OpenStreetMap

Improving the OSM map - why don't we? [12]

Posted by marczoutendijk on 24 December 2015 in English (English)

What or who is our source?

When we map, we use something (or someone) to base our mapping on. Preferably in such a way that other mappers can verify what we added to the map.
To help with that task, we are requested to add a source=* tag to whatever we put on the map.
What are the most frequent used sources on OSM?
The top 3 (all used more than 10 000 000 times):
1. BAG - 18 840 435
2. cadastre-dgi-fr - 12 150 135
3. Bing - 10 695 411
You can see the full list on taginfo yourself.

The BAG is a large import of all buildings in the Netherlands. wiki is here.
In France the same was done. wiki is here.
Do I have to explain Bing?


The tag source=* can be used in numerous combined tags (like source:date) as can be seen here.
For this article I only used the plain source=* tag.
However, I was not interested in the source=* tags that were most used, but on the ones used less. And even more so in source=* tags that were incomprehensible. For instance, what should we think of this: Am I supposed to call that telephone number if I want to check the details?
I found quite a few instances of such source-tagging where a telephone number was used. And quite often this was used on bars and clubs. Some of them "members only".


A telephone number is at least something that could be used as a source of information, although it is not very informational. But what about this one? What source of information is hidden in 0,4? This one is also not very helpful: And sometimes mappers have some sense of humor, like: "Q" (think James Bond) is of course a wealthy source of information: And this one surely used a source, but left us just guessing: And this one? It is clearly a mistake, because the mapper use a colour code in the source field. Finally, the weirdest I saw: I contacted the mapper to ask what it meant, and it must have been some mistake. He maybe corrected it already at this time. To see to the situation it refers to, check this.

The list is endless, sometimes hilarious, but should we do something about it? I don't think so, relatively those errors are a minority, because they are used very seldom. Mostly just once, hence they don't take up very much space.
But using a reliable, understandable source key, would surely help improving OSM! Think about your source the next time you let other mappers now where your data came from!

All screenshots with openpoimap.

Comment from SimonPoole on 26 December 2015 at 10:03

Actually ... the concept of attaching a source tag (except in extraordinary) circumstances to an object has been defunct since the introductions of changesets,, 5 years or so ago and it has been not considered good practice for multiple years.

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Comment from marczoutendijk on 27 December 2015 at 16:17

@Simon,

It doesn't matter very much where (and how) the source information is stored if it is incomprehensible to others...

Btw, your opinion and the wiki differ.
Moreover, general use (see taginfo) shows that the source tag (and the source namespace) is in wide use.

Marc.

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Comment from SomeoneElse on 27 December 2015 at 22:12

I reckon that it's almost always possible to figure out what even "incomprehensible" tags actually mean (or at least what the mapper was trying to do at the time). A while back SimonPoole mentioned "yes=no": https://twitter.com/sp8962/status/643894726366789632 . I had a look at those, and even something with something as nonsensical as that it was possible to figure out the meaning (e.g. a mapper was trying to change tiger:reviewed=no to tiger:reviewed=yes). That's part of the reason why I find "tagfiddlers" (people who just remove tags that they don't understand, without asking the previous mapper) annoying.

On the more general point about source tags, if you really do use only one source for an entire changeset then it might make sense to use a changeset source tag (and when I do, that's what I do) but most of the time the source of anything is much more complicated (for me usually some combination of notes, new GPS traces, previous GPS traces, imagery and government open data). That's when I find element source tags useful.

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Comment from Vincent de Phily on 28 December 2015 at 23:39

iD still can't set anything else than a comment on changesets, so its numerous users can only tag source=* on individual objects (and also tend to overload the changeset comment with stuff that'd be better placed elsewhere, like hashtags).

IMHO taging the objects is useful when things are not straightforward (typically when multiple source:= tags are needed), but that's an optional addition to tagging the changeset.

When you want to know the source of some data, the source*=* tags on an object are never as good as looking up the object's history anyway. Object's source=* tags are rarely updated. The changesets date, user, comment, source, and imagery tags are invaluable and easy to fetch.

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Comment from dieterdreist on 11 January 2016 at 20:00

actually I agree with Simon, a source tag on the object might at most make sense at version 1 but as soon as you modify something pre-existing you come into trouble: you have to weigh the importance of your modification and its source against the source of the previous edits. What do you do, add your source to the existing one? Replace the old one with the new one? Conceptually the source belongs to the edit, not to the object.

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Comment from marczoutendijk on 14 January 2016 at 10:21

Reactions to this topic show different visions and point to various problems with a proper way of recording the source of our mapping. Suppose this:

A new building (in my hometown) appears on the map because it was imported under the BAG rules that were accepted in the Dutch community. One of the tags added with this import is: source=BAG.

Next, a grocery is starting buisiness in that building and is added by me (to the existing tags) and I can do this because of local knowledge. But at the same time I add the phone-number for that shop, using the yellow pages as source and from an advertisement for that grocery I can learn that they only sell natural grown groceries.
I have now 3 different sources at the same time.
Should I:

  • record all that source-information in the changeset comment?

or

  • add this information to the existing source=BAG tag, separated by ";"?

or

  • create phone:source=yellow pages etc, etc, for all the other sources?
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