[Proj] Google Earth Accuracy

Irwin Scollar al001 at uni-koeln.de
Mon Dec 1 14:28:08 EST 2008

MarKus Neteler wrote:

>How about precision in mountainous regions such as the Southern Alps?
>PROJ4/OGR reprojected mountain peak points from geonames.org as
>well as known points from our province are off for > 100m when
>visualized in GE which they match our local orthophotos. Is the positional
>error related to the height of the point (here, satellite orthorectification)?

In GE go to View, Sidebar and turn it on if it's not already 
on.  There, go to Layers, Primary Database, and uncheck the Terrain 
option checkbox.  That gives an orthophoto-like view even in the 
Alps.  You can check this by turning the GE Grid off and on in the 
View menu or pressing Ctrl + L.

It's far from perfect, but it's good enough for my application of 
mapping archaeological sites visible as crop or soil markings in the 
GE images over modest areas.  Cliff dwellings are fortunately rare 
exceptions. :-D

A more important concern is to increase the apparent resolution of GE 
imagery which is normally limited to the number of pixels of the 
display screen to approach that of a cadastral map of a small 
area.  Using the GE lat/lons of image corners which can be found via 
KML API routines, one can mosaic multiple zoomed detail images if the 
GE raw data is originally from a single good orthophoto or a mosaic 
made from several of these or even from a single Digital Globe strip 
in some parts of the world like the Nile valley.  I have had good 
results with this in moderately hilly terrain up to a factor of as 
much as 6 in some places on my 1600x1200 display.

Irwin Scollar 

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