[OSM-talk] Aerial photo offsets

[Andrew Errington]

Hi everyone,

Forgive me if this has been covered elsewhere, but I am wondering what can be 
done about the need for aerial photo offsets.  There's very little in the 
wiki[1]

In Korea (as elsewhere, I assume) the aerial photos are not always accurately 
aligned with reality.  Across the country different offsets are needed 
(sometimes no offset).  What I do is look for a set of GPS traces from a very 
visible landmark and then use them to align the aerials.  If there are none I 
will often find a visible landmark, such as a park, or a running track in a 
stadium, and make several GPS traces of my own on different days, so that I 
can use them to line up the aerials.  I know that one trace is insufficient.

I know there are tools in JOSM and Potlatch to move the aerial photo layer 
around, and JOSM even stores them for future use, but surely we must all have 
the same problem around the world?  Is there any project underway to record 
the offsets centrally, so that we don't all have to have our own (slightly 
different) offsets recorded?  It's a big problem when mappers map something 
*only* from aerials, and don't compensate for the offset (this is different 
from the parallax error from tall buildings).  When I map the same area later 
with GPS traces I often find that I have to move many objects.

So, I have two suggestions:
1) Have all mapping software pop up a warning when a mapper chooses to display 
aerial photos.  The warning could say "Warning: Aerial photos may be slightly 
offset.  Please check photo alignment before tracing aerial features".
2) Have a central repository of offsets for the main aerial photo providers.  
In the application (JOSM, for example) we can choose to automatically use the 
offset for the current viewport, or our own local one, or submit a better 
one.

Any comments?  Any suggestions to improve my technique?  Any pointers to 
something that already does the Right Thing?

Thank you,

Andrew

[1] 
<http://wiki.openstreetmap.org/wiki/Yahoo!_Aerial_Imagery/Accuracy#Mapping_with_an_offset>