[OSM-talk] finding overlapping buildings

john whelan jwhelan0112 at gmail.com
Tue Nov 28 23:46:13 UTC 2017


I don't have an interest in buildings as such what a lot of my mapping is
concerned with validating HOT projects.  I generally sit on three and check
the work as its done.  Working like this I can catch new mappers making
mistakes and give them gentle feedback to get the overall quality of
mapping up a little.  It also helps get the project completed.

We are talking very new mappers here doing things like labeling buildings
name=road.  building=unclassified, highway=yes.  area=yes has dropped off
recently from the new mappers but its still fairly common.

As a percentage the errors aren't that high but from a data quality point
of view every little helps and if you catch them young they'll add fewer
errors in future or at least that is the hope.

Maproulette I seem to recall will spot crossing highways with no node.
Currently there are about a thousand of these in Egypt looking at
unclassified and above.  JOSM validation does quite a nice job of detecting
these and its a lot quicker than running maproulette.  Occasionally I'd
come across a pocket of duplicated buildings where one mapper had drawn
over the buildings a second time.  Sometimes it was even the same mapper
and that's where the interest came from.

I am aware that the area and number of buildings is used to estimate
population the exact incantations needed to do this I'm not familiar with
but duplicated buildings throw these calculations off because the same
building is counted twice and that's why I'd like to get rid of them.

One problem with using an XML dump is they age very quickly.  I use country
dumps, for adding crossing nodes and untagged ways they are fine for
roughly four days.  After that you start to get conflicts.  When deleting
the last thing you want to do is delete one of a pair of buildings if the
other has already been deleted.  So for deleting duplicate buildings it's
very much a case of go to the area and freshly download it.

For the case of twenty buildings mapped as one.  I really don't know what
to do.  From a population count point of view the area is roughly right and
generally I leave them alone.

Again for badly mapped buildings where they aren't square what do you do?
If you square them its an approximation and my science background says
anytime you approximate something you are adding in errors.

For ones that are three times the size of the buildings and look like a
flag blowing in the wind?  Again what do you do?  I think these are close
to junk and should probably be deleted.  Others will say no they should be
cleaned up.  Fine except cleaning them up takes far more effort than it
would take to remap them with the building_tool plugin and they really
screw up the population estimates.  Many of these have a HOT project number
in the tag by the way.

Cheerio John

On 28 November 2017 at 17:57, SandorS <sandors39 at gmail.com> wrote:

> John,
>
> yes, the buildings in Africa are really messed up. But yet, they are there
> in contrast to some largest digital map-makers. Just take a look on some
> examples like here https://goo.gl/CYVahc or here https://goo.gl/UWuBcf or
> here https://goo.gl/9iNYCm or here https://goo.gl/jUuqmS. There are
> several 10s of thousands similar errors like in the examples. But even
> though the absolute number is large, it is only a percent or two compared
> to the total number of buildings in Africa. And as I told you it is
> possible to repair almost all the above illustrated anomalies. It is
> challenging and I am working on it using the source data (XML dump) from
> some weeks ago.
>
> So, if you are patient for several days, you will get the buildings for
> Africa probably with the best preparation and based on the potential in the
> source data. The data will be in shp format accompanied by some statistics,
> additional geometries for control and a short documentation.
>
> Of course, if you don need these I will not bother you. But at least, you
> could use the stuff as an option to compare if you use other models.
> Anyway, the subject might not be of interest for other members so if you
> still want the buildings we could just switch to bilateral communication
> about some details (for instance Africa, the continent and Madagascar,
> eventually some small islands, in which projection(s) and so on).
>
> Regards, Sandor.
>
>
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows 10
>
>
>
>
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