HOW A SAND HEAP IS BUILT ?
A sand particule slides towards the nearest edge.
A ridge separates the two sides.
A ridge is therefore at equal distance of the edges of the two sides.
Top side view of a sand heap
The ridges are thick marked.
The arrows indicate the path of the sand when it flows.
Two corresponding arrows are equidistant.
POSITION OF THE RIDGES
When ridges are projected onto a plate, the bisector of the angle
formed by the edges of the two sides is obtained.
Starting from a quadrilateral, and by moving one side, two possible
orientations for the the ridges are obtained.
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