rework admin boundaries and admin labels on lower zooms

[nebulon42]

Fixes #907, #1571.
Adresses part of #622 (the part that is similar to #907, #1571)

Changes are concerned with reduction of complexity and better rendering of country/state labels as well as boundary lines, especially in maintaining a visual hierarchy of admin levels 2,3,4.

Details:

• country lines are thinner on z1,2,3 but get thicker quickly and maintain a hierarchy with level 3 and 4
• level 3 and 4 are much thinner earlier
• line thickness of admin entities on higher zooms is unchanged
• line rendering only uses data derived from relations (osm_id < 0) which seems to avoid some duplicate rendering
• simplification of admin lines starting at z4 using ST_simplify with a initial tolerance of ~ 7800 and decreasing linearly with scale denominator until the layer no longer gets rendered at z10
• omission of admin levels 3,4 for smaller countries on z4, using a way_area factor of 9000000000000, which should only include Russia, Canada, China (?), United States of America, Brazil, Australia and Greenland (only if their country geometries are correct and therefore in the polygon table)
• placename labels (city, town)
  • are dropped from z3 as I think it is too early to show them
• country labels
  • are dropped from z2 (here e.g. continent labels could be rendered)
  • are darker with more char and line spacing
  • have now a semi-transparent halo
  • they wrap earlier and have larger font-size from z3
  • generally they stay bigger than state labels all the time
  • they are rendered for smaller countries earlier and stay longer (I tuned it so that the US is visible on z4)
• state labels
  • have a semi-transparent halo now
  • wrap earlier
• all boundary lines use now bevel joins as suggested by @imagico

Problems:

• usage of ST_simplify is not undisputed (see borders at zoom 4-6 #907), but I think it is a usable possibility until we have something better
• performance of SQL query deciding if subnational entities should be rendered on z4 seems suboptimal fixed, thanks @pnorman, @imagico
• since country boundaries are thicker later on sea boundaries are further emphasised

possible Improvements:

• exponential decline of tolerance parameter when using ST_simplify - if somebody wants to experiment with it

Previews (only after the changes, available data: admin entities incl. names and place names [city, town]):

z1

z2

z3

z4
Europe

Africa

Americas

unfortunately the USA geometry seems to be broken, but see this other screenshot in #907 (comment).

Oceania/Asia

z5 (from here roads are missing)

z6

Testing:
If you need test data you can download it from [link no longer active]. This file contains global admin boundaries of level 2,3,4 as well as place nodes of cities and towns. Some country geometries like France, Belgium ... and unfortunately also the US are broken so their name (and sub-national entities at z4 in case of US) won't appear.

[imagico]

usage of ST_simplify is not undisputed (see #907), but I think it is a usable possibility until we have something better

A general remark here: While this is an understandable approach in principle it is important to also keep in mind the likely negative consequences:

• establishing a superficial solution to a problem will make it less likely that someone will develop a more thorough one in the future
• the disadvantages of the presented approach are not immediately apparent. It affects readability but this is not clearly visible because the mechanisms are subtle and non-obvious. A map user will, when presented with a better rendering, likely immediately see the improvement but will usually not be able to articulate the need for it a priori.

Specific suggestions:

• use another line-join - since the corners of the boundaries are artificial anyway after ST_simplify there is no need to add even more high frequency noise.
• although not directly related it would be a good opportunity to give important cities priority over subnational admin unit labels.
• performance: i will leave the detailed analysis to others but ST_Within(b.way, c.way) is likely unneccessarily complicated, you'd just need to check the centroid of the level 3/4 polygon being within the level 2 polygon. I am not sure if there is a way to use the simplified level 2 polygons you generate anyway for the test.
• What's with China at z4?

[kocio-pl]

In general I like z4 with just the country borders. I suppose USA label should be visible if not the broken geometry you mentioned. However why Australia is different on that zoom level from the rest of the world (sub-country borders are visible)?

[pnorman]

Should this be sqrt(!pixel_width!*!pixel_height!)*...?

[imagico]

I guess the idea is to make the amount of simplifications independent of the rendering resolution.

[pnorman]

Thinking about this, I'm not sure what way works best.

Does this work appropriately when generating high resolution images?

[nebulon42]

TBH I don't know, but if using pixel dimensions is the more common approach I'm happy to change it.

[pnorman]

It depends on rationale for simplifying.

[pnorman]

I'd probably do the line SQL something like this for lines

(SELECT
    *
  FROM planet_osm_roads l
  WHERE boundary = 'administrative'
    AND (admin_level = '2'
      OR (admin_level IN ('3', '4')
        AND EXISTS 
          (SELECT 1 
            FROM planet_osm_polygon p
            WHERE admin_level = '2'
              AND way_area > !pixel_width!::real*!pixel_height!::real*something
              AND ST_Within(l.way, p.way))
  ORDER BY admin_level ASC, way_area DESC
) AS admin_very_low_zoom

[pnorman]

If you're returning functions(way) AS way you need to add && conditions to appropriate WHERE clauses to check for intersection with !bbox! And if !bbox! appears in the query string, Mapnik will not add an outer bbox check so you have to make sure to add all of the checks you need

[nebulon42]

re SQL: seems I was thinking a bit to complicated when it came to separating the less costly parts from the more expensive ones, thanks @pnorman
Using only polygon centroids speeded it up also, thanks @imagico

establishing a superficial solution to a problem will make it less likely that someone will develop a more thorough one in the future

I don't disagree with your arguments, but waiting for the perfect solution, when the problem has been there for years already is also not desirable IMO. But of course the maintainers are free to delay or decline this, if they think there will be better solutions. Also, anybody with more expertise is free to step forward.

use another line-join

I have now switched to bevel joins for all boundaries. Maybe better anyway since it helps to avoid something like this:

it would be a good opportunity to give important cities priority over subnational admin unit labels

I decided against including it as there would be too many changes, but it is on my list.

What's with China at z4?

Sub-national entities should be displayed, but in the last two planet extracts I obtained China was not in the polygon table, same with Russia.

why Australia is different on that zoom level from the rest of the world (sub-country borders are visible)?

While Australia is a bit smaller than Brazil (which I have selected as cut-off country) according to https://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_area it is either the sea boundaries or the projection or both that distorts the ranking.

If I query the database for the biggest countries (way_area) I get the following list from the data upon which the previews are based (smallest on top):

"Brasil";9.29986e+12
"United States of America";1.0177e+13
"Australia";1.06476e+13
"Kalaallit Nunaat";4.31275e+13
"Canada";7.36066e+13

You see that China and Russia are missing, USA is too small because only Alaska made it into the table and also Greenland is in the list because it extends into low latitudes. I personally have no problem with Australia getting sub-national entities on z4. We would have to hardcode names or (if we would have hstore) iso codes otherwise, but that would be getting close to country specific renderings, which I wanted to avoid.

[nebulon42]

@pnorman I cannot quite follow your last remark, can you point me towards some examples or documentation about it?

[pnorman]

https://github.com/mapnik/mapnik/wiki/PostGIS#bbox-token

An outer filter on way will be unable to use indexes because the way returned by the query is computed, and there's no suitable indexes

[pnorman]

How's this going to work in countries with multiple disjoint areas like the US or Russia?

[nebulon42]

I'll read up on it.

Next good point raised. Currently it would work only for those parts which exceed the threshold. So e.g. the overseas territories that belong to the USA would not get sub-entities if they have some. Alaska has none at z4 anyway. I'm not familiar with the structure of the Russian Federation so I'm not sure if this would be a problem.

[imagico]

I don't disagree with your arguments, but waiting for the perfect solution, when the problem has been there for years already is also not desirable IMO.

This is not what i am suggesting. But introducing an approach that can definitely not be recommended to other map designers in the vague hope that someone will some time in the future come up with something better is equally not desirable IMO.

I consider my objection essentially to be in line with what @gravitystorm told me when i made my first suggestion to deal with the Antarctic ice sheet in #646 (comment) - this essentially amounts to the demand that a solution should offer a certain level of applicability beyond the precise situation it is used in.

Please don't view this as a blanket criticism of your work here - this is an interesting approach to the problem - but it seems it only works given the very special circumstances here, i.e. rendering as polygons, styling with fairly thick lines and using the trick in https://github.com/gravitystorm/openstreetmap-carto/blob/master/admin.mss#L56

Beyond these general remarks it also occurred to me that since you apply ST_Simplify up to z10 this change would seriously affect mapper feedback by showing a modified geometry at scales where the difference is already going to be clearly visible, like here:

http://www.openstreetmap.org/#map=10/69.6781/30.6299

I'm not familiar with the structure of the Russian Federation so I'm not sure if this would be a problem.

None of the Russian arctic islands is a separate level 4 entity i think, neither is the western hemisphere part. Labeling however is probably an issue (i.e Alaska etc. would not get a correct label) But admin unit labeling is already a complete mess anyway so this will probably hardly be noticeable.

[sommerluk]

Looks clean and much more organized than the current rendering!

My suggestions:
– Simply drop country border at z1 in favour of continent labels
– Continent labels z1–z2
– On z5, for sub-national entities, use the abbreviated name instead of the normal name (like at z4)
– And, as @imagico said: give important cities priority over subnational admin unit labels

[nebulon42]

I'm actually not quite attached to the simplification. I see the main improvements with the labels, line styles and also with removing subnational units from z4. We could also leave out the simplification, but the map would still look quite ugly.

you apply ST_Simplify up to z10

I see the problem, that's why I wrote: "exponential decline of tolerance parameter when using ST_simplify". Still have to think about it, input welcome.

Please do not expect too much out of this PR. At this stage I'm not going to add more features, it is more likely that some changes are removed.

Simply drop country border at z1 in favour of continent labels

Might be a good idea, but I only want to do that when we render something to replace them (e.g. those labels).

On z5, for sub-national entities, use the abbreviated name instead of the normal name (like at z4)

I'm not sure, will have to test this.

[pnorman]

I would drop the simplification. It adds complexity, and I'm not certain the results are what we really want. We can look at it independently.

I wouldn't worry much about z0-z1. The use-cases for that are extremely limited, and it's always going to be a bad map at such a low zoom thanks to Mercator.

I am still concerned about complexity, but it might be okay with those changes.

[dieterdreist]

sent from a phone

Am 02.11.2015 um 22:14 schrieb Michael Glanznig notifications@github.com:

Simply drop country border at z1 in favour of continent labels

I did this on my own rendering but switched to a small shapefile because of phantasy continents popping up from time to time.

[kocio-pl]

@sommerluk I also made a quick list of low zoom levels based on French style few months ago, maybe there are things we could use.

[HolgerJeromin]

Rendering false continents is a good idea. At least if a fix in the database is rerendered faster than once a month (current render cycle on low zoom)

[nebulon42]

I have removed the controversial simplification.

[matthijsmelissen]

I haven't had time to look at it yet.

[pnorman]

The cartography looks fine. I'm checking into some z3/z4 performance issues with a whole planet DB.

[pnorman]

admin level 0, 1, and 3 labels will disappear on zooming in. I suggest only doing 2 and 4

[nebulon42]

Good catch, I missed that.

[pnorman]

Did the PR nebulon42#1 to fix some issues. There's still an admin 0,1,2,3,4 vs 2,4 mismatch to be fixed, but I think that's the last of the technical issues.

When those are fixed, I'm okay with it.

We're adding more layers, but I'm okay with that. It comes with our choices about how to handle layers.

The admin 4 in large admin 2 logic is a bit annoying to have to add, but it's not a complexity cost we're imposing on others. Most styles will use Natural Earth and special case iso_code2='USA' and a couple of other large places.

[nebulon42]

Thanks @pnorman. I have addressed the remaining issues.

[matthijsmelissen]

Thanks @nebulon42. I haven't studied the code in detail, but the output certainly looks much better.

[pnorman]

I used the index CREATE INDEX planet_osm_polygon_low_admin ON planet_osm_polygon USING gist (way) WITH (fillfactor=100) WHERE boundary = 'administrative' AND osm_id < 0 AND admin_level IN ('0','1','2','3','4'); while working on this - since in development you're constantly re-rendering low zooms instead of caching them for a month.

[matthijsmelissen]

Is there a reason to keep the #necountries layer and shapefile, or would we be able to get rid of this and render the low-level admin borders based on osm-data?

[nebulon42]

I think it would be great to replace the Natural Earth data with OSM data. But this would need generalization and should be done with a shapefile since Douglas-Peucker (ST_Simplify) was not well-received. This would need some logic similar to coastline processing and could be hosted on openstreetmapdata.com (?), but someone would need to provide the update and error logic.

[pnorman]

think it would be great to replace the Natural Earth data with OSM data. But this would need generalization and should be done with a shapefile since Douglas-Peucker (ST_Simplify) was not well-received. This would need some logic similar to coastline processing and could be hosted on openstreetmapdata.com (?), but someone would need to provide the update and error logic

We don't use dashed lines at the even lower zooms, so the issues with dashes don't apply here. But we can handle this in a new issue.

[nebulon42]

We don't use dashed lines at the even lower zooms, so the issues with dashes don't apply here. But we can handle this in a new issue.

Yes, we should do that. But I think at least @imagico's concerns were not mainly about dashes.

[matthijsmelissen]

Is it just me, or is zoom level 4 not being properly updated with the new rendering on the openstreetmap.org servers? Is this a glitch on the servers or in the stylesheet?

[gravitystorm]

I had to revert this pull request, since it was bringing the OSMF servers to a halt (see the revert PR #1987 ). This is the query that had been running for more than 14 hours for one request:

openstreetmap-carto/project.yaml

Lines 1322 to 1342 in 7b8998c

	(SELECT
	way,
	admin_level
	FROM planet_osm_roads o
	WHERE boundary = 'administrative'
	AND (admin_level IN ('0', '1', '2')
	OR (admin_level IN ('3', '4')
	AND EXISTS
	(SELECT 1
	FROM planet_osm_polygon i
	WHERE boundary = 'administrative'
	AND admin_level = '2'
	AND way_area > 9e+12
	AND ST_Within(o.way, i.way)
	AND osm_id < 0
	)
	)
	)
	AND osm_id < 0
	ORDER BY admin_level DESC
	) AS admin_very_low_zoom

I suspect this needs to be tested further with databases containing more representative data - it appears the query runs fine when there's a small amount of boundary-only data, but not when the tables contain lots of non-boundary data too. I think @pnorman is making some investigations regarding the query and the behaviour of the query planner.

[imagico]

The shapefile layer changes which were positive on their own could be restored independently by the way.

[pnorman]

One of the longest queries was

SELECT ST_AsBinary("way") AS geom,"admin_level" FROM (SELECT
    way,
    admin_level
  FROM planet_osm_roads o
  WHERE boundary = 'administrative'
    AND (admin_level IN ('0', '1', '2')
      OR (admin_level IN ('3', '4')
        AND EXISTS
          (SELECT 1
            FROM planet_osm_polygon i
            WHERE boundary = 'administrative'
              AND admin_level = '2'
              AND way_area > 9e+12
              AND ST_Within(o.way, i.way)
              AND osm_id < 0
          )
      )
    )
    AND osm_id < 0
  ORDER BY admin_level DESC
  ) AS admin_very_low_zoom WHERE "way" && ST_SetSRID('BOX3D(-1252344.271424999 -1252344.271424999,20037508 20037508)'::box3d, 900913);

This is the z4 query, which gets z2 admin or z4 admin in large z2.

The query plan on one of the servers was http://explain.depesz.com/s/VgNj

The query plan is moderately stupid, involving a bitmap and of a 20GB gist and 5GB btree index for the subplan, and isn't great for the roads part of the plan, though indexes are smaller there.

[pnorman]

Running on my server with only standard indexes, I get

http://explain.depesz.com/s/laJE

30 minute query time isn't great, but there are only 4 z4 metatiles, and this is the big one that covers most of Europe.

The query on production with the different plan was running for >14h before terminated.

[matthijsmelissen]

Is that a server with full planet loaded?

[pnorman]

Yes. My setup has a few differences

• Single-user, faster CPU, but much less RAM. Probably not significant
• PostgreSQL 9.4 instead of 9.1 (?). (Actually, it's the same version)
• No btree index on planet_osm_roads (osm_id). I do have one on planet_osm_polygon (osm_id)
• Data from this summer. Not a significant difference
• No slim tables. Doesn't matter
• No updates to the render tables. Not significant, though it does make it probably 25-75% faster without bloat

Since the query takes >14h to run on production, I'm not going to get any selectivity estimates to see why it's doing the BitmapAnd plan or if it's a good plan, but my instincts are telling me it's a horrible plan.

My inclination is to fix this through #207, because a suitable partial index brings query times down into the 1 minute range, and even an ill-suited one is way better.

[nebulon42]

Oh dear, I will provide a new PR with just the style updates as I think these are worthwhile to have in also without the new logic on z4.