[OSM-talk] satellite precision

[Lars Aronsson]

Dave wrote:

> I think it more likely they just haven't bothered to sync them up very well,
> or for some reason they can't.

There's a kind of visual error called a parallax, which can happen 
if you try to measure a drawing with a ruler that isn't flat on 
the paper, http://en.wikipedia.org/wiki/Parallax

I think some of the Google maps error between map and satellite 
image can be explained by an error in the altitude estimate:

                   S
                  /
                 /
     --------z--Y------- A
               /
              /
     - - - - X - - - - - B

S = satellite; B = real altitude of the ground; X = real object on 
the ground; A = ground altitude estimated by the satellite 
company; Y = image of object X on the satellite image; z = where 
the object should have been instead of Y; distance z--Y = error.

You can see from the satellite imagery that the satellite isn't 
straight above the target (in zenith).  If it is off to the south, 
displaying the southern facade of tall buildings, the parallax 
error described above should also be off to the north or south, to
the extent that A>B or A<B.


-- 
  Lars Aronsson (lars at aronsson.se)
  Aronsson Datateknik - http://aronsson.se