How to make a line style for drawing the ground line?

Question

I'd like to draw a piece of "ground line" (picture below), but I don't know, how to make a line style and how to ensure proper localizations of the picture boundary. The solution of achieving an expected line style I found in the following discussion: laser beam drawing - the line having a gradient of colours which is perpendicular in each point to the line axis.

Is there any way of creating a customized line style, that will be executed by the command like \ground (0,0) -- (0.5,1) -- (1,0.25); or by option (for instance with the name ground) in the optional argument of \draw command?

According to @Loop Space answer (tikz: double lines are shifted) the bounding box covers all paths and is extra enlarged with the half of the line width. So by default any line might be placed within the bounding box while line join=round option is chosen (I prefer miter style). How to find the line boundaries to avoid unwanted cutting of the line?

In MWE below I applied an extension of lines with the use of point symmetry to solve the problem of vertical line endings (the best other solution given by @Loop Space in vertical line endings leads to not smooth line boundaries - as he demonstrated it).

MWE:

\documentclass{standalone}
\usepackage{amsmath}
\usepackage{pgfplots}
\usetikzlibrary{calc}

\newcommand\groundthick{12} % ground thickness
\newcommand\linesamount{30} % number of coaxial gray lines having various hue

\newcommand\groundpath[1]{
    \begin{scope}
        % start and end point
        \path #1 coordinate (rend) -- cycle
              -- (current subpath start) coordinate (lend);
        \begin{scope}
            \clip #1 |- (current bounding box.south) -| cycle;
            \foreach \lineno [evaluate=\lineno as \shader using \lineno*100/\linesamount] in {0,...,\linesamount}{
                \def\thicker{\groundthick*2-\lineno*\groundthick*2/\linesamount}
                % ground line member
                \draw [line width=\thicker,color=gray!\shader,
                       line join=miter] #1;
                % left ground line extension
                \draw [line width=\thicker,color=gray!\shader,
                       line join=miter,scale around={-1:(lend)}] #1;
                % right ground line extension
                \draw [line width=\thicker,color=gray!\shader,
                       line join=miter,scale around={-1:(rend)}] #1;
            }
        \end{scope}
        \draw #1;   % pavement
    \end{scope}
}

\begin{document}
    \begin{tikzpicture}
    \groundpath{
        (0,0) -- +(0.5,0.15) -- +(1.0,0.65) -- +(1.5,-0.12)
        -- +(2.0,0.56) -- +(2.5,0.58) -- +(3.0,0.65) -- +(3.5,0.6)
        -- +(4.0,0.98) -- +(4.5,0.55) -- +(5.0,0.95)
    }
    \end{tikzpicture}
\end{document}

EDIT:

After @cfr comment and answer I decided to try correct my question. The problem is connected with automatic finding of coordinates of the most external parts of lines. What I am doing in the code is depicted below:

After adding only the line in which the line is drawing with opacity=0, as proposed by @cfr below, the problem follows from the addition of unwanted margins, as well as from the removing of one line tip (generally of more tips):

And I also would like to ask, if the line:

\groundpath{(A) -- (B) -- (C)}

could be replaced by something like that:

\groundpath (A) -- (B) -- (C);

or like that:

\draw [groundpath] (A) -- (B) -- (C);

Answer 1

Updated version

This is more of a faff but it does clip the path correctly on at least 3 sides (north, west and east). It is a little harder to tell in the case of south because the path fades to white in this direction anyway. (So maybe this is less important - not sure.)

Faffy code:

\documentclass[tikz,border=5pt]{standalone}

\newcommand\groundthick{12}% ground thickness
\newcommand\linesamount{30}% number of coaxial gray lines having various hue
\begin{document}
  \begin{tikzpicture}
    [
      ground/.pic={
        \path #1 coordinate (rend) -- cycle -- (current subpath start) coordinate (lend);
        \foreach \lineno
        [
          evaluate=\lineno as \shader using \lineno*100/\linesamount,
          evaluate=\lineno as \thicker using (1-\lineno/\linesamount)*2*\groundthick
        ] in {0,...,\linesamount}{%
          \path [line width=\thicker pt, draw=gray!\shader, opacity=0] #1;
          \path [line width=\thicker pt, draw=gray!\shader, scale around={-1:(lend)}, opacity=0] #1;
          \path [line width=\thicker pt, draw=gray!\shader, scale around={-1:(rend)}, opacity=0] #1;
        }
        \coordinate (c) at (current bounding box.south);
        \pgfresetboundingbox
        \begin{scope}
          \clip #1 |- (c) -| cycle;
          \foreach \lineno
          [
          evaluate=\lineno as \shader using \lineno*100/\linesamount,
          evaluate=\lineno as \thicker using (1-\lineno/\linesamount)*2*\groundthick
          ] in {0,...,\linesamount}{%
            \draw [line width=\thicker pt, color=gray!\shader] #1;
            \draw [line width=\thicker pt, color=gray!\shader, scale around={-1:(lend)}] #1;
            \draw [line width=\thicker pt, color=gray!\shader, scale around={-1:(rend)}] #1;
          }
        \end{scope}
      }
    ]
    \pic {ground={(0,0) -- +(0.5,0.15) -- +(1.0,0.65) -- +(1.5,-0.12) -- +(2.0,0.56) -- +(2.5,0.58) -- +(3.0,0.65) -- +(3.5,0.6) -- +(4.0,0.98) -- +(4.5,0.55) -- +(5.0,0.95)}};
    \draw (current bounding box.north west) rectangle (current bounding box.south east);
  \end{tikzpicture}
\end{document}