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Reconstruction of the Great Wall


	Historically, the Great Wall stretches over approximately 6,400 km in total from Shanhai Pass in the east to Jiayu Pass in the west, along a rough arc. After hundreds of years of natural erosion and human activity, the Great Wall is vanishing. A survey shows 20% of it is in ruins and another 50% has already disappeared. The existing locations of the Great Wall are shown as points. Their GPS positions come from Google Earth. What can those points tell us? What was the length of the Great Wall? This Demonstration shows a method for reconstructing the Great Wall by using a minimum spanning tree from the GPS points, giving a maximum distance of 3365 km.


	Contributed by: Frederick Wu

The graphics show randomly selected points connected using a minimum spanning tree by Kruskal's algorithm.

Snapshot 1: nine random points (10% of the points) with a connected length of 2465 km

Snapshot 2: 45 random points (50% of the points) with a connected length of 2853 km

Snapshot 3: all 90 existing points (100% of the points) gives the maximum length of 3365 km

The initial location data of the Great Wall is based on GPS coordinate points from Google Earth. Most of the existing GPS points are distributed to the north of Beijing.

For more information on the Great Wall of China, see the Wikipedia entry.

Connecting Towns Using Kruskal's Algorithm (The Wolfram Demonstrations Project)

Shortest Path between Two Points on a Sphere (The Wolfram Demonstrations Project)

Spanning Tree of Points on Sphere (The Wolfram Demonstrations Project)

"Reconstruction of the Great Wall" from The Wolfram Demonstrations Project
http://demonstrations.wolfram.com/ReconstructionOfTheGreatWall/

Contributed by: Frederick Wu

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