[HOT] How bad is mapping from offset imagery?

john whelan jwhelan0112 at gmail.com
Tue Apr 4 22:55:11 UTC 2017

When I validate or go searching for untagged ways I quite often come across
a building mapped two or three times its actual size.

If its important for estimating population etc perhaps this could be
mentioned in the instructions for the project.

For the moment I'm just adding building=yes tags.  There are too many to
correct manually and I must confess it takes longer than to delete and
remap with the JOSM building plugin.

Thanks John

On 4 April 2017 at 13:54, Andrew Buck <andrew.r.buck at gmail.com> wrote:

> For almost everything HOT does, and really OSM in general alignment is
> really not that big of a deal.  Today, almost any imagery we get access
> to is aligned to within at most 10 meters (and lots of it is within 2 or
> 3 meters) of reality and in sparsely populated areas a few meters offset
> is not a big deal at all (especially considering the accuracy of a gps
> is only a few meters anyway).
> For most of what we do, building counts are the main thing humanitarian
> groups are interested in.  The second most important thing is the size
> of the buildings (for example not counting really small buildings in
> population estimates, or marking really large buildings as likely
> candidates for visits by aid workers since they are likely schools,
> etc).  Neither of these are affected at all by offsets which are
> measured in meters.
> The only place offsets really become important is in urban areas, or
> where different "layers" of data were mapped at different times (for
> example roads done a few years ago and buildings done today).  In these
> cases large offsets can mean that a building on the east side of a road,
> gets mapped on the west side of the road due to the offset.  In these
> situations consistency is the main concern so that is why we have
> instructions to try to align the imagery.
> Regarding the accuracy of Bing, we don't know that it is good
> everywhere, however just about anywhere we compare it to gps data it
> lines up nearly perfect (like to within a meter).  So given that they
> have a great track record so far, we tend to defer to them as a baseline
> if no other information is available.  Also, since bing is in all the
> editors and is nearly worldwide it just makes for a consistent baseline.
>  I.E. it is not necessarily perfect, but since consistency is more
> important than absolute accuracy, bing wins just by being the most
> consistent over large areas.
> So the short answer is, we always try to be as accurate as possible, but
> small offsets are not worth losing sleep over.  Don't spend a lot of
> time trying to match up trees in two sets of imagery so that you can
> correct the 5 m offset on the only road for miles.
> -AndrewBuck
> On 04/04/2017 11:07 AM, Florian Niel wrote:
> > I often see that other mappers don't align the background imagery. I am
> > wondering how a big deal this is.
> >
> > Aligning the imagery in tasks like the malaria elimination projects is
> > difficult, if the square does not contain any data (and no gps traces).
> > I do it (in JOSM) by clicking some points of a road or path or something
> > other visible in both Bing and the recommended imagery, and check if
> > there is an offset and then align it to Bing. Then I delete the path I
> > just mapped and start mapping the buildings.
> >
> > With this method I need more time for the alignment than for mapping the
> > two or three buildings to finish the task.
> >
> > So I have questions:
> > 1) Is there a faster way to align the imagery on empty squares?
> > 2) Is it really necessary to align it for just a few buildings and a lot
> > of nothing (mapped) around? As I dont know what the data is used for so
> > I cannot tell. The size and the shape of buildings are correct anyways
> > and my intention would say it is no big deal if it is a bit offset.
> > 3) Is it possible to align the imagery in advance before opening the
> > task for HOT so mappers do not have to care about it?
> > 4) Is Bing really always better aligned or are we doing just a shift
> > from one unknown offset to another?
> >
> > Another comment on alignment: I just looked at the video from Blake
> > (https://www.youtube.com/watch?v=1Pcj_wFTHeE) about how to align imagery
> > in iD. I think we need a better one. One thing is that it is not
> > comparable to empty squares (like in many malaria-project squares) and
> > the other thing is that he comes to the conclusion that he should not
> > align the imagery at all (because it is a messed up square with objects
> > mapped from different imageries).
> >
> > Best,
> > Florian
> >
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