[OSM-talk] satellite precision

[Iván Sánchez Ortega]

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El Viernes 26 de Enero de 2007 22:51, Nic Roets escribió:
> Perhaps Google is creating "satellite images that lie" by varying the
> offsets.

As R. J. Halos sad, "Never attribute to malice, that which can be explained by 
incompetence."

Keep in mind that the sat images have to be orthorectified[1] and 
georeferenced. That means that the imaes get rotated, scaled, distorted and 
filtered; I *guess* that this process is driven by visually locating geodeic 
landmarks and referencing those pixels to lat-long positions (or, better 
said, to UTM coordinates in most cases). If you simply don't care much about 
precision, you can get 4 or 5 geodetic landmarks, feed the lat-long positions 
to your software, and "get the job done".

Also, it is not google, but some sat imaginery companies who do this. Think 
TeleAtlas, NavTeq, GeoGlobe, iEye, etcetera. Tight schedules, lack of 
personnel, and differences between the different companies' datasets, or 
differences between the distortion algorithms seem to me the most probable 
cause of offsets in the sat images. Not malice.


[1] http://en.wikipedia.org/wiki/Orthophoto


Cheers,
- -- 
- ----------------------------------
Iván Sánchez Ortega <ivansanchez at escomposlinux.org>

http://acm.asoc.fi.upm.es/~mr/ ; http://acm.asoc.fi.upm.es/~ivan/
MSN:i_eat_s_p_a_m_for_breakfast at hotmail.com
Jabber:ivansanchez at jabber.org ; ivansanchez at kdetalk.net
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