tikz syntax name dot number

Question

I found a code example http://www.texample.net/tikz/examples/inertial-navigation-system/

where I see that positioning:

\node (naveq) [naveqs] {Navigation equations};
\path (naveq.140)+(-\blockdist,0) node (gyros) [sensor] {Gyros};

What is the meaning of the "name.value" syntax mean? I see it positions to the left side. If I change to:

 \path (naveq.1cm)+(-\blockdist,0) node (gyros) [sensor] {Gyros};

it positions to the top.

Can both be done simultaneously and can measurement units also used for horizontal positions?

Answer 1

In a coordinate like (<nodename>.<value>), the <value> indicates an anchor of the node. It can either be a compass direction (north, north east, east etc.) or a number, corresponding to an angle in degrees. That is, foo.0 is the same as foo.east, i.e. the right side of the node named foo, and foo.90 is the same as foo.north, i.e. the top of foo.

Hence,

\path (naveq.140)+(-\blockdist,0) node ...

means start at the point that is about north west on the naveq node, then move \blockdist to the left, and place the node here.

However, when you do naveq.1cm, the 1cm is converted to pt (the basic unit for TikZ), and the point value is used for the angle. As 1cm is about 28.45pt, naveq.1cm is the same as naveq.28.45, as illustrated by this example:

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\node[circle,minimum size=3cm,draw,name=a]  {};
\fill [red] (a.1cm) circle[radius=5pt];
\fill [blue] (a.28.45) circle[radius=3pt];
\end{tikzpicture}
\end{document}

The output is:

As you can see both the red and blue dots are placed at the same place.