\documentclass{beamer}

\usepackage[english]{babel}
\usepackage[utf8]{inputenc}
\usepackage[orientation=portrait,size=a4,scale=2]{beamerposter}
\usepackage[absolute,overlay]{textpos}
\usepackage{url}

\usepackage{tikz}

\parindent=0pt
\setbeamertemplate{navigation symbols}{}

\setlength{\TPHorizModule}{1cm}
\setlength{\TPVertModule}{1cm}

\def\minor #1{{\small\color{gray}#1}}
\def\remark #1{\minor{(#1)}}

\begin{document}

\begin{frame}[t]{}

  %%
  %% drawing grid
  %%

  \begin{textblock}{21}(0,0)
    \begin{tikzpicture}[yellow!70,yscale=-1]
      \draw[densely dotted] (0,0) grid (21,30);
      \foreach \x in {1,...,20} {
        \node at (\x,.5) {\x};
        }
      \foreach \y in {1,...,29} {
        \node at (.5,\y) {\y};
        }
    \end{tikzpicture}
  \end{textblock}

  %%
  %% title & logos
  %%


\end{frame}
\end{document}