On 4/9/2012 11:01 PM, Russ Nelson wrote:
Nathan Edgars II writes:
> It's not as bad as it seems. Imagery is adjusted using an elevation
> dataset. Since this data doesn't (and shouldn't) include buildings and
> bridges, these appear distorted. You'll also see problems where recent
> heavy construction has caused changes in topography.
Or where the elevation dataset doesn't include a deep canyon, which
causes a straight bridge to appear curved. If it's a railroad, you can
be pretty sure it isn't. If it's a road bridge, you have to rely on
what you saw when you were there.
I think this is because the elevation data *does* include the canyon.
Since the image was taken at an angle, the bridge appears at a different
place in the canyon, and must curve to reach the correct location at the
top.
Here, for example: http://mapper.acme.com/?ll=38.06661,-81.08022&z=16&t=O
The camera was to the northwest, so the bridge was on a line with the
canyon southeast of its actual location. The bridge down in the bottom
of the canyon is also curved, but much less so because it's smaller.
It's also in essentially the correct place (as seen by comparing to
Google's photos and USGS topos).
(crossposted to talk-us because who knows when the talk@ mods will let
this through)
_______________________________________________
Talk-us mailing list
Talk-us@openstreetmap.org
http://lists.openstreetmap.org/listinfo/talk-us
