[HOT] Question about background image alignment

[Erik Walthinsen]

On 08/13/2014 08:44 AM, Dan S wrote:
> The answer is no, when you first load an imagery layer into JOSM it
> does not offset it, and there may be a systematic misalignment
> relative to GPS coordinates.

OK, I guess I wasn't quite precise enough in my query:

What I'm seeing is that much of the data in e.g. Monrovia is offset by 
7.5m relative to the Bing imagery.  That means that either Bing is 
misaligned, or the imagery being used by many of the people entering 
data is misaligned (or both, most likely).

Potlatch2, iD, and JOSM all show the same offset vs. Bing, so we can 
eliminate *those* editors as the issue.

Even worse, the particular ways I'm looking at (e.g. way 296966938) 
claims a source of Bing, which means that either they *aren't* using 
Bing imagery yet claiming to do so, or they are using some other mapping 
program that's faulty.

Another example would be node 1102111387 which I corrected a few days 
ago.  It was last edited in 2012 and also claims a Bing source, yet was 
offset the same ~7.5m east relative to what JOSM shows.

A couple days ago I agreed to a license for an alternate imagery set, 
but the TMS link didn't work in JOSM, so I dropped it.  I can't find it 
anymore on the hotosm page, so I can't determine if it's the original 
source for this data.

Whether or not Bing or this other imagery is more correct isn't 
necessarily the issue right now.  The problem is that data is being 
entered with one, then corrected against another, then maybe realigned 
back to the original offset.  Where one region is adjusted, it screws up 
everything on its edit boundary.

*One* of them should be deemed "canonical" in the short term, and the 
Bing imagery seems to be the best candidate because it's the default in 
all the editors.

> More generally, you can use the Imagery Offset Database to find out if
> other people have suggested an offset for the area+imagery you're
> working with: http://wiki.openstreetmap.org/wiki/Imagery_Offset_Database
> In particular, check out the JOSM plugin which checks the database for
> you very simply.
There are no offsets for the Bing imagery anywhere in the area....