Friday, September 23, 2011

Ground Truth

Geographers who work with remote sensing data call "ground truth" the final empircal test.  You never know for sure what the instruments are telling you until and unless you go there with your own natural-born senses. 

Over lunch, a couple of months ago, one of my former geography professors told me that despite two years on this field project, one of the graduate assistants became lost when faced with a detour.  Always focused on the vehicle's GPS, the driver had no sense of geographic context.  This is not unsual.

Last month, my wife had a similar experience.  Returning to her apartment from the airport, she told the taxicab driver which turn to make.  Focused on the GPS, the driver insisted on the direct route. Faced with orange barrels, they had to go four blocks around and the difference was the driver's lost tip.  The tip was included; the driver just burned it up. 

This happened to me twice when I first came to Austin.  Eyes nailed on the GPS, drivers from two different companies drove in circles looking for addresses that I could see from the back seat.  They ignored my help - eventually, my pleas.  In the first case, after my meeting, I walked back to the motel, a direct backstreet distance of a quarter-mile that cost me $10 in a taxicab via the freeways, offramps, and access roads.  After the second incident, I decided that I will never take a taxicab in Austin again.  I would rather spend the time waiting for buses, reading a book, than to spend the money on drivers who do not know their city.

I drove a cab three times in the 70s and 80s.  The first time, I was new in town. I depended on the map, of course, but as Alfred Korzybski told us, the map is not the territory.  I made notes.  My third time, though I knew the town much better, I still started out with a fresh map in booklet form, which I annotated, drawing in new subdivisions as I discovered them.  I do not blame a driver for not knowing every street, but I do hold any person morally responsible to use their own senses.

In remote sensing, we learn six ways to create "yellow." 
  • A direct radiation of 5600 angstroms.
  • Reflection: all other wavelengths absorbed; yellow reflected.
  • Full Red and Full Blue with No Green in an RGB display
  • Full Yellow and No Cyan and No Magenta in a CMYK display
  • Passing White light through a Yellow filter
  • Refraction of White light at an appropriate angle

It is a prinicple of objectivism (and empiricism) that the senses cannot be fooled.  When you see an "optical illusion" the trick is in the mind, not the eye.  When you see yellow, yellow is what you see.  However, as the rationalists warn us, we do not necessarily know how the yellow was created.  For that, you need "ground truth."

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