18
Here's one possibility:
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{intersections,shapes.arrows}
\newcommand\Dist[1]{\phantom{\rule{#1}{4pt}}}
\begin{document}
\begin{tikzpicture}
% the bottom left border of the surface
\path[name path=border1] (0,0) to[out=-10,in=150] (6,-2);
...