[OSM-talk] A forest ... what?

Mon Apr 10 08:45:06 UTC 2017

Hi sandor:

to long - did not read it.

keep it simple please,

regards
walter

Am 10.04.2017 um 10:04 schrieb Sandor Seres:
>
> Three weeks ago I posted some multipolygon related notes. This mail 
> is, in a way, an addition to that former mail.
>
> My first note was triggered by some user worries about poorer maps if 
> they use data from the osm2ogsql preparation. Dropping “broken 
> multipolygons” will result in many and large empty/white places with 
> long reparation period. Strengthening the preparation on the subject 
> might be a better option in my opinion (I know, I was there). However, 
> at the end, how this subject will be handled is perfectly up to the 
> authors of the osm2pgsql application.  Users starting from the OSM 
> source data will not be affected whatever strategy will be selected.
>
> The second note was related to the mass/programmatic correction of the 
> source data. This could have dangerous/damaging impact on many OSM 
> users. Fortunately, the replays say that programmatic correction is 
> not a strategy in the “fixing multypolygons” actions. I have mentioned 
> the “self-crossings” issue which is not an error for many users 
> (depending on what notion interpretations and tools one uses). To 
> clean up the confusion, this note needs some additional words. Assume 
> someone would correct all polygon self-crossings in the source data. 
> Assume, the selected fixing model is the popular dividing model (the 
> polygon is divided into new polygons between self-crossings). The 
> “fix” will be correct but the consequences damaging. Namely, in 
> scaling and rendering the new small areas quickly  reach 
> ignorable/collapsing size causing brakes. Here, it is worth noting, 
> that the self-crossing issue is a topic in the modern vector based 
> digital mapping even if all self-crossings are somehow resolved in the 
> source data. Namely, while scaling and doing edge-smoothing in data 
> generalisation, self-crossings on thin area sections (like fiords, 
> peninsulas, rivers and so on) are unavoidable and dividing produces 
> many tiny areas. High fragmentation of the source data and freedom of 
> tag selection (river sections tagged as lakes) make the issue even 
> worse. Just look  at the Amazonas river-system rendering from a 
> popular vector map-maker her http://goo.gl/bT1Bu9
>
> (the screen dump is from yesterday, from a demo system, in roughly 
> 1:6.7 mill scale). There are really many and large unacceptable 
> breaks. However, from the same data source, using topology geometry as 
> suggested in my former mail, it is possible to create a compact 
> minimal coverage for the same river system like this 
> https://goo.gl/pNQwDm . Note that the river system her is one simple 
> area (one outer and many inner borders never touching each other) from 
> Peru to the Atlantic. To be on the fair side the last image should be 
> rendered from a zoom/scale level that corresponds to the 1:6.7 mill 
> scale. This is done here https://goo.gl/eaAWNy and the zoom level 
> contains approximately 250 times less nodes than the level used for 
> the previous image. The area connectivity is still perfectly preserved 
> and the image is much cleaner in this scale extract. Finally, if a 
> user is still insist on fixing the polygon self-crossings, exchanging 
>  and reversing the poly-lines between two consecutive self-crossings 
> (eventually just reversing the end loop after a self-crossing) should 
> be a much better strategy.
>
> However, the third, the last note was my major point. Just to remind. 
> There is a large set of area related anomalies caused by relations 
> between objects from different classes (between seas, forests, lakes, 
> rivers…). The extent and complexity of this set is far beyond the 
> “broken polygons” issue  and should be more in the development focus. 
> Even if the areas/multipolygons within a class are in perfect 
> conformity with the strongest OSM and OGC rules, still these anomalies 
> are there, though sometimes hardly visible in maps. Therefor many 
> map-makers tolerate them but in GIS systems they appear as strong 
> limitations and should not be tolerated. In the former mail  I have 
> presented many examples and some hints how these anomalies could be 
> resolved. Unfortunately, the discussion went in a wrong direction, 
> about the Scandinavian forests, while the region selection is 
> irrelevant for the subject. To avoid much repetition I will present 
> further examples without details in procedures. The illustrations are 
> from the area of Japan (one of the best mapped areas) and the source 
> is the standard OSM dump from some week ago.
>
> Honestly, I am not sure what a forest is. More precisely, if you ask 
> me – I know, if you ask me to tell what it is – I do not know. 
> However, among the many interpretations, I am closest to accept the 
> topology interpretation of the notion. The green area in the front 
> page map (or in other OSM based maps) usually covering the areas 
> tagged as forest and/or wood. In Japan, as everywhere, forests are 
> uploaded highly fragmented, they overlap in the most strange 
> combinations, the same with river and lake area objects. The most 
> common case is when borders of neighbouring objects run in and out of 
> each other. The fragmentation itself is causing lots of problems even 
> in rendering. Just look at these examples (the well-known light/dark 
> stripes) here http://osm.org/go/7WCEND?layers=H or here 
> http://osm.org/go/7WCzACu--?layers=C or here https://goo.gl/JVI1E7 or 
> here https://goo.gl/Xhv1nq . Extending the areas within the object 
> classes may help in rendering but still the fragmentation is there.
>
> Assume, we have managed to remove all redundancy, repair most of the 
> “broken polygons” and perform full defragmentation within area 
> classes: forests, lakes, rivers and land masses. Besides, we managed 
> to recognize and replace missing river sections, missing islands in 
> lakes and rivers. So, within any of these object classes we have the 
> best data presentation that is potentially possible from the source 
> data. Yet, we quickly discover that there are forests overwritten by 
> lakes, rivers running over forests, borders of lakes running in and 
> out of forests and so on (the inter class anomalies). While these 
> anomalies are not show stoppers in rendering, they limit the 
> corresponding GIS’s quality, statistics, quantitative analyses and 
> forecasts (number of trees in forests, CO2 consumption per year, 
> oxygen production per year and so on).  Let us assume, we have managed 
> to repair all these anomalies by using the topology geometry/calculus 
> as hinted in my previous mail. Then some of the results are like these:
>
> The country’s land area created from the coastline data is here 
> http://goo.gl/O1L60r , the border polygons are disjunctive and there 
> are no holes at all. Subtracting all inland water areas and adding the 
> islands within these, we get the land-masses illustrated here 
> http://goo.gl/OM2dqn <http://goo.gl/OM2dqn>. The yellow areas 
> represent a minimal simple/compact land-masses coverage. The inland 
> waters make only about 0.5% of the land area.
>
> The countries forest coverage is pretty high https://goo.gl/HU63M7 . 
> The forests cover around 63.6% of the land-masses, though there are 
> still some forests to be mapped (see the Kyushu island). The largest 
> compact/simple forest area, here https://goo.gl/4yzeyC, by size equals 
> to 24% of all forests. It consists of one outer/container and 25831 
> inner/excluding polygons. All polygons are disjunctive and from any 
> point A to any point B in this area one can go walking exclusively 
> through the forest (hm, the shortest way?). However, the holes of this 
> largest simple area contain additional 2892 new (small) “forests”. An 
> extract from this complete, largest reginal forest is presented here 
> https://goo.gl/mzgDRg . The light green is the largest simple forest 
> area while the dark green represents the smaller forests in holes. One 
> can see that there are even holes in these small forests and new 
> forests in their holes and so on. Similar inclusions sometimes go up 
> to 6 levels. The ten largest simple areas make 70.2% of all forests in 
> the country.
>
> Finally, extending the case to other object types and/or larger areas 
> like continents or the Planet, one can feel the huge potential of OSM, 
> especially in the future with growing content. Simply, it is difficult 
> not to be an enthusiast of it.
>
> Regards, Sandor
>
>
>
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