[Tagging] Map maintenance with StreetComplete - Preferred tagging

Tobias Zwick osm at westnordost.de
Thu Jul 23 16:06:24 UTC 2020

Hello everyone

As you may or may not know, my microgrant proposal "Map maintenance with
StreetComplete" [1] was selected to be funded by the OSMF. So, I am
happy to have the oppurtunity to invest the time  extending the app,
hoping that it will help to keep the map up-to-date and unburden people
and communities invested in that topic.

I am pitching this here because there are three details on which I would
like to hear your input. Two of these are about tagging.

But first, how will it work?

So, what StreetComplete will do is to scan the map for whether certain
properties (tags) of map features haven't been surveyed for a certain
time. If this is the case, users will be prompted to answer the question
for that property again. For example, if the app ascertained that the
smoothness of a road hasn't been changed for 5 years, it would ask users
again about the smoothness of the road.


Now, you might imagine that this is not so straightforward to implement,
and you would be right, for several reasons:

Firstly, the OSM API has no notion about when a particular property of
an element has last been changed, only for when the whole element has
last been changed. But elements are changed all the time for various
reasons. Roads for example tend to be changed especially often, splitted
up to accomodate bus routes, turn restrictions, when detailing
intersections, etc.

Secondly, there is only the date of the last change, but that doesn't
mean that is the date of the last survey. Even though that would be the
information we are interested in: when has the element last been checked

Thirdly, the OSM API does not offer users to record solely that they
checked something and that it is still up to date. Any new record in OSM
must always come with a tagging change.


In the absence of direct API support, fortunately the community came up
with a solution: Add the check_date tag to the element that has been
checked on a survey - or with the namespace if it concerns only a
certain tag of a map feature.

This works well and is actually already used by Streetcomplete in the
"Is this construction site finished?"-quest:
If the element as a whole hasn't been changed for 6 months *or* the
check_date key is present and its value is older than 6 months, the
quest is eligible to be asked again. When the user answers the question,
the check_date is set to current date.

Your input

Now here is where I would like your input:

1. Use check_date:smoothness or smoothness:check_date?
check_date with a namespace isn't used all that often yet, both variants
are used and thus there is no real winner yet. What variant do you
prefer and why? And most importantly, which variant would be most
consistent with existing tagging practices?

2. Always record check_date or avoid tagging it where not absolutely
If something is resurveyed and it is acknowledged that nothing changed,
it is absolutely necessary to tag check_date. If something changed, it
is not strictly necessary because that the element changed as a whole is
itself also recorded.
But as already mentioned, elements can and do change all the time. One
can not assume that if an element has been changed that it has been
checked on-site. And even if one could, maybe not all the things were
checked but for example only the bus route relation, or maybe only the
presence of a sidewalk, or someone made the way a little more detailed etc.

The topic whether StreetComplete should only tag the minimum of what is
necessary to ensure functionality or always tag check_date (at least for
properties that are eligible for re-survey) was already subject to
discussion in the issue tracker here:

Maybe it is obvious that my opinion is that StreetComplete should always
tag check_date as it also adds valuable information for other surveyors
that do not use StreetComplete. Nevertheless, in the GitHub ticket
linked above, I played a bit of a devils advocate for the other point of
view - for being frugal with such meta-tags.

So, I'd like to collect what are the advantages and drawabacks of adding
check_date to all the tags surveyed on-site, with your help.

3. At what intervals should questions be asked?
Certain properties can be expected to usually not change in tangible
amount of time. For example, properties such as the structure of a
bridge, the levels and roof shape of buildings, street names and
housenumbers don't or change so infrequently that it is not worth
sending users every once in a blue moon to re-check the data. Others
will change more frequently, such as the smoothness of roads and ways or
anything related to businesses as they close and other shops open in
their place. Sometimes, it is also dependent on how it is currently
tagged whether it is likely that it changes within a number of years: If
a road already has a paved surface such as asphalt, it is less likely to
change than if it was just a compacted road. Same with facilities for
the blind or other things that "upgrade" infrastructure. The issue was
already d

Long story short, not all quests [2] would be eligible for re-survey and
those who are will have different intervals, partly also based on how
they are tagged now. I could use your input on how long these intervals
should be. The issue was already discussed in a GitHub ticket [3], but
now prepared a wiki page here in which further discussion could take place:



So, that's all for now. Looking forward to read your constructive input!


[2] https://wiki.openstreetmap.org/wiki/StreetComplete/Quests
[3] https://github.com/westnordost/StreetComplete/issues/1766

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