lvl1middle) {lvl1middle} ;
\node at (8, 6) (lvl2left) {lvl2left} ;
\node at (13, 5) (lvl2right) {lvl2right} ;
\node at (10, 5) (lvl2middle) {lvl2middle} ;
\draw (root) -- (lvl1middle);
\draw (lvl1middle) -- (lvl2left);
\draw (lvl1middle) -- (lvl2middle);
\draw (lvl1middle) -- (lvl2right);
\end{tikzpicture}
\end{figure}
\end{document}

With trees:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{trees} 
\begin{document}

\begin{figure}
\begin{tikzpicture}
\node at (10, 10) (root) {root}
    [edge from parent fork down]
    child { node at (10, 7) (lvl1middle) {lvl1middle} 
        child { node at (8, 6) (lvl2left) {lvl2left} }
        child { node at (13, 5) (lvl2right) {lvl2right} }
        child { node at (10, 5) (lvl2middle) {lvl2middle} }
    }; 
\end{tikzpicture}
\end{figure}
\end{document}

Answer 1

The edge from parent fork down simply installs a special edge from parent path that we can use for a to path. The original definition is

\tikzset{edge from parent fork down/.style={
  edge from parent path={(\tikzparentnode\tikzparentanchor) -- +(0pt,-.5\tikzleveldistance) 
                                                      -| (\tikzchildnode\tikzchildanchor)}}}

The default \tikzleveldistance is 15mm.

One solution with a to path would be

\tikzset{
  edge down and up again/.style={
    to path={
       |- ([shift={(\tikztotarget.south)}] +0pt,+-2.5mm) -- (\tikztotarget) }}}

In your example, I had to switch back to the default line to (i.e. --) path as the edge would have to go around the target node in that way.

Code

\documentclass[tikz,convert=false]{standalone}
\tikzset{
  edge down and up again/.style={
    to path={
       |- ([shift={(\tikztotarget.south)}] +0pt,+-2.5mm) -- (\tikztotarget) }}}
\begin{document}
\begin{tikzpicture}
\node at (10, 10) (root) {root} ;
\node at (10, 7) (lvl1middle) {lvl1middle} ;
\node at (8, 6) (lvl2left) {lvl2left} ;
\node at (13, 5) (lvl2right) {lvl2right} ;
\node at (10, 5) (lvl2middle) {lvl2middle} ;

\path[edge down and up again] (root) edge [line to] (lvl1middle)
                        (lvl1middle) edge           (lvl2left)
                                     edge [line to] (lvl2middle)
                        (lvl2middle) edge           (lvl2right);
\end{tikzpicture}
\end{document}

Output