[HOT] Question about background image alignment

Wed Aug 13 18:38:40 UTC 2014

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Yes looking at the GPX traces in the area the current Bing looks very
well aligned (by the way that imagery layer is awesome, thanks for the
url on that; been looking for something like that for a while).

Regarding the things which are offset and showing Bing as a source, I
would say it was probably some older bing imagery that was not as well
aligned.  Bing updated their process in the past couple years and the
imagery they put online now is much better aligned than it used to be.

I would say it is safe to align all the data in the city to the
current Bing imagery.

Regarding different editors: if you are not using a manually set
offset (or the offset DB) then all of the editors will show the
imagery aligned the same, so that is not something that needs to be
checked.

- -AndrewBuck

On 08/13/2014 11:54 AM, Dan S wrote:
> 2014-08-13 17:20 GMT+01:00 Erik Walthinsen <omega at omegacs.net>:
>> 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).
> 
> Those people *may* have used bing but corrected the imagery offset 
> before tracing, by aligning the imagery to some GPS. I don't know,
> I haven't looked, but that would be the most optimistic state of 
> affairs! It's the best way to work.
> 
>> 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.
> 
> OR they are indeed using bing and are offsetting the imagery
> before doing their work.
> 
> If you look at the history for the changeset in which your way was
> added: http://www.openstreetmap.org/changeset/24645722 you see
> "created_by JOSM/1.5 (7347 en)" so we know it was JOSM.
> 
> I suspect they manually aligned bing before tracing.
> 
> (You could even ask them, I guess!)
> 
> You're right that they may also have used the alternate imagery
> source that you mention, and forgotten to fix the source tag. But
> the solution isn't to pick which imagery is canonical, the solution
> is to work out how much offset the imagery in your area needs, in
> order to align it to GPS.
> 
> When I look at the GPS tracks in the area you mention, I see
> pretty good alignment between the road data and GPS. (I like to use
> the rainbow GPS tiles to view this: 
> <tms:http://{switch:a,b,c}.gps-tile.openstreetmap.org/lines/{zoom}/{x}/{y}.png>.)
>
> 
There are not many tracks in the area, but the data doesn't look to me
> to be systematically misaligned, it looks to be probably correct.
> 
> Hope this helps! Best Dan
> 
> 
>> 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....
> 
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