[HOT] Question about background image alignment

[Dan S]

Hi -

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.

If you are in an area where enough GPS traces are available, you can
move the imagery to match those before you trace. GPS traces are
canonical, not imagery.

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.

Best
Dan


2014-08-13 15:51 GMT+01:00 Erik Walthinsen <omega at omegacs.net>:
> [Not on the the list, just trying to get an idea of what is 'correct']
>
>> JOSM is better to do such a Job.
>
> I'm also looking at a ~7.5m misalignment in parts of Monrovia, when viewed
> on Bing imagery in JOSM.
>
> Should I assume then that JOSM alignment is "always correct" or at least
> always *canonical*?  [e.g. if there really is an offset it can be corrected
> *en-masse* when either better imagery or real measurements become available]
>
> If so, there's a significant error source somewhere in the HOT datapath.  I
> haven't done any editing via the website since I work in JOSM, so I don't
> know what editor it brings up and what imagery, but it seems like any offset
> in that path is systemic and should be corrected ASAP before all these areas
> get mapped with mixed offsets.  I've realigned a number of roads in that
> area and drawn stuff in outlying areas that may get "messed up" by future
> edits if not done in JOSM....
>
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