Wednesday, February 5, 2014

County Borders in OpenStreetMap

I have had a fairly steady relationship with county borders in OSM. They were originally imported before I joined the project in 2010. However they were imported as overlapping closed ways which is not ideal. In some places people had started working on de-duplicating the ways and turning them into relations. However this is tedious work and some of the people who attempted it didn't quite understand how relations worked so it was pretty much a big mess.

Throughout 2011 and 2012 I periodically came back and worked on county borders and eventually got to a place where there was a relation for every county in the nation. There are still some minor differences in tagging but I wasn't as concerned with that and tried not to change the work of other mappers as long as the relation existed and had valid geometry. At the same time I also reduced complexity at state borders. There used to be a mess of admin boundary ways. One for the state, one for each of the counties on either side of the state border and possibly also some city boundaries. They usually didn't match up very well either so it looked terrible. I didn't mess with cities but at least I made the state and county borders share a single way to reduce clutter.

At some point I also noticed (thanks to the MapQuest Open rendering which renders county names) that the place=county nodes were all placed at the extreme eastern border of all the counties. This caused the county names to be rendered in unexpected locations. So I ended up fixing those as well. Here is a screenshot of the last changeset where I moved over 1,100 nodes to the middle(ish) of their respective counties:



The Effects

So that was nice. But there was still some weirdness with counties, particularly when geocoding in Nominatim. Especially if there was a city with the same name as a county. For example, I searched for "Kearney, NE" and the first result was the place=county node for Kearney County in Nebraska. What I was looking for and expected to find first was the city of Kearney which is in Buffalo county. Then there is the issue of Nominatim finding both the county node and the boundary relation when you search for a county. The same thing often happens with cities. Really, they are the same thing and should show up as one result. And sometimes it might. Nominatim tries to be smart about this and does remarkably well but sometimes it needs help.

You might say we should just delete the node since we already have the boundary information. But very few renderers use boundary relations to render place names from. They all rely on these nodes. The same goes for cities, states and even countries I believe. There is also the oddness of what to use for county name. The relations all have a "name=Kearney County" tag while the nodes just have "name=Kearney".

After poking around a little and bugging lonvia (maintainer of nominatim.osm.org) about it a couple of times, I learned a few things about Nominatim and was able to make some fixes. It was my intent to come back to this some day and figure it all out and fix up county relations once and for all. I still hadn't gotten around to that but just in the last few days, new user revent was getting annoyed at the geocoding problems in Texas and really dove into Nominatim to figure out the best practices. He wrote up a diary entry on the subject and we have since then talked together some more on IRC and since I had work cancelled today because of snow I thought I would write up this post about my current understanding of things:
  • Nominatim is able to "link" multiple elements into a single feature. It tries to do this automatically but doesn't always get it right.
  • Nominatim uses Wikipedia to help determine the importance of a place.
  • Nominatim makes use of the alt_name tag.
  • After editing the map, Nominatim does not always fully reindex an area. It might depend on current workload of the server so things can seem inconsistent. But a manual reindex can be forced by an admin after a large area has undergone a lot of edits.

The linking is important because this allows the county border and the node to be linked into a single feature so that only one result is returned when you search for a particular county. It also improves addressing since Nominatim no longer has to choose between two different things to associate addresses with. This linking can be accomplished by adding the node to the county border relation with a role of "label".

The use of Wikipedia is interesting and (I think) the source of my problems with Kearney, NE that I mentioned above. Nominatim was linking the county node to the Kearney, Nebraska Wikipedia page based on name tag. This helped bump its importance up to where it was returned before the city. Now that I have added the Kearney county node to the Kearney county relation and specified a "wikipedia=en:Kearney County, Nebraska" tag, it links things to the correct Wikipedia page and the search results return the city first, as expected.


The Fix

So to summarize, I believe revent and I have settled on the ideal way to map a county. Using Kearney county as an example:
One relation with the following tags:
type=boundary
boundary=administrative
admin_level=6
name=Kearney County
alt_name=Kearney
wikipedia=en:Kearney County, Nebraska

In addition, the following tags are in widespread use:
border_type=county
nist:state_fips=31
nist:fips_code=31099

I'm not sure the border_type tag is actually used for much but it has a fair amount of existing use and seems to further differentiate borders that are the same admin_level based on local conventions - county vs borough and such. The two nist: tags were part of the import and I have actually used the state_fips as an easy way to download all county relations in a given state to work on. These FIPS codes have been withdrawn by NIST recently but are still commonly used. We may need to look at replacing or augmenting these tags with an ISO standard at some point but for now they work.

In addition to the relation tags the following relation memberships are recommended:
  • Obviously the ways that make up the border with a role of "outer" in the correct order so that they form a closed ring
  • The node with the following two tags: place=county and name=Kearney with a role of "label"
  • The place=city/town node of the county seat with a role of "admin_centre"

The county seat city node as an admin centre (yes, use the British spelling) is kind of a new thing to me. I'm not 100% sold on it but it does seem like useful information to have.

If you have looked at very many county nodes you might notice that the "is_in" tag is conspicuously absent. It turns out, it causes more problems than it solves and should probably be left off or removed. I have always found it a bit silly anyway. OSM is a geodatabase - there is no need to indicate location in a tag.

I have also been promoting any county seats that were place=village to place=town. I know I saw this as a thing on the OSM wiki at one point although I can't find it right now. While they may not meet the population guidelines of a "town" according to the wiki, I think county seats carry extra importance due to their governmental functions. It also means that at least some city names are rendered on maps at low zoom levels in low population areas. Without this promotion you could pan around in the Kansas/Nebraska/Dakotas area for several screens at zoom level 9 or 10 and not see a single city name on the map on osm.org. In some ways this is a rendering issue but I don't feel like this is an egregious case of tagging for the renderer.

Your Turn!

So if you are looking for something to do, jump in and make sure your county is correctly linked and tagged. I have already done Kansas, Nebraska and South Dakota but I probably won't have time to do everything in the near future and I will concentrate on lower population areas first so please feel free to jump in! If there is interest, I may do another post about my workflow to do a state-wide edit. Otherwise you can always find me on the #osm or #osm-us IRC channels if you have questions.

UPDATE! I have since written another post detailing my workflow: http://ksmapper.blogspot.com/2014/02/workflow-for-fixing-county-borders.html

Incidentally, a lot of the same concepts apply to cities as well. If you do a Nominatim search for "Los Angeles, CA" right now it will return two results. One for the node and one for the boundary relation. Making the node a member of the relation should fix that.
(sigh. In doing that search I just saw that the relation for LA is completely broken anyway. Admin boundaries suck.)

In closing, here is a screen shot of the completed county relations in South Dakota:

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