TikZ drawing with Geometry package

Question

I have the following test document:

\documentclass[11pt,a4paper,landscape]{article}

\usepackage[
    left=0.500cm,
    right=0.500cm,
    top=1.00cm,
    bottom=0.800cm,
]{geometry} % turning on showframe here makes
            % the thing even more puzzling (to me)
\usepackage{tikz}

\begin{document}
    
    \thispagestyle{empty}
    \begin{tikzpicture}
        \draw (0,0) rectangle (1,1);
        \draw (2,2) rectangle (3,3);
        \draw (4,4) rectangle (5,5);
        \draw (0,0) rectangle (27.9,20);
    \end{tikzpicture}

\end{document}

The first thing I don't know, and I didn't find any clue in the documentation to geometry, is if the margins (top, bottom, left, right) relate to the rotated page (because of landscape) or to the original page.

Whatever the case may be, I tried both: subtracting 1cm from the height of the landscape page and from the width. It does make a difference, but I still can't get what I would like: a rectangle that's exactly 0.5cm from the top and the bottom and exactly 1cm and 0.8cm from the right and left respectively. The current one (27.9 and 20; I tried other values) just goes to the very bottom of the page and doesn't stop at 0.5cm from the paper edge.

Sure enough I could do it with the page anchors of TikZ, but then I wouldn't know exactly how big they are and how they relate to the margins of geometry, which would be nice to know.

Answer 1

I think the first thing you missed is \noindent which shifts the whole tikz picture rightwards by 15pt, that is ~ 0.5cm.

There are other extra gaps such as \topskip added before the first line and lineskip. After adding \vspace*{-\topskip} and \nointerlineskip, it seems the tikz picture is now in place

\documentclass[11pt]{article}

\usepackage[
    left=0.5cm,
    right=0.5cm,
    top=1cm,
    bottom=0.8cm,
    a4paper,
    landscape,
]{geometry} % turning on showframe here makes
            % the thing even more puzzling (to me)
\usepackage{tikz}

\thispagestyle{empty}

\begin{document}
\nointerlineskip\noindent%
\vspace*{-\topskip}%
\begin{tikzpicture}
    \draw (0,0) rectangle (1,1);
    \draw (2,2) rectangle (3,3);
    \draw (4,4) rectangle (5,5);
    \draw (0,0) rectangle (28.7,19.2);
\end{tikzpicture}
\end{document}

Answer 2

If (as I suspect) you are just trying to draw a single drawing on a single page, and not trying to make this part of a larger document, then you can do this much more simply with the standalone class:

\documentclass[tikz, border=5mm]{standalone}
\begin{document}
\begin{tikzpicture}
    \draw (0,0) rectangle (1,1);
    \draw (2,2) rectangle (3,3);
    \draw (4,4) rectangle (5,5);
    \draw (0,0) rectangle (28.7,19.2);
\end{tikzpicture}
\end{document}

which you can compile to produce:

If you just want it to be A4 with arbitrary content, then try something like this (no margins, but an invisible A4 box).

\documentclass[tikz]{standalone}
\begin{document}
\begin{tikzpicture}
    \draw (0,0) rectangle (1,1);
    \draw (2,2) rectangle (3,3);
    \draw (4,4) rectangle (5,5);
    \draw[draw=none] (0,0) rectangle (29.7,21);
\end{tikzpicture}
\end{document}