Viewshed Analysis, or What Could Sauron’s Eye Really See?

Author: Amy Rock

Adapted from the blog post by Robert Rose, College of William and Mary, and with the kind permission of  jvangeld and the ME-GIS team for use of their data.

Introduction

Frodo and Sam, in their perilous journey across Middle Earth, faced many dangers, not the least of which was being spotted by Sauron before they could reach Mount Doom and destroy the One Ring.  While Sauron could track the One Ring while it was being worn, and had various spies tracking the Fellowship, it was harder for him to know where they were without direct line of sight. In this lab, you’ll determine how much of Middle Earth that terrible eye could really see.

Data

Middle Earth DEM and the citadels layer: https://bit.ly/Mordor_Data

Modeling the terrain

1.      Add the Middle Earth DEM to your map.

2.      Verify that the DEM (and your data frame) are projected to UTM_Zone_31_Northern_Hemisphere; project if necessary.

3.      Make a Hillshade from the DEM (Spatial Analyst > Surface > Hillshade). Put it in your working folder, and leave all settings on default.  (Did you remember to turn on Spatial Analyst?)

4.      To enhance the appearance of the hillshade, go to Symbology and set the stretch type to Standard Deviations.

5.      Set this aside for later, to use in your map

What could the Eye see?

1.      Add the Citadels layer.

2.      Select Barad-dûr from the Citadels layer (Select by Attributes, or manual selection)

3.      Export Barad-dûr to a new shapefile and add it to your map.

4.      Remove Citadels from your map.

5.      Run the Viewshed tool  (Spatial Analyst > Surface > Viewshed), with the DEM as your input raster and Barad-dûr as your input point. Save the output to your working folder and leave all other settings on default.

Notice that the visible areas are largely confined to a few wedges, mostly north of the tower.  This doesn’t seem especially menacing, does it?  However, recall that the Viewshed tool assumes you have a height of 0. To correct this, we need to add a property called OFFSETA to account for the height of the tower.  Estimates put Barad-dûr around 5000 feet, but your map units are in meters, so you’ll need to convert that.

6.      Open the attribute table on the Barad-dûr shapefile and add a field named OFFSETA with a field type of Short Integer.

7.      Use the field calculator to update the value to your calculated value.

8.      Close the attribute table and run Viewshed again. You should now be able to see a considerable portion of the map.

Sample result map

“It’s the job that’s never started as takes longest to finish.” — Samwise Gamgee