list manipulation - How to extract the edge from a set of points

I have an accumulation of points representing a body as you can see in the image below. From this data I want to generate a list of points or a curve, which is the envelope.

The first idea was a convex hull, but the shape of the body is not convex.

Is there a way to extract the points at the border?

click here for the data

Answer

One very simple basic approach is the following (I assume you already loaded the variable spiel)

With[{m = Normal@SparseArray[spiel -> 1]}, 
 boundary = Position[m - Erosion[m, 1, Padding -> 0], 1];

 ListPlot[{Position[m, 1], boundary}, PlotStyle -> {Gray, Red}]]