[Tagging] Imagery variations/misalignments in iD - which to use?

André Pirard a.pirard.papou at gmail.com
Sat Jan 12 17:40:14 UTC 2019

On 2019-01-12 06:33, Eugene Alvin Villar wrote:
> On Sat, Jan 12, 2019 at 8:26 AM John Willis <johnw at mac.com 
> <mailto:johnw at mac.com>> wrote:
>     The issue I am facing is that, even after some adjustment of the
>     angle of bing imagery, there seems to be some distortion. things
>     don’t line up well between the Bing and ortho maps in some places,
>     and are much closer in others.  a *lot* of the mapping aligns with
>     the bing imagery, but there are areas of obvious 2-3m distortion
>     in places (the road is wavy), but other areas of newer/clearer
>     imagry align with the ortho imagry.
Bing is sometimes (often? I almost never use it) inconsistent (offset) 
with itself at different zoom levels.
> I also encounter this "wavy" roads in imagery. I think they are the 
> result of improper orthorectification by the imagery provider. 
> Satellite imagery is often off-nadir (not photographed straight down) 
> so providers correct for differences in terrain elevation by 
> rectifying them based on available elevation models. Unfortunately, 
> many elevation models like SRTM cannot distinguish between buildings 
> and hills and so roads are often distorted around tall buildings in 
> many parts of the world.
I don't think that orthorectification uses elevation maps as correction. 
Rather, they combine shots from different angles to compute the position 
of objects and that computes their elevation. A nadir photograph of a 
building is uninteresting because it only shows the roof. Side shots 
show the walls and allow computation of their height and ground 
location.  Aerial imagery shows only one of the many shots they have.
> I don't have any good solution for this aside from trying to get 
> access to better imagery so I just try to map things as best as I can. 
> It may also help to avoid micromapping unless you are sure that the 
> imagery is really good.
The orthorectification we can use in Wallonia (PICC) is really perfect. 
Of the lines I see it computes in places I know, almost only the center 
of the road can be not wrong but debatable. Sometimes lanes that have 
been changed to parking or things like that are not up to date, but 
that's manual fixing, not orthorectification.

All the best,


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