Monday, February 24, 2020

Thoughts about intervisibility, Part 2

In my last post I tried to suggest that intervisibility was a human idea and not one strictly mathematical.  Sure, if the math shows a big obstacle between sites A and B then they are not intervisible.  There are gradations however.  Suppose a town is just barely behind an obstacle (like a ridge).  Mathematically speaking your town and that town are not intervisible.  But you can see the smoke from their fires during the day and a glow in the sky at night.  If, imagining the worst, an enemy destroys that town you will surely see the smoke from the fires and know that something is wrong.

I've tried to make allowances for such situations in the intervisibility software on the Mycenaean Atlas.  Of course I add 1.8 m to the elevation of the source site (site A) in order to mimic the height of an observer.  And now I've also added 3 meters to the elevation of the target site (site B) in order to get rid of some edge cases where site B sits juuuussst below the horizon but really should be counted as intervisible.

The point is that this intervisibility page is a tool for exploring intervisibility.  The mathematics cannot definitively make a decision about a fundamentally human idea.  If you see a result that you don't agree with then you should pursue it further and make up your own mind.

It's easier to do that now because my friend Xavier Fischer, of elevationapi.com, has provided an additional tool.  Now you can generate an intervisibility graph that addresses the sites pairwise.  Here's an example:

This shows the intervisibility sightline from, on the left, Kastro (C714) in the neighborhood of Gla to Magoula   Kavkala (C984) about 2.25 km. distant.  Because no obstacles intrude on the red line then the sites are intervisible.  The bottom scale is distance in km.  The left side scale is elevation in meters.  Notice that I add 1.8 m. to the starting elevation (on the left) and a few meters to the right.  That helps to tip the two sites into intervisibility.

So how do you exercise this fine new tool?




On the new intervisibility page you simply click on the 'true' or 'false' values in column 3 of the intervisibility list.  When you do that the graph will display. 

Here is an example of the I. graph between Kastro (C714) and one of the cemeteries at Hagia Marina (C728) directly to its east.  These two sites are intervisible; the resulting graph is:



You should keep in mind that this graph's x-axis (distance) is compressed in comparison to the y-axis (elevation).  In this particular graph one major division in the vertical (5 m) has the same size as 250 m. in the horizontal - a ratio of 1:50.  So all these graphs are what statisticians call 'Oh boy!' graphs.  Always be sure that you understand the relative scales on these graphs before trying to interpret them.

News of the Mycenaean Atlas Project

On February 12 I delivered version 131a of the database.  It contains many additions and minor changes.  At that time a new version of the software was delivered in order to support intervisibility.

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And, by the way, friends don't let friends use Facebook.



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